- 


F  THE 

/ERSH7 
OF 


. 


J 


* 


ELEMENTS 


OP 


PHYSICS; 


OR, 

NATURAL  PHILOSOPHY, 

GENERAL    AND    MEDICAL: 

WRITTEN  FOE  UNIVERSAL  USE, 

IN 

PLAIN  OR  NON-TECHNICAL  LANGUAGE; 

AND.  CONTAINING 

NEf  DISQUISITIONS  AND  PRACTICAL  SUGGESTIONS. 

COMPRISED  IN  FIVE  PARTS: 


1.   SOMATOLOGY,    STATICS   AND 


DYNAMICS. 


2.   MECHANICS. 


3.  PNEUMATICS,    HYDRAULICS 

AND    ACOUSTICS. 

4.  HEAT  AND  LIGHT. 


5.  ANIMAL  AND  MEDICAL  PHYSICS. 


BY  NEILL  AKNOTT,  M.  D.     , 

OF  THE  ROTALJ30LLEGE  OF  PHYSICIANS. 


L  Xjpl 


A  NEW  EDITION,  REVISED  AND  CORRECTED  FROM  THE  LAST  ENGLISH  EDITION. 

WITH  ADDITIONS. 
BY    ISAAC    HAYS,   M.  D. 

COMPLETE  IN  ONE  VOLUME. 

i 

PHILADELPHIA: 

BLANCHARD    AND    LEA. 

1856. 


Entered  according  to  Act  of  Congress,  in  the  year  1841,  by 
LEA   AND   BLANCHARD, 

in  'the  Clerk's  Office  of  the  District  Court  of  the  United  States,  in  and  for 
the  Eastern  District  of  Pennsylvania. 


GIFT 


Al 


ADVERTISEMENT 


AMERICAN    PUBLISHERS 


THE  very  valuable  and  popular  work  of  Dr.  Arnott,  has  passed 
through  several  editions  in  this  country,  in  the  form  in  which  it 
was  originally  published  by  the  author,  in  separate  parts.  A  new 
edition  being  now  called  for,  the  work  has  been  carefully  revised 
and  corrected,  and  the  whole  condensed  into  one  volume.  In  tkis 
form  it  cannot  fail  to  be  more  acceptable  to  the  public,  and  rendered 
more  convenient  and  useful  for  the  purposes  of  instruction  in  the 
various  Colleges  and  Seminaries  of  Learning  that  have  adopted  it 
as  a  Class  Book  for  their  pupils.  This  volume  embraces  all  that 
has  been  prepared  or  published  by  the  author. 


VI  INTRODUCTION. 

knowledge.  Language  thus,  at  the  present  moment  of  the  world's  existence, 
may  be  said  to  bind  the  whole  human  race  of  uncounted  millions,  into  one 
gigantic  rational  being,  whose  memory  reaches  to  the  beginnings  of  written 
records,  and  retains  imperishably  the  important  events  that  have  occurred ; 
whose  judgment  analyzing  the  treasures  of  memory,  has  discovered  many 
of  the  sublime  and  unchanging  laws  of  nature,  and  has  buift  on  them  all 
the  arts  of  life,  and  through  them  piercing  far  into  futurity,  sees  clearly 
many  of  the  events  that  are  to  come,  and  whose  eyes  and  ears,  and  obser- 
vant mind  at  this  moment,  in  every  corner  of  the  earth,  are  watching  and 
recording  new  phenomena,  for  the  purpose  of  still  better  comprehending  the 
magnificence  and  beautiful  order  of  creation,  and  of  more  worthily  adoring 
its  beneficent  author. 

It  might  be  very  interesting  to  show  here,  in  minute  detail,  how  the  arts 
and  civilization  have  progressed  in  accordance  with  the  gradual  increase  of 
man's  knowledge  of  the  universe;  but  to  do  so  would  lead  too  far  from  the 
main  subject.  We  deem  it  right,  however,  to  make  evident  to  the  student 
the  arousing  truths,  that  the  progress  is  not  yet  at  an  end;  that  it  has  been 
vastly  more  rapid  in  recent  times  than  ever;  and  that  it  seems  still  to  pro- 
ceed with  increasing  celerity  : — and  we  know  not  where  the  Creator  has  fixed 
the  limits  of  the  change  !  Although  there  are  thousands  of  years  on  the 
records  of  the  world,  our  BACON,  who  first  taught  the  true  way  to  investi- 
gate nature,  lived  but  the  other  day.  NEWTON  followed  him,  and  illustrated 
his  precepts  by  the  most  sublime  discoveries  which  one  man  has  ever  made. 
HARVEY  detected  the  circulation  of  the  blood  only  two  hundred  years  ago. 
ADAM  SMITH,  DR.  BLACK  and  JAMES  WATT  were  friends,  and  the  last, 
whose  steam-engines  are  now  changing  rapidly  the  condition  of  empires,  may 
be  said  to  be  scarcely  cold  in  his  grave.  JOHN  HUNTER  died  not  long  ago ; 
HERSCHEI/S  accounts  of  newly-discovered  planets,  and  of  the  sublime  struc- 
ture of  the  heavens,  and  DAVY'S  account  of  chemical  discoveries  not  less 
important  to  man,  are  in  the  late  numbers  of  our  scientific  journals; — illus- 
trious Britons  these,  and  who  have  left  worthy  successors  treading  in  their 
steps.  On  the  continent  of  Europe  during  the  same  period,  a  corresponding 
constellation  of  genius  has  shone ;  and  LAPLACE  was  lately  the  bright  star 
shining  between  the  future  and  the  past. 

But  there  is  a  change  going  on  in  the  world,  connected  closely  with  the 
progress  of  science,  yet  distinct  from  it,  and  more  important  than  a  great  part 
of  the  scientific  discoveries; — it  is  the  diffusion  of  existing  knowledge  among 
the  mass  of  mankind.  Formerly,  knowledge  was  shut  up  in  convents  and 
universities,  and  in  books  written  in  the  dead  languages — or  in  books  which, 
if  in  the  living  languages,  were  so  abtruse  and  artificial,  that  only  a  few  per- 
sons had  access  to  their  meaning ;  and  thus  the  human  race  being  considered 
as  one  great  intellectual  creature,  a  smalljfraction  only  of  its  intellect  was 
allowed  to  come  into  contact  with  science,  and  therefore  into  activity.  The 
progress  of  science  in  those  times  was  correspondingly  slow,  and  the  evils  of 
general  ignorance  prevailed.  Now,  however,  the  strong  barriers  which  con- 
fined the  stores  of  wisdom  have  been  thrown  down,  and  a  flood  is  overspread- 
ing the  earth;  old  establishments  are  adapting  themselves  to  the  spirit  of 
the  age ;  new  establishments  are  arising ;  the  inferior  schools  are  introducing 
improved  systems  of  instruction ;  and  good  books  are  rendering  every  man's 
fireside  a  school.  From  all  these  causes  there  is  growing  up  an  enlightened 
public  opinion,  which  quickens  and  directs  the  progress  of  every  art  and 
science,  and  through  the  medium  of  a  free  press,  although  overlooked  by 
many,  is  now  rapidly  becoming  the  governing  influence  in  all  the  affairs  of 


INTRODUCTION.  VH 

• 

man.  In  Great  Britain,  partly  perhaps  as  a  consequence  of  its  insular  situa- 
tion, which  lessened  among  its  inhabitants  the  dread  of  hostile  invasion,  and 
sooner  formed  them  into  a  united  and  compact  people,  the  progress  of  enlight- 
ened public  opinion  had  been  more  decided  than  in  any  other  state.  The 
early  consequences  were  more  free  political  institutions ;  and  these  gradually 
led  to  greater  and  greater  improvements,  until  Britain  became  an  object  of 
admiration  among  the  nation*.  A  colony  of  her  children,  imbued  with  her 
spirit,  now  occupies  a  magnificent  territory  in  the  new  world  of  Columbus ; 
and  although  it  has  been  independent  as  yet  for  only  half  a  century,  it  already 
counts  more  people  than  Spain,  and  will  soon  be  second  to  no  nation  on 
earth.  The  example  of  the  Anglo-Americans  has  aided  in  rendering  their 
western  hemisphere  the  cradle  of  many  other  gigantic  states,  all  free,  and 
following,  although  at  a  distance,  the  like  steps.  In  the  still  more  recently 
discovered  continent  of  Australasia,  which  is  nearly  as  large  as  Europe,  and 
is  empty  of  men,  colonization  is  spreading  with  a  rapidity  never  before  wit- 
nessed ;  and  that  beautiful  and  rich  portion  of  the  earth  will  soon  be  covered 
with  the  descendants  of  free-bore  and  enlightened  Englishmen.  Thence, 
still  onward,  they  or  their  institutions  will  naturally  spread  over  the  vast 
archipelago  of  the  Pacific  Ocean,  a  tract  studded  with  islands  of  paradise. 
Such,  then,  is  the  extraordinary  moment  of  revolution,  or  transit,  in  which 
the  world  at  present  exists  !  And  where,  we  may  ask  again,  has  the  Creator 
predestined  that  the  progress  shall  cease  ?  Thus  far  at  least  we  know,  that 
he  has  made  our  hearts  rejoice  to  see  the  world  filling  with  happy  human 
beings,  and  to  observe  that  the  increase  of  the  sciences  can  make  the  same 
spot  maintain  thousands  in  comfort  and  godlike  elevation  of  mind,  where 
with  ignorance  even  hundreds  had  found  but  a  scanty  and  degrading  supply. 

The  progress  of  knowledge  which  has  thus  led  from  former  barbarism  to 
present  civilization,  has  gone  on  by  certain  remarkable  steps,  which  it  is  easy 
to  point  out  ;  and  which  it  is  very  useful  to  consider,  because  we  thereby 
discover  the  nature  of  human  knowledge,  with  the  relations  and  importance 
of  its  different  branches;  and  we  obtain  great  facilities  for  studying. science, 
and  for  quickening  its  farther  progress. 

The  human  mind,  when  originally  directed  to  the  almost  infinity  of  objects 
in  the  universe  around  it,  must  soon  have  discovered  that  there  were  resem- 
blances among  them  ;  in  other  words,  that  the  infinity  was  only  a  repetition 
of  a  certain  number  of  kinds.  Among  animals,  for  instance,  it  would  distin- 
guish the  sheep,  the  dog,  the  horse ;  among  vegetables,  the  oak,  the  beech, 
the  pine  ;  among  minerals,  lime,  flint,  the  metals,  and  so  forth.  And  becom- 
ing aware  that  by  studying  an  examplar  of  each  kind,  its  limited  power  of 
memory  might  acquire  a  tolerably  correct  knowledge  of  the  whole,  while 
this  knowledge  would  enable  the  possessors  more  easily  to  obtain  what  was 
useful  to  them,  and  to  avoid  what  was  hurtful,  the  desire  for  such  knowledge 
must  have  arisen  with  the  first  exercise  of  reason.  Accordingly,  the  pursuit 
of  it  has  been  unremitting,  and  the  labour  of  ages  has  at  last  nearly  com- 
pleted an  arrangement  of  the  constituent  materials  of  the  universe,  under 
three  great  classes  of  MINERALS,  VEGETABLES  and  ANIMALS;  commonly 
called  the  three  kingdoms  of  Nature,  and  .of  which  the  minute*  description 
is  termed  NATURAL  HISTORY  :  and  museums  of  natural  history  have  been 
formed,  which  contain  a  specimen  of  almost  every  object  included  in  these 
classes,  so  that  now,  a  student  within  the  limits  of  an  ordinary  garden,  may 
be  said  to  be  able  to  examine  the  whole  of  the  material  universe. 

While  men  are  examining  the  forms  and  other  qualities  of  the  bodies 
around  them,  they  could  not  avoid  noticing  also  the  motions  or  changes  going 


Vlll  INTRODUCTION. 

• 

on  among  bodies ;  and  here,  too,  they  would  soon  make  the  grand  discovery 
that  there  were  resemblances  in  the  multitude.  Self-interest,  as  in  the  case 
of  the  bodies  themselves,  having  prompted  to  careful  classification,  in  the 
present  day,  as  the  result  of  countless  observations  and  experiments  made 
through  the  series  of  ages,  we  are  enabled  to  say,  that  all  the  motions,  or 
changes,  or  phenomena  (words  synonymous  here)  of  the  universe,  are  merely 
a  repetition  and  mixture  of  a  few  simple  manners,  or  kinds  of  motion  or 
change,  which  are  as  constant  and  regular  in  every  case  as  where  they  pro- 
duce the  returns  of  day  and  night,  and  of  the  seasons.  All  these  phe- 
nomena are  referable  to  four  distinct  classes,  which  we  call  Physical,  Chemi- 
cal, Vital  and  Mental.  The  simple  expressions  which  describe  them  are 
denominated  General  Truths  or  Laws,  of  Nature,  and  as  a  body  of  know- 
ledge, they  constitute  what  is  called  SCIENCE  OF  PHILOSOPHY,  in  contra- 
distinction of  NATURAL  HISTORY,  already  described.  Now  as  man  cannot, 
independently  of  a  supernatural  revelation,  learn  anything  but  what  respects, 
1st,  the  momentary  state,  past  or  present,  of  himself  and  the  objects  around 
him;  and  2d,  the  manner  in  which  the  states  are  changed;  Natural  His- 
tory and  Science,  in  the  sense  now  explained,  make  up  the  whole  sum  of  his 
knowledge  of  nature. 

To  exemplify  the  process  by  which  a  general  truth  or  law  of  nature  is 
discovered,  we  shall  take  the  physical  law  of  gravity  or  attraction.  1st.  It 
was  observed  that  bodies,  in  general,  if  raised  from  the  earth,  and  left  un- 
supported, fell  towards  it;  while  flame,  smoke,  vapors,  &c.,  if  left  free, 
ascended  away  from  the  earth.  It  was  held,  therefore,  to  be  a  very  general 
law,  that  things  had  weight;  but  that  there  were  exceptions  in  such  mat- 
ters, as  now  mentioned,  which  were  in  their  nature  light  or  ascending.  2d. 
It  was  discovered  that  our  globe  of  earth  is  surrounded  by  an  ocean  of  air, 
having  nearly  fifty  miles  of  altitude  or  depth,  and  of  which  a  cubic  foot 
taken  near  the  surface  of  the  earth,  weighs  about  an  ounce.  It  was  then 
perceived  that  flame,  smoke,  vapor,  &c.,  rise  in  the  air  only  as  oil  rises  in 
water,  viz.,  because  not  so  heavy  as  the  fluid  by  which  they  are  surrounded  ; 
it  followed,  therefore,  that  nothing  was  known  on  earth  naturally  light,  in 
the  ancient  sense  of  the 'word.  3d.  It  was  found  that  bodies  floating  in 
water,  near  to  each  other,  approached  and  feebly  cohered ;  that  any  contigu- 
ous hanging  bodies  were  drawn  towards  each  other,  so  as  not  to  hang  quite 
perpendicularly ;  and  that  a  plummet  suspended  near  a  hill  was  drawn  to- 
wards the  hill  with  force  only  so  much  less  than  that  with  which  it  was 
drawn  towards  the  earth,  viz ,  the  weight  of  the  plummet,  as  the  hill  was 
smaller  than  the  earth.  It  was  then  proved  that  weight  itself  is  only  an 
instance  of  a  more  general  mutual  attraction,  operating  between  all  the  con- 
stituent elements  of  this  globe ;  and  which  explains,  moreover,  the  fact  of 
the  rotundity  of  the  globe,  all  the  parts  being  drawn  towards  a  common  cen- 
tre, as*  also  the  •  form  of  dew-drops,  rain-drops,  globules  of  mercury,  and 
of  many  other  things ;  which,  still  further,  is  the  reason  why  the  distinct 
particles  of  which  any  solid  mass,  as  a  stone  or  piece  of  metal,  is  composed, 
cling  together  as  a  mass,  but  which,  when  overcome  by  the  repulsion  of  heat, 
allows  the  game  particles  to  assume  the  form  of  a  liquid  or  air.  4th.  It 
was  further  observed,  that  all  the  heavenly  bodies  are  round,  and  must, 
therefore,  consist  of  material  obeying  the  same  law.  5th.  And  lastly,  that 
these  bodies,  however  distant,  attract  each  other ;  for  that  the  tides  of  our 
ocean  rise  in  obedience  to  the  attraction  of  the  moon,  and  become  high  or 
spring-tides,  when  the  moon  and  sun  operate  in  the  same  direction.  Thus 
the  sublime  truth  was  at  last  made  evident,  and  by  the  genius  of  the  immor- 


INTRODUCTION.  IX 

tal  Newton,  that  there  is  a  power  of  attraction  connecting  together  the  bodies 
of  this  solar  system  at  least,  and  probably  limited  only  by  the  bounds  of  the 
universe. 

Acquaintance  with  the  laws  of  nature  has  been  very  slowly  obtained,  owing 
to  that  complexity  of  ordinary  phenomena,  which  is  produced  by  several 
laws  operating  together,  and  under  great  variety  of  circumstance.  With 
respect  to  many  laws  of  Chemistry  and  Life,  men  seem  to  be  yet  little  far- 
ther advanced  than  they  were  with  respect  to  the  physical  law  of  attraction, 
when  they  knew  only  that  heavy  things  fell  to  the  earth.  But  we  have 
learned  enough  to  perceive  that  the  great  universe  is  as  simple  and  harmo- 
nious as  it  is  immense ;  and  that  the  Creator,  instead  of  interposing  sepa- 
rately, or  miraculously,  in  the  common  sense  of  the  word,  to  produce  every 
distinct  phenomenon,  has  willed  that  all  should  proceed  according  to  a  few 
general  laws.  There  is  nothing  in  nature  so  truly  miraculous  and  adorable 
as  that  the  endless  and  beneficent  variety  of  results  which  we  see,  should 
spring  from  such  simple  elements.  In  times  of  ignoranee,  men  naturally 
regarded  every  occurrence  which  they  did  not  understand,  that  is  to  say, 
which  they  could  not  refer  to  a  general  law,  as  arising  from  a  direct  inter- 
ference of  supreme  power ;  and  thus,  for  many  ages,  among  some  nations 
still,  eclipses  and  earthquakes,  and  many  diseases,  particularly  those  of  the 
mind,  and  the  winds  and  weather,  were  or  are  accounted  miraculous.  Hence 
arose,  among  heathens,  many  ceremonies,  and  sometimes  even  barbarous 
sacrifices,  for  propitiating  or  appeasing  their  offended  deities ;  but  founded 
on  expectations  no  more  reasonable  than  if  we  should  now  pray  to  have  the 
day  of  the  year  made  shorter,  or  to  have  a  coming  eclipse  averted.  They 
had  not  yet  risen  to  the  sublime  conception  of  the  one  God,  who  said,  "  Let 
there  be  light,"  and  the  light  was;  and  who  gave  the  whole  of  nature  per- 
manent laws,  which  he  allows  men  to  discover  for  the  direction  of  their  con- 
duct in  life — laws  so  unchanging,  that  by  them  we  can  calculate  eclipses 
backward  or  forward  for  thousands  of  years,  almost  without  erring,  by  the 
time  of  one  beat  of  a  pendulum  ;  and  as  our  knowledge  of  nature  advances, 
we  can  anticipate  and  explain  other  events  with  equal  precision.  Even  the 
wind  and  the  rain,  which,  in  common  speech,  are  the  types  of  uncertainty 
and  change,  obey  laws  as  fixed  as  those  of  the  sun  and  moon  ;  and  already, 
as  regards  many  parts  of  the  earth,  man  can  foretell  them  without  fear  of 
being  deceived.  He  plans  his  voyages  to  suit  the  coming  monsoons,  and  he 
prepares  against  the  floods  of  the  rainy  seasons. 

The  general  laws  of  Nature,  divisible,  as  stated  above,  into  the  four  classes 
of,  1st,  Physics,  often  called  Natural  Philosophy ;  2d,  of  Chemistry ;  3d, 
of  Life,  commonly  called  Physiology  ;  and  4th,  of  Mind,  may  be  said  to 
form  the  pyramid  of  Science,  of  which  Physics  is  the  base,  while  the  others 
constitute  succeeding  layers  in  the  order  now  mentioned ;  the  whole  having 
certain  mutual  relations  and  dependencies  well  figured  by  the  parts  of  a 
pyramid.  We  must  describe  them  more  particularly,  to  show  these  relations. 

Physics. — The  laws  of  Physics  govern  every  phenomenon  of  nature  in 
which  there  is  any  sensible  change  of  place,  being  concerned  alone  in  the 
greater  part  of  these  phenomena,  and  regulating  the  remainder  which  origi- 
nate from  chemical  action,  and  from  the  action  of  life.  The  great  physical 
truths,  as  comprehended  in  the  present  day  by  man,  are  reduced  to  four,  and 
are  referred  to  by  the  words  atom,  attraction,  repulsion  and  inertia.  It 
gives  an  astonishing,  but  true  idea  of  the  nature  and  importance  of  methodi- 
cal Science,  to  be  told  that  a  man,  who  understands  these  words,  viz.,  how 
the  ATOMS  of  matter  by  mutual  ATTRACTION  approach  and  cling  together 


X  INTRODUCTION. 

to  form  masses,  which  are  solid,  liquid,  or  aeriform,  according  to  the  quan- 
tity or  REPULSION  of  heat  among  them,  and  which,  owing  to  their  INERTIA 
or  stubbornness,  gain  and  lose  motion,  in  exact  proportion  to  the  force  of 
attraction  or  repulsion  acting  on  then, — understands  the  greater  part  of  the 
phenomena  of  nature ;  but  such  is  the  fact !  Solid  bodies  existing  in  con- 
formity with  these  truths,  exhibit  all  the  phenomena  of  Mechanics  ;  Liquids 
exhibit  those  of  Hydrostatics  and  Hydraulics  ;  Airs,  those  of  Pneumatics  ; 
and  so  forth,  as  seen  in  the  table  of  heads  given  below,  at  page  xii.  And 
the  whole  of  this  work  is  merely  a  list  of  the  most  interesting  physical 
phenomena,  arranged  in  classes  under  these  heads. 

Chemistry. — Had  there  been  only  one  kind  of  substance  or  matter  in  the 
universe,  the  laws  of  Physics  would  have  explained  all  the  phenomena;  but 
there  are  iron,  and  sulphur,  and  charcoal,  and  about  fifty  others,  which  to 
the  present  state  of  science,  appear  essentially  distinct.  Now  these,  when 
taken  singly,  obey  the  laws  of  Physics ;  but  when  two  or  more  of  them  are 
placed  in  contact,  under  certain  circumstances,  they  exhibit  a  new  order  of 
phenomena.  Iron  and  sulphur,  for  instance,  brought  together  and  heated, 
disappear  as  individuals,  and  unite  into  a  yellow  metallic  mass,  which,  in 
most  of  its  properties,  is  unlike  to  either : — under  other  new  circumstances, 
the  two  substances  will  again  separate,  and  assume  their  original  forms.  Such 
changes  are  called  chemical,  (from  an  Arabic  word  signifying  to  burn,  be- 
cause so  many  of  them  are  effected  by  means  of  heat,)  but  during  the 
changes,  the  substances  are  not  withdrawn  from  the  influence  of  the  physi- 
cal laws, — their  weight  or  inertia,  for  instance,  is  not  altered  ;  and  indeed 
the  phenomenon  is  merely  a  modification  of  general  attraction  and  repul- 
sion. Many  chemical  changes,  besides,  are  only  the  beginnings  of  purely 
mechanical  changes,  as  when  the  new  chemical  arrangement  produced  by 
heat  among  the  intimate  atoms  of  gunpowder,  causes  the  mechanical  or 
physical  motion  of  the  sudden  expansion  or  explosion.  And  all  the  mani- 
pulations of  Chemistry,  as  the  transferring  of  gases  from  vessel  to  vessel, 
the  weighing  of  bodies,  pounding,  grinding,  &c.,  are  directed  to  Physics 
alone.  Chemistry,  then,  is  truly,  as  figured  above,  a  superstructure  on 
Physics,  and  cannot  be  understood  or  practised  by  a  person  who  is  ignorant 
of  Physics.  The  chief  departments  of  study  involving  the  consideration  of 
Chemical  in  conjunction  with  Physical  laws,  are  enumerated  in  the  table 
below,  under  the  head  of  CHEMISTRY. 

Life. — The  most  complicated  state  in  which  matter  exists,  is  where,  under 
the  influence  of  life,  it  forms  bodies  with  a  curious  internal  structure  of  tubes 
and  cavities,  in  which  fluids  are  moving,  and  producing  incessant  internal 
change.  These  are  called  Organized  Bodies,  because  of  the  various  distinct 
parts  or  organs  which  they  contain ;  and  they  form  two  remarkable  classes, 
the  individuals  of  one  of  which  are  fixed  to  the  soil,  and  are  called  Vegeta- 
bles ;  and  of  the  other,  are  endowed  with  power  of  locomotion,  and  are 
called  Animals.  The  phenomena  of  growth,  decay,  death,  sensation,  self- 
motion,  and  many  others,  belong  to  life,  but  from  occurring  in  material 
structures  which  subsist  in  obedience  to  the  laws  of  Physics  and  Chemistry, 
the  life  is  truly  a  superstructure  on  the  other  two,  and  cannot  be  studied 
independently  of  them.  Indeed  the  greater  part  of  the  phenomena  of 
organic  life  are  merely  chemical  and  physical  phenomena,  modified  by  an 
additional  principle.  The  science  of  Life  is  divided  into  animal  and  vege* 
table  Physiology  (see  the  table  below). 

Mind. — The  most  important  part  of  all  science  is  the  knowledge  which 
man  has  obtained  of  the  laws  governing  the  operations  of  his  own  MIND. 
This  department  stands  eminently  distinct  from  the  others,  on  several 


INTRODUCTION.  XI 

accounts.  Unlike  that  of  organic  life,  which  could  not  be  understood  until 
physics  and  chemistry  had  been,  previously  investigated,  this  had  made 
extraordinary  advances  in  a  very  early  age,  when  the  others,  as  methodical 
sciences,  had  scarcely  begun  to  exist.  In  proof  of  this  assertion  we  need 
only  refer  to  the  writings  of  the  Greek'  philosophers.  The  most  brilliant 
discoveries  and  applications,  however,  were  reserved  for  the  moderns,  as  will 
occur  to  many  readers,  on  perusing,  in  the  table  below,  the  several  divisions 
of  the  subject,  and  recollecting  the  honoured  names  which  are  now  associa- 
ted with  each.  It  is  truly  admirable  to  see  the  modern  analysis,  deducing 
from  a  few  simple  laws  of  mind,  act  the  subordinate  departments,  just  as  it 
deduces  mechanics,  hydrostatics,  pneumatics,  &c.,  from  the  laws  of  physics : 
and  let  us  hope  that  sound  opinions  on  this  subject,  ensuring  human  happi- 
ness, and  therefore,  beyond  comparison,  more  important  than  any  other 
knowledge,  will  soon  be  widely  spread.  The  crowning  science  of  Mind, 
although  in  certain  respects  independent  of  the  science  of  Matter,  is  still 
closely  allied  to  them  in  the  following  ways.  The  faculties  of  the  mind  are 
originally  awakened  or  called  into  activity  solely  by  the  impressions  of  mat- 
ter or  external  nature ;  all  the  language  used  in  speaking  of  mind  and  its 
operations,  is  borrowed  from  matter ;  and  many  mental  emotions  are  entirely 
dependent  on  bodily  conditions.  The  science  of  Mind,  therefore,  cannot  be 
studied  until  after  knowledge  acquired  of  an  external  nature;  and  cannot 
be  studied  extensively  until  that  knowledge  be  extensive. 

Quantity. — To  express  most  of  the  facts  an  J  laws  of  Physics,  Chemistry 
and  Life,  terms  of  QUANTITY  are  required,  as  when  we  speak  (  f  the  magni- 
tude of  a  body,  or  say,  that  the  force  of  attraction  between  two  bodies 
diminishes,  in  a  certain  proportion,  as  their  distance  increases.  Hence 
arises  the  necessity  of  having  a  set  of  fixed  measures  or  standards,  with  which 
to  compare  all  other  quantities.  Such  measures  have  been  adopted;  and 
they  are,  for  NUMBERS,  the  fingers,  Jives  and  tens;  for  LENGTH,  the  human 
foot,  cubit,  pace,  &c.  ;  and  lately  the  second' s  pendulum  and  the  French  metre, 
(taken  from  the  magnitude  of  our  globe);  for  SURFACES,  the  simplest  forms 
of  circle,  square,  triangle,  &c.,  compared  among  themselves  by  the  lengths 
of  their  diameters  or  other  suitable  lines;  and  for  SOLID  BULK,  the  corres- 
ponding simple  solids,  of  globe.,  cube,  pyramid,  cone,  &c.t  similarly  compared 
by  the  lengths  of  diameters  or  of  other  lines  of  dimension.  The  rules  for 
applying  these  standards  to  ail  possible  cases,  and  for  comparing  all  kinds 
of  quantities  with  each  other,  constitute  a  body  of  science,  called  the  Science 
of  Quantity,  the  Mathematics.  It  may  be  considered  as  a  subsidiary  depart- 
ment of  human  science,  created  by  the  mind  itself,  to  facilitate  the  study  of 
the  others. 

Supposing  description  of  particulars,  or  Natural  History,  to  be  studied 
along  with  the  different  parts  of  the  System  of  Science  sketched  in  the  table, 
there  will  be  included  in  the  scheme  the  whole  knowledge  of  the  universe 
which  man  can  acquire  by  the  exercise  of  his  own  powers :  that  is  to  say, 
what  he  can  acquire  independently  of  a  supernatural  Revelation.  And 
on  this  knowledge  all  his  arts  are  founded, — some  of  them  on  the  single 
part  of  Physics,  as  that  of  the  machinist,  architect,  mariner,  carpenter,  &c. ; 
some  on  Chemistry,  (which  includes  Physics,)  as  that  of  the  miner,  glass- 
maker,  dyer,  brewer,  &c. ;  and  some  on  Physiology,  (which  includes  much  of 
Physics  and  Chemistry,)  as  that  of  the  scientific  gardener  or  botanist,  agri- 
culturist, zoologist,  &c.  The  business  of  teachers  of  all  kinds,  and  of  governors, 
advocates,  linguists,  &c.,  &c.,  respects  chiefly  the  science  of  Mind.  The  art 
of  medicine  requires  in  its  professor  a  comprehensive  knowledge  of  all  the 
departments. 


xu 


INTRODUCTION. 


TABLE  OF  SCIENCE  AND  ART. 
1.  PHYSICS.  2.  CHEMISTRY. 


Mechanics, 
Hydrostatics, 
Hydraulics, 
Pneumatics, 
Acoustics, 
Heat, 
Optics, 
Electricity, 
Astronomy, 
&c. 


Simple  substances, 
Mineralogy, 
Geology, 
Pharmacy, 
Brewing, 
Dyeing, 
Tanning, 
&c. 


3.  LIFE. 

Vegetable  Physiology. 
Botany, 
Horticulture, 
Agriculture, 
&c. 

Animal  Physiology, 
Zoology, 
Anatomy, 
Pathology, 
Medicine 
&c. 


4.  MIND. 

Intellect, 
Logic, 

Mathematics, 
&c. 

Motives  to  action, 

Emotions  and  Passions, 

Morals, 

Government, 

Political  Economy, 

Theology, 

Education. 


In  the  first  stages  of  education,  viz.,  during  the  years  of  childhood  and 
youth,  the  learning  acquired  is  necessarily  of  the  most  mixed  kind,  and 
much  of  it  is  determined  by  what  is  called  accident;  but  from  the  mutual 
dependence  of  the  different  departments  of  science,  as  explained  in  the  pre- 
ceding paragraphs,  it  follows  that  with  a  view  to  complete  erudition',  the 
order  exhibited  in  "The  Table/'  is  that  in  which  they  should  afterwards  be 
studied,  so  as  to  prevent  repetitions  and  anticipations,  and  to  diminish,  as 
much  as  possible,  the  labor  of  acquirement. 

Every  man  may  be  said  to  begin  his  education,  or  acquisition  of  know- 
ledge, on  the  day  of  his  birth.  Certain  objects,  repeatedly  presented  to  the 
infant,  are,  after  a  time,  recognized  and  distinguished.  The  number  of 
objects  thus  known  gradually  increases,  and  from  the  constitution  of  the 
mind,  they  are  soon  associated  in  the  recollection,  according  to  their  resem- 
blances, or  obvious  relations.  Thus,  sweetmeats,  toys,  articles  of  dress,  &c., 
soon  form  distinct  classes  in  the  memory  and  conceptions.  At  a  later  age, 
but  still  very  early,  the  child  distinguishes  readily  between  a  mineral  mass, 
a  vegetable,  and  an  animal;  and  thus  his  mind  has  already  noted  the  three 
great  classes  of  natural  bodies,  and  has  acquired  a  certain  degree  of  acquaint- 
ance with  Natural  History.  He  also  soon  understands  the  phrases  aa  falling 
body,"  "the  force  of  a  moving  body,"  and  has  therefore  a  perception  of  the 


INTRODUCTION.  Xlll 

great  physical  laws  of  gravity  and  inertia.  Then  having  seen  sugar  dissolved 
in  water,  and  wax  melted  round  the  wick  of  a  burning  candle  he  has  learned 
some  phenomena  of  Chemistry.  And  having  obs'erved  the  conduct  of  the 
domestic  animals,  and  of  the  persons  about  him,  he  has  begun  his  acquaint- 
ance with  Physiology,  and  the  science  of  mind.  Lastly,  when  he  has  learned 
to  count  his  fingers  and  his  sugar  plums,  and  to  judge  of  the  fairness  of  the 
division  of  a  cake  between  himself  and  brothers,  he  has  advanced  into  Arith- 
metic and  Geometry.  Thus  within  a  year  or  two,  a  child  of  common  sense 
has  made  a  degree  of  progress  in  all  the  great  departments  of  human  science ;' 
and  in  addition  has  learned  to  name  objects,  and  to  express  feelings,  by  the 
arbitrary  sounds  of  language.  Such,  then,  are  the  beginnings  or  founda- 
tions of  knowledge,  on*  which  'future  years  of  experience,  or  methodical 
education,  must  rear  the  superstructure  of  the  more  considerable  attain- 
ments which  befit  the  various  conditions  of  men  in  a  civilized  com- 
munity. 

In  the  course  of  the  preceding  disquisition,  we  have  seen  that  Physics,  or 
Natural  Philosophy,  the  subject  of  the  present  volume,  is  fundamental  to 
the  other  parts,  and  is  therefore  that  of  which  a  knowledge  is  indispensable. 
Bacon  truly  calls  it  "  the  root  of  the  sciences  and  arts."  That  its  import- 
ance has  not  been  marked  by  the  place  which  it  has  held  in  common  systems 
of  education,  is  owing  chiefly,  1st,  to  the  misconception  that  a  knowledge 
of  technical  mathematics  was  a  necessary  preliminary ;  and,  2d,  to  an 
opinion,  also  erroneous,  that  the  degree  of  acquaintance  with  Physics  which 
all  persons  acquire  by  common  experience,  is  sufficient  for  common  pur- 
poses ;  now  it  is  true,  that  the  toys  of  childhood,  as  the  windmill,  ball, 
syphon,  tube,  and  a  hundred  others,  furnish  so  many  exemplifications  of 
the  laws  of  Physics,  and  may  well  be  called  a  philosophical  apparatus ;  but 
they  give  information  which  is  exceedingly  vague,  and  not  at  all  such  as  is 
absolutely  requisite  in  the  practice  of  many  of  the  arts.  If,  then,  the  study 
of  Physics  be  so  easy  as  now  appears,  and  so  important  as  we  shall  try  still 
farther  to  show,  there  can  be  no  excuse  for  neglecting  it. 
•=-  The  greatest  sum  of  knowledge  acquired  with  the  least  trouble  is,  perhaps, 
that  which  comes  with  the  study  of  the  few  simple  truths  of  Physics.  To 
the  man  who  understands  these,  very  many  phenomena,  which,  to  the  unin- 
formed, appear  prodigies,  are  only  beautiful  illustrations  of  his  fundamental 
knowledge,  and  this  he  carries  about  with  him,  not  as  an  oppressive  weight, 
but  as  a  charm  supporting  the  weight  of  other  knowledge,  and  enabling  him 
to  add  to  his  valuable  store  every  new  fact  of  importance  which  may  offer 
itself.  With  such  a  principle  of  arrangement,  his  information,  instead  of 
resembling  loose  stones  or  rubbish  thrown  together  in  confusion,  becomes 
as  a  noble  edifice,  of  correct  proportions  and  firm  contexture,  and  is  acquiring 
greater  strength  and  consistency  with  the  experience  of  every  day.  It  has 
been  a  common  prejudice,  that  persons  thus  instructed  in  general  laws,  had 
their  attention  too  much  divided,  and  could  know  nothing  perfectly.  But 
the  very  reverse  is  true;  for  general  knowledge  renders  all  particular 
knowledge  more  clear  and  precise.  The  ignorant  man  may  be  said  to  have 
charged  his  hundred  books  of  knowledge,  to  use  a  rude  simile,  with  single 
objects,  while  the  informed  man  makes  each  support  a  long  chain,  to  which 
thousands  of  kindred  and  useful  things  are  attached.  The  laws  of  Philosophy 
may  be  compared  to  keys  which  give  admission  to  the  most  delightful  gar- 
dens that  fancy  can  picture ;  or  to  a  magic  power,  which  unveils  the  face 
of  the  universe,  and  discloses  endless  charms  of  which  ignorance  never 
dreams.  The  informed  man,  in  the  world,  may  be  said  to  be  always  sur- 


xiv  INTRODUCTION. 

rounded  by  what  is  known  and  friendly  to  him,  while  the  ignorant  man  is 
as  one  in  a  land  of  strangers  and  enemies.  A  man  reading  a  thousand 
volumes  of  ordinary  botfks  as  agreeable  pastime,  will  receive  only  vague 
impressions;  but  he  who  studies  the  methodized  Book  of  Nature,  converts 
the  great  universe  into  a  simple  and  sublime  history,  which  tells  of  God, 
and  may  worthily  occupy  his  attention  to  the  end  of  his  days. 

We  have  said  already,  that  theJaws  of  Physics  govern  the  great  natural 
phenomena  of  Astronomy,  the  tides,  winds,  currents,  &c.  We  will  now 
mention  some  of  the  artificial  purposes  to  which  man's  ingenuity  has  made 
the  same  laws  subservient. 

Nearly  all  that  the  civil  engineer  accomplishes,  ranges  under  the  head  of 
Physics.  Let  us  take,  for  instance,  the  admirable  specimens  scattered  over 
the  British  Isles : — the  numerous  canals  for  inland  traffic ;  the  dock  to 
receive  the  riches  of  the  world,  pouring  towards  us  frbm  every  quarter;  the 
many  harbours  offering  safe  retreat  to  the  storm-driven  mariner ;  the  mag- 
nificent bridges  which  every  where  facilitate  intercourse ;  hills  bored  through 
to  open  ways  for  commerce  by  canals,  common  roads  and  rail-roads,  the 
canals  in  some  places  being  supported,  like  the  roads,  on  arches  across  valleys 
or  above  rivers,  so  that  here  and  there  the  singular  phenomenon  is  seen  of  one 
vessel  sailing  directly  over  another ;  vast  tracts  of  swamps  or  fen-land  drained 
and  now  serving  for  agriculture ;  the  noble  light-house,  rearing  its  head 
amidst  the  storm,  while  the  dweller  within  trims  his  lamp  in  safety,  and 
guides  his  endangered  fellow-creature  through  the  perils  of  the  night,  &c.,  &c. 

In  Holland,  great  part  of  the  country  has  been  won  and  is  now  preserved 
from  the  sea,  by  the  same  almost  creating  power,  and  now  rich  cities  and 
an  extended  garden  smile,  where,  as  related  by  Caesar,  were  formerly  only 
bogs  and  a  dreary  waste. 

As  a  general  picture,  it  is  interesting  to  consider,  that  in  many  situations 
on  earth  where  formerly  the  rude  savage  beheld  the  cataract  falling  among 
the  rocks,  and  the  wind  bending  the  trees  of  the  forest,  and  sweeping  the 
clouds  along  the  mountain's  brow,  or  whitening  the  face  of  the  ocean,  and 
regarding  these  phenomena  with  awe  and  terror,  as  marking  the  agency  of 
some  great  but  hidden  power,  which  might  destroy  him;  in  the  same  situa- 
tions now,  his  informed  son,  who  works  with  the  laws  of  nature,  can  lead 
the  waters  of  the  cataract,  by  sloping  channels,  to  convenient  spots,  where 
they  are  made  to  turn  his  mill-wheel,  and  to  do  his  multifarious  work ;  the 
rushing  winds,  also,  he  makes  his  servant,  by  rearing  in  their  course  the 
broad-vaned  wind-mill,  which  then  performs  a  thousand  offices  for  its  master, 
man;  and  the  breezes  which  whiten  the  ocean  are  caught  in  his  expanded 
sails,  and  are  made  to  waft  their  lord  and  his  treasures  across  the  deep,  for 
his  pleasure  or  his  profit. 

In  Architecture,  also,  Physics  in  supreme,  and  has  directed  the  construc- 
tion of  the  temples,  pyramids,  domes  and  palaces  which  adorn  the  earth. 

In  respect  to  machinery,  generally,  Physics  is  the  guiding  light.  There 
are,  for  instance,  the  mighty  steam-engine;  machines  for  spinning  and. 
weaving,  and  for  moulding  other  bodies  into  various  shapes,  yea,  even  iron 
itself,  as  if  it  were  plastic  clay;  wind-mills  and  water-mills,  and  wheel 
carriages;  the  plough  and  implements  of  husbandry ;  artillery  and  the  fur- 
niture of  war;  the  balloon,  in  which  man  rides  triumphantly  above  the 
clouds,  and  the  diving-bell,  in  which  he  penetrates  the  secret  caverns  of  the 
deep ;  the  implements  of  the  intellectual  arts,  of  printing,  drawing,  painting, 
sculpture,  &c. ;  musical  instruments,  optical  and  mathematical  instruments, 
and  a  thousand  others. 


INTRODUCTION.  XV 

But  Physics  is  also  an  important  foundation  of  the  healing  art.  The  medi- 
cal man,  indeed,  is  the  engineer  pre-eminently  ;  for  it  is  in  the  animal  body 
that  true  perfection  and  the  greatest  variety  of  mechanism  are  found.  Where, 
to  illustrate  Mechanics,  is  to  be  found  a  system  of  levers  and  hinges,  and 
moving  parts,  like  the  limbs  of  an  animal  body  ;  where  such  an  hydraulic 
apparatus,  as  in  the  heart  and  blood-vessels  ;  such  a  pneumatic  apparatus, 
as  in  the  breathing  chest  ;  such  acoustic  instruments,  as  in  the  ear  and 
larynx ;  such  an  optical  instrument,  as  in  the  eye ;  in  a  word,  such  variety 
and  perfection,  as  in  the  whole  of  the  visible  anatomy  ?  All  these  struc- 
tures, then,  the  medical  man  should  understand,  as  the  watchmaker  knows 
the  parts  of  a  time-piece  about  which  he  is  employed.  The  watchmaker, 
unless  he  can  discover  where  a  pin  is  loose,  or  a  wheel  injured,  or  a  particle 
of  dust  adhering,  or  oil  wanting,  &c.,  would  ill  succeed  in  repairing  an 
injury;  and  so,  also,  of  the  ignorant  medical  man  in  respect  to  the  human 
body.  Yet  will  it  be  believed,  that  there  are  many  medical  men  who 
neither  understand  mechanics,  nor  hydraulics,  nor  pneumatics,  nor  optics, 
nor  acoustics,  beyond  the  merest  routine ;  and  that  systems  of  medical 
education  are  set  forth  at  this  day  which  do  not  even  mention  the  depart- 
ment of  Physics  !  That  such  is  the  case,  furnishes  an  illustration  of  what  is 
stated  in  the  beginning  of  this  essay,  viz.,  that  the  sciences  and  arts  are 
progressive,  and  that  perfect  methods  of  education  must  arise  gradually, 
like  all  other  things  of  human  contrivance.  It  is  within  the  recollection  of 
persons  now  living,  that  political  economy  was  discovered  to  be  a  grand 
foundation  of  the  art  of  government,  indicating  means  of  security  against 
many  national  misfortunes  common  in  former  times,  yea,  even  against  famine 
and  war.  And  the  day  is  not  distant,  when  the  members  of  the  medical 
pro^ssion  generally  will  understand  how  much  the  correct  knowledge  of 
animal  structure  and  function,  and  of  many  remedies,  must  depend  on  pre- 
cise acquaintance  with  Physics. — Besides  the  more  strictly  professional 
matters  contained  in  the  medical  sections  of  the  present  work,  there  are 
many  others  scattered  through  it  which  greatly  interest  the  medical  man  ; 
such  are  the  subjects  of  meteorology,  climate,  ventilation  and  warming  of 
dwellings,  specific  gravities,  &c.,  &c. 

The  laws  of  Physics  having  an  influence  so  extensive  as  appears  from 
these  paragraphs,  it  need  not  excite  surprise  that  all  classes  of  society  are  at 
last  discovering  the  deep  interest  they  have  to  understand  them.  The  lawyer 
finds  that  in  many  of  the  causes  tried  in  his  courts,  an  appeal  must  be  made 
to  Physics, — as  in  cases  of  disputed  inventions;  accidents  in  navigation,  or 
among  carriages,  steam-engines,  and  machines  generally  ;  questions  arising 
out  of  the  agency  of  winds,  rains,  water-currents,  &c. :  the  statesman  is  con- 
stantly listening  to  discussions  respecting  bridges,  roads,  canals,  docks,  and 
the  mechanical  industry  of  the  nation  :  the  clergyman  finds  ranged  among 
the  beauties  of  nature,  the  most  intelligible  and  striking  proof  of  God's  wis- 
dom and  goodness;  the  sailor  in  his  ship  has  to  deal  with  one  of  the  most 
admirable  machines  in  existence  :  soldiers,  in  using  their  projectiles,  in 
marching  where  rivers  are  to  be  crossed,  woods  to  be  cut  down,  roads  to  be 
made,  towns  to  be  besieged,  &c.,  are  dependent  chiefly  on  their  knowledge 
of  Physics  :  the  land-owner,  in  making  improvements  on  his  estates,  building, 
draining,  irrigating,  road-making,  &c.  :  the  farmer  equally  in  these  particu- 
lars, and  in  all  the  machinery  of  agriculture;  the  manufacturer,  of  course; 
the  merchant  who  selects  and  distributes  over  the  world  the  products  of 
manufacturing  industry — all  these  are  interested  in  Physics ;  then  also  the 
man  of  letters,  that  he  may  not,  in  drawing  his  illustrations  from  the  material 


XVI  INTRODUCTION. 

world,  repeat  the  scientific  heresies  and  absurdities  which  have  heretofore 
prevailed,  and  which,  by  shocking  the  now  better-informed  public,  exceed- 
ingly lower  the  estimation  in  which  such  specimens  of  the  Belles  Lettres 
are  held,  and  lessen  their  general  utility;  and  lastly,  parents  of  either  sex, 
whose  conversation  and  example  have  such  powerful  effect  on  the  character 
of  their  children,  who  when  grown  up,  are  to  fill  all  the  stations  in  society; 
all  should  study  Physics,  as  one  important  part  of  their  education. 

And  it  is  for  such  reasons  that  Natural  Philosophy  is  becoming  daily  more 
and  more  a  part  of  common  education.  In  our  cities  now,  and  even  in  an 
ordinary  dwelling-house,  men  are  surrounded  by  prodigies  of  mechanic  art, 
and  cannot  submit  to  use  these,  regardless  of  how  they  are  produced,  as  a 
horse  is  regardless  of  how  the  corn  falls  into  his  manger.  A  general  diffu- 
sion of  knowledge,  owing  greatly  to  the  increased  commercial  intercourse  of 
nations,  and  therefore  to  the  improvements  in  the  physical  departments  of 
astronomy,  navigation,  &c.,  is  changing  every  where  the  condition  of  man, 
and  elevating  the  human  character  in  all  ranks  of  society.  In  remote  times 
the  inhabitants  of  the  earth  were  generally  divided  into  small  states  or  socie- 
ties, which  had  few  relations  of  amity  among  themselves,  and  whose  thoughts 
and  interests  were  confined  very  much  within  their  own  little  territories  and 
rude  habits.  In  succeeding  ages,  men  found  themselves  belonging  to  larger 
communities,  as  where  the  English  heptarchy  was  united ;  but  still  distant 
kingdoms  and  quarters  of  the  world  were  of  no  interest  to  them,  and  were 
often  totally  unknown.  Now,  however,  every  one  feels  that  he  is  a  member 
of  one  vast  civilized  society,  which  covers  the  face  of  the  earth ;  and  no  part 
of  the  earth  is  indifferent  to  him.  In  England,  for  instance,  a  man  of  small 
fortune  may  cast  his  looks  around  him,  and  say  with  truth  and  exultation, 
"  I  am  lodged  in  a  house  which  affords  me  conveniences  and  comforts  w^ich 
some  centuries  ago,  even  a  king  could  not  command.  Ships  are  crossing 
the  seas  in  every  direction,  to  bring  me  what  is  useful  to  me  from  all  parts 
of  the  earth.  In  China,  men  are  gathering  the  tea-leaf  for  me  ;  in  America, 
they  are  planting  cotton  for  me ;  in  the  West  Indies,  they  are  preparing 
my  sugar  and  my  coffee;  in  Italy,  they  are  feeding  silk-worms  for  me;  in 
Saxony,  they  are  shearing  the  sheep  to  make  me  clothing;  at  home,  power- 
ful steam-engines  are  spinning  and  weaving  for  me,  and  making  cutlery  for 
me,  and  pumping  the  mines  that  minerals  useful  to  me  may  be  procured. 
Although  my  patrimony  was  small,  I  have  post-coaches  running  day  and 
night  on  all  the  roads  to  carry  my  correspondence ;  I  have  roads  and  canals, 
and  bridges,  to  bear  the  coal  for  my  winter  fire ;  nay,  I  have  protecting  fleets 
and  armies  around  my  happy  country,  to  secure  my  enjoyments  and  repose. 
Then  I  have  editors  and  printers,  who  daily  send  me  an  account  of  what  is 
going  on  throughout  the  world,  among  all  these  people  who  serve  me.  And 
in  a  corner  of  my  house  I  have  BOOKS  !  the  miracle  of  all  my  possessions, 
more  wonderful  than  the  wishing-cap  of  the  Arabian  Tales  :  for  they  trans- 
port me  instantly,  not  only  to  all  places,  but  to  all  times.  By  my  books  I 
can  conjure  up  before  me,  into  vivid  existence,  all  the  great  and  good  men 
of  antiquity ;  and  for  my  individual  satisfaction  I  can  make  them  act  over 
again  the  most  renowned  of  their  exploits,  the  orators  declaim  for  me ;  the 
historians  recite ;  the  poets  sing ;  and  from  the  equator  to  the  pole,  or  from 
the  beginning  of  time  until  now,  by  my  books,  I  can  be  where  I  please." 
This  picture  is  not  overcharged,  and  might  be  much  extended,  such  being 
God's  goodness  and  providence,  that  each  individual  of  the  civilized  millions 
dwelling  on  the  earth,  may  have  nearly  the  same  enjoyment  as  if  he  were 
the  single  lord  of  all. 


INTRODUCTION.  Xvij 

Reverting  to  the  importance  of  Natural  Philosophy  as  a  general  study,  it 
may  be  remarked  that  there  is  no  occupation  which  so  much  strengthens  and 
quickens  the  judgment.  This  pra;se  has  usually  been  bestowed  on  the  Ma- 
thematics, although  a  knowledge  of  abstract  Mathematics  existed  with  all  the 
absurdities  of  the  dark  ages ;  but  a  familiarity  with  Natural  Philosophy 
which  comprehends  Mathematics,  and  gives  tangible  and  pleasing  illustra- 
tions of  the  abstract  truths,  seems  incompatible  with  the  admission  of  any 
gross  absurdity.  A  man  whose  mental  faculties  have  been  sharpened  by 
acquaintance  with  these  exact  sciences  in  their  combination,  and  who  has 
been  engaged,  therefore,  in  contemplating  real  relations,  is  more  likely  to 
discover  truth  in  other  questions,  and  can  better  defend  himself  against 
sophistry  of  every  kind.  We  cannot  have  clearer  evidence  of  this  than  in 
the  history  of  the  sciences,  since  the  Baconian  method  of  reasoning  by  indi- 
cation took  place  of  the  visionary  hypotheses  of  preceding  times.  Until  then, 
even  powerful  minds  did  not  recoil  from  the  most  absurd  theories  on  all  sub- 
jects. Astronomy  was  mixed  with  Astrology ;  Chemistry  with  Alchemy ; 
Physiology  with  the  singular  hypotheses  which  preceded  the  discovery  of  the 
circulation  of  the  blood ;  Politics  with  the  errors  of  monopolies,  prohibitions, 
balance  of  trade,  &c.  Even  Religion  itself,  in  various  ages  and  countries, 
has  felt  the  influence  of  the  state  of  the  public  mind  as  to  solid  attainments. 
To  a  man  with  the  knowledge  of  nature  which  we  now  possess,  the  fables 
.and  licentious  abominations  of  the  Greek  and  Roman  theologies  are  shocking 
indeed ;  as  are  the  religions  of  the  God  of  Fire  in  China,  of  Vishnoo  in 
India,  of  Mahomet's  imposture  and  pretended  miracles,  &c.  But  the  enlight- 
ened Christian  minister  earnestly  recommends  the  study  of  nature ;  first 
because  from  contemplating  the  beauty  of  creation,  with  the  wisdom  and 
benevolent  design  manifest  in  all  its  parts,  there  spring  up  in  every  unde- 
praved  mind  those  feelings  of  admiration  and  gratitude,  which  constitute  the 
adoration  of  natural  religion,  and  which  form,  as  shown  by  many  estimable 
writers  on  Natural  Theology,  a  fit  foundation  for  the  sublime  doctrine  of 
immortality,  and  secondly,  because  a  Revelation  being  probable  only  by  the 
miracles  occurring  at  its  establishment;  to  enable  men  to  distinguish  between 
miracles  and  the  usual  course  of  nature,  a  perfect  knowledge  of  that  course, 
or  of  Natural  Philosophy,  is  essential :  all  the  false  religions  of  antiquity 
were  founded  on,  and  upheld  by  pretended  miracles.  As  regards  the  ques- 
tion of  immortality,  even  independently  of  Revelation,  no  man  who  con- 
templates the  order  and  beauty  of  the  material  world,  and  then  thinks  on  the 
hideous  deformities  of  the  moral  world — where  vice  so  often  triumphs,  and 
modest  virtue  pines  and  dies — can  for  a  moment  believe  that  they  are  the 
work  of  the  same  author,  unless  there  be  a  hereafter  of  retribution ;  and 
feeling  thus  that  eternal  justice  requires  another  state  for  man,  he  embraces 
with  delight  the  cheering  promises  of  immortality.  There  have  been,  how- 
ever, at  various  times,  even  among  Christians,  sincere,  but  weak-minded  or 
ill-informed  men,  who  decried  the  study  of  the  natural  sciences,  as  inimical 
to  true  religion ;  as  if  God's  ever-visible  and  magnificent  revelation  of  his 
attributes  in  the  structure  of  the  universe  could  be  at  variance  with  any 
other  revelation.  But  such  prejudices  are  now  quickly  passing  away. 
Wherever  considerable  knowledge  of  nature  exists,  debasing  and  gloomy 
superstition  must  cease.  It  is  not  the  abject  terror  of  a  slave  which  is  in- 
spired by  contemplating  the  majesty  and  power  of  our  God,  displayed  in  his 
works,  but  a  sentiment  akin  to  the  tender  regard  which  leads  a  favored  child 
to  approach  with  confidence  a  wise  and  indulgent  parent. 

It  remains  for  the  author  now  only  to  say  a  few  words  with  respect  to  the 

2 


XV111  INTRODUCTION. 

present  work.  He  was  originally  led  to  the  undertaking  with  the  view  of 
supplying  the  desideratum  in  medical  literature,  of  a  treatise  on  Medical 
Physics;  but  soon  perceiving  that  the  preliminary  investigation  of  General 
Physics,  necessary  to  adapt  the  work  to  medical  readers,  would  require  to 
be  nearly  as  extensive  as  it  would  for  general  readers,  and  reflecting  that 
every  person  of  liberal  education  must  now  possess  such  a  book,  not  to  be 
read  once  and  then  thrown  aside  as  a  novel  is,  but  to  be  frequently  consulted 
as  a  manual,  he  determined  to  make  his  book  as  complete  and  as  extensively 
useful  as  possible.  He  has  been  encouraged  during  his  labor,  by  the  belief 
that  the  growing  light  of  science,  which  now  exhibits  more  clearly  the  na- 
tural relations  of  the  different  departments  of  study,  as  attempted  to  be  por- 
trayed in  the  preceding  pages,  might  enable  him  to  avoid  some  of  the  defects 
of  former  elementary  treatises,  and  to  add  features  of  novelty  and  improve- 
ment to  his  own.  The  sections  on  Animal  Physics  were,  of  course,  written 
for  medical  men ;  and  a  great  service  will  be  rendered  by  the  work,  if  it 
only  awakens  them  to  a  just  sense  of  the  importance  of  Physics  as  one  of 
the  foundations  of  their  art.  But  even  for  general  readers  there  are  few 
parts  of  these  sections  which  the  author  would  exclude.  There  is  nothing 
more  admirable  in  nature  than  the  structure  and  functions  of  the  human 
body,  and  there  are  many  reasons  why  no  liberal  mind  should  be  careless  of 
the  study.  The  details  here  given  are  not  more  anatomical  than  the  illus- 
trations from  the  animal  economy  contained  in  the  common  treatises  on 
Natural  Theology.  From  the  attempt  in  this  work  to  compress  into  the 
smallest  possible  ppace  the  greatest  possible  sum  of  scientific  information, 
few  historical  details  have  been  admitted,  whether  relating  to  the  distin- 
guished men  who  have  benefitted  the  world  as  authors  or  inventors,  or  to 
the  history  of  the  progress  of  science : — such  details  form  an  interesting,  but 
distinct  branch  of  study. 

The  author  must  not  conclude  without  observing,  that  no  treatise  on  Na- 
tural Philosophy  can  save,  to  a  person  desiring  full  information  on  the  sub- 
ject, the  necessity  of  attendance  on  experimental  lectures  or  demonstrations. 
Things  that  are  seen,  and  felt,  and  heard,  that  is,  which  operate  on  the  ex- 
ternal senses,  leave  on  the  memory  much  stronger  and  more  correct  impres- 
sions than  where  the  conceptions  are  produced  merely  by  verbal  description, 
however  vivid.  And  no  man  has  ever  been  remarkable  for  his  knowledge 
of  Physics,  Chemistry,  or  Physiology,  who  has  not  had  practical  familiarity 
with  the  objects.  With  reference  to  this  familiarity,  persons  who  take  a 
philanthropic  interest  in  the  affairs  of  the  world,  must  observe,  with  much 
pleasure  the  now  daily  increasing  facilities  of  acquiring  useful  knowledge, 
afforded  by  the  scientific  institutions  formed  and  forming,  not  only  through 
this  kingdom,  but  through  most  civilized  nations. 

Bedford  Square,  1st  March,  1827. 


ELEMENTS 


NATURAL    PHILOSOPHY. 


SYNOPSIS,  OR  GENERAL  REVIEW. 

IF  it  excite  our  admiration  that  a  varied  edifice,  or  even  a  magnificent 
city  can  be  constructed  of  stone  from  one  quarry,  what  must  our  feeling  be 
to  learn  how  few  and  simple  the  elements  are  out  of  which  the  sublime  fab- 
ric of  the  universe,  with  all  its  orders  of  phenomena,  has  arisen,  and  is  now 
sustained,  These  elements  are  general .  facts  and  laws  which  human 
sagacity  is  able  to  detect,  and  then  to  apply  to  endless  purposes  of  human 
advantage. 

Now  the  four  words,  atom,  attraction,  repulsion,  inertia,  point  to  four 
general  truths,  which  explain  the  greater  part  of  the  phenomena  of  nature. 
Being  so  general,  they  are  called  physical  truths,  from  the  Greek  word 
signifying  nature  as  also  "  truths  of  Natural  Philosophy,"  with  the  same 
meaning,  and  sometimes  "  mechanical  truths/'  from  their  close  relation  to 
ordinary  machinery.  These  appellations  distinguish  them  from  the  remain- 
ing general  truths,  namely  the  chemical  truths,  which  regard  particular 
substances,  and  the  vital  and  mental  truths,  which  have  relation  only  to 
living  beings.  And  even  in  the  cases  where  a  chemical  or  vital  influence 
operates,  it  modifies,  but  does  not  destroy,  the  physical  influence.  By 
fixing  the  attention,  then,  on  these  four  fundamental  truths.,  the  student 
obtains,  as  it  were,  so  many  keys  to  unlock,  and  lights  to  illuminate  the 
secrets  and  treasures  of  nature. 

1st.  ATOM.  Every  material  mass  in  nature  is  divisible  into  very 
minute  indestructible  and  unchangeable  particles, — as  when  a  piece  of  any 
metal  is  bruised,  broken,  cut,  dissolved,  or  otherwise  transformed,  a 
thousand  times,  but  can  always  be  exhibited  again  as  perfect  as  at  first. 
This  truth  is  conveniently  recalled  by  giving  to  the  particles  the  name  atom, 
which  is  a  Greek  term,  signifying  that  which  cannot  be  further  cut  or  divided, 
or  an  exceeding  minute  resisting  particle. 

2d.  ATTRACTION.  It  is  found  that  the  atoms  above  referred  to,  whether 
separate  or  already  joined  into  masses,  tend  towards  all  other  atoms  or 
masses, — as  when  the  atoms  of  which  any  mass  is  composed  are,  by  an  in- 
visible influence,  held  together  with  a  certain  degree  of  force ;  or  when  a 
block  of  stone  is  similarly  held  down  to  the  earth  on  which  it  lies;  or  when 
the  tides  on  the  earth  rise  towards  the  moon.  These  facts  are  conveniently 


20  SYNOPSIS. 

/ 

recalled  by  connecting  with  them  the  word  Attraction  (a  drawing  together) 
or  gravitation. 

3d.  REPULSION.  Atoms  under  certain  circumstances,  as  of  heat  diffused 
among  them,  have  their  mutual  attraction  countervailed  or  resisted,  and 
they  tend  to  separate ; — as  when  ice  heated  melts  into  water,  or  when  water 
heated  bursts  into  steam,  or  when  gunpowder  ignited  explodes.  Such 
facts  are  conveniently  recalled  by  the  term  Repulsion  (a  thrusting  asunder.) 

4th.  INERTIA.  As  a  fly-wheel  made  to  revolve,  at  first  offers  resistance  to 
the  force  moving  it,  but  gradually  acquires  speed  proportioned  to  that  force, 
and  then  resists,  being  again  stopped,  in  proportion  to  its  speed,  so  all  bodies 
or  atoms  in  the  universe  have  about  them,  in  regard  to  motion,  what  may 
be  figuratively  called  a  stubb.orness,  tending  to  keep  them,  in  their  existing 
state,  whatever  it  may  be — in  other  words,  they  neither  acquire  motion,  nor 
lose  motion,  nor  bend  their  course  in  motion,  but  in  exact  proportion  to 
some  force  applied.  Many  of  the  motions  now  going  on  in  the  universe 
with  such  regularity — as  that  turning  of  the  earth  which  produces  the  phe- 
nomena of  day  and  night — are  motions  which  began  thousands  of  years 
ago,  and  continue  unvarying  in  this  way.  Such  facts  are  conveniently 
recalled  by  the  term  inertia  applied  to  them. 

A  person  comprehending  fully  the  import  of  these  four  words,  that  is  to 
say,  having  present  to  his  mind  numerous  good  types  or  exemplars  of  the 
facts  referred  to  them,  may  predict  or  anticipate  correctly,  and  may  control 
very  many  of  the  facts  and  phenomena  which  the  extended  experience  of  a 
life  can  display  to  him ;  and  such  a  person  is  commonly  said  to  know  the 
causes  or  reasons  of  things  and  events.  Now  it  is  important  here  to  observe, 
that  when  a  person  gives  a  reason  or  explanation  of  any  fact,  other  than 
that  it  is  a  fact,  or  than  that  the  Creator  has  willed  it,  he  is  merely,  although 
he  may  not  be  aware  of  this,  showing  its  resemblance  to  many  other  facts, 
no  one  of  which  he  understands  better  than  itself — and  what  he  calls  a 
general  truth,  or  law,  or  principle,  is  merely  an  expression  for  the  observed 
but  unaccountable  resemblance  of  the  facts.  Thus,  when  a  man  says  that 
a  stone  falls  because  of  attraction  or  gravitation,  he  only  uses  a  word  which 
recalls  thousands  of  instances  which  he  has  witnessed  of  one  body  approach- 
ing another;  but  by  any  cause  of  the  approach,  other  than  that  God  has 
willed  it,  is  to  him  utterly  unknown.  Should  men,  in  the  course  of  their 
researches,  discover  that  the  phenomena  now  classed  by  them  under  the 
heads  of  attraction  and  repulsion,  although  apparently  opposite,  are  really 
as  closely  allied  as  they  already  know  the  rising  of  a  balloon  and  the  falling 
of  a  stone  to  be  (the  balloon  rises  like  a  cork  in  water,  being  pushed  up  by 
the  fluid  air  around  it,  heavier  than  it,  and  seeking  to  descend,)  they  will 
not  have  discovered  a  new  cause,  but  a  new  resemblance,  (new  to  them) 
among  phenomena,  and  will  only  have  advanced  one  step  farther  in 
perceiving  the  simplicity  of  creation.  In  accordance  with  these  views,  it 
will  be  found  that  this  volume  is  chiefly  an  extensive  display  of  the  ^rnost 
important  phenomena  of  nature  and  art,  classified  so  as  to  be  explained  by  the 
four  physical  truths,  and  mutually  to  illustrate  one  another.  They  will  be 
listributed  under  the  following  heads  or  divisions : 


SYNOPSIS.  21 

* 

PAKT   I. 

CONSTITUTION   OP   MASSES,    MOTIONS  AND   FORCES. 

The  four  fundamental  truths  extensively  examined,  and  used  to  explain 
generally,  in 
Section 

1.  The  nature  or  constitution  of  the  material  masses  which  compose  the 
universe ;  (a  department  technically  called  SOMATOLOGY,  from  Greek 
words  signifying  a  discourse  on  body.} 

2.  The  motions  or  phenomena  going  on  among  the  masses; — a  department 
including  the  common  divisions  of  STATICS  (things  stationary  or  at 
rest,)  and  DYNAMICS  (what  relates  to  force  or  power.} 

PART  II. 

PHENOMENA  OP    SOLIDS. 

The  four  truths  explaining  the  peculiarities  of  state  and  motion  among 
solid  bodies : — a  department  called,  in  a  restricted  sense,  MECHANICS,  (from 
the  Greek,  and  signifying  machine.) 

PART  III. 

PHENOMENA   OP   FLUIDS. 

The  truths  explaining  the  peculiarities  of  state  and  motion  among  fluid 
bodies  : — a  department  called  HYDRODYNAMICS  (from  Greek  words  signify- 
ing water  &M&  force) 
Section 

1.  HYDROSTATICS  (water  at  rest  or  in  equilibrium.) 

2.  PNEUMATICS  (air  phenomena.) 

3.  HYDRAULICS  (water  or  fluid  in  motion.) 

4.  ACOUSTICS  (phenomena  of  sound  and  hearing.) 

PART  IY. 

PHENOMENA   OP    IMPONDERABLE    SUBSTANCES. 

The  truths  aiding  to  explain  the  more  recondite  phenomena  of  IMPON- 
DERABLE SUBSTANCES,  under  the  heads  of 
Section 

1 .  HEAT  or  Caloric. 

2.  LIGHT  or  Optics. 

PART  Y. 

ANIMAL   AND    MEDICAL   PHYSICS. 

In  this  part  will  be  ranged  the  most  interesting  illustrations  afforded  by 
the  animal  economy,  constituting — ANIMAL  AND  MEDICAL  PHYSICS. 

As  no  man  can  well  understand  a  subject  of  which  he  does  not  carry  a 
distinct  outline  in  his  mind,  it  is  recommended  to  the  reader  of  this  work 
to  study  the  general  synopsis,  and  the  analysis  placed  at  the  heads  of  the 
chapters  and  sections,  until  the  memory  be  well  impressed  with  them. 


22  .     CONSTITUTION    OF    MASSES 


PART  I. 


THE  FOUR  FUNDAMENTAL  TRUTHS  MINUTELY  EXAMINED,  AND  USED  TO 
EXPLAIN  GENERALLY,  FIRST,  THE  NATURE  OR  CONSTITUTION  OF  THE 
MATERIAL  MASSES  WHICH  COMPOSE  THE  UNIVERSE,  AND  SECONDLY, 
THE  MOTIONS  OR  PHENOMENA  GOING  ON  AMONG  THEM. 


SECTION  L— THE  CONSTITUTION  OF  MASSES. 


ANALYSIS   OF   THE   SECTION. 

The  visible  universe  is  built  up  of  very  minute  indistructible  ATOMS  called 
matter,  which  by  mutual  ATTRACTION,  cohere  or  cling  together  in  masses 
of  various  form  and  magnitude.  The  atoms  are  more  or  less  approxi- 
mated, according  to  the  quantity  or  REPULSION  of  heat  among  them,  and 
hence  arise  the  three  remarkable  forms  in  the  masses,  of  solid,  liquid  and 
air,  which  mutually  change  into  each  other  with  change  in  the  quantity 
of  heat.  Certain  modifications  of  attraction  and  repulsion  produce  the 
subordinate  peculiarities  of  state  called  crystal,  dense,  hard,  elastic,  brittle, 
malleable,  ductile  and  tenacious. 


"  Minute  Indestructible  ATOMS."* 

THAT  the  smallest  portion  of  any  substance  which  the  human  eye  can  per- 
ceive, is  still  a  mass  of  many  ultimate  atoms  or  particles,  which  may  be 
separated  from  each  other,  or  newly  arranged,  but  which  cannot  individu- 
ally be  hurt  or  destroyed,  is  deduced  from  such  facts  as  the  following : 

A  particle  of  powdered  marble,  hardly  visible  to  the  naked  eye,  still  ap- 
pears to  the  microscope  a  block  susceptible  of  indefinite  division ;  and,  when 
it  is  broken  by  fit  instruments,  until  the  microscope  can  hardly  discover  the 
separate  particles  of  fine  powder,  these  may  be  yet  further  divided,  by 
solution  in  an  acid;  the  whole  becoming  then  absolutely  invisible,  as  part 
of  a  transparent  liquid. 

A  small  mass  of  gold  may  be  hammered  into  thin  leaf,  or  drawn  into  fine 
wire,  or  cut  into  almost  invisible  parts,  or  liquefied  in  a  crucible,  or  disolved 
in  an  acid,  or  dissipated  by  intense  heat  into  vapour ;  yet,  after  any  and  all 
these  changes,  the  atoms  can  be  collected  again  to  form  the  original  mass 
of  gold,  without  the  slightest  diminution  or  change.  And  all  the  substance  of 

*  The  different  heads  or  titles,  which  appear  thus,  throughout  the  work,  between  in- 
verted commas,  are  the  successive  portions  of  the  Analysis,  detached  for  separate 
consideration.  The  reader  is  particularly  requested  to  re-peruse  the  analysis  at  the 
several  interruptions,  that  he  may  have  constantly  before  him  that  clear  view  of  the 
general  relations  among  the  different  parts  of  the  subject,  which  is  essentially  to  a  per- 
fect understanding  of  it. 


CONSTITUTION    OF    MASSES.  23 

elements  of  which  our  globe  is  composed,  may  thus  be  cut,  torn,  bruised, 
ground,  &c.,  a  thousand  and  a  thousand  times,  but  are  always  recoverable  as 
perfect  as  at  first. 

And  with  respect  to  delicate  combinations  of  these  elements,  such  as  exist 
in  animal  and  vegetable  bodies,  although  it  be  beyond  human  art,  originally 
to  produce,  or  even  closely  to  imitate  many  of  them — for  we  cannot  build  up 
a  feather  or  a  rose — still,  in  their  decomposition  and  apparent  destruction, 
the  accomplished  chemist  of  the  present  day  does  not  lose  a  single  atom. 
The  coal  which  burns  in  his  apparatus,  until  only  a  little  ash  remains  behind, 
or  the  wax -taper  that  seems  to  vanish  altogether  in  flame,  or  the  portion  of 
animal  flesh  which  putrefies,  and  gradually  dries  up  and  disappears — present 
to  us  phenomena  which  are  now  proved  to  be  only  changes  of  connection 
and  arrangement  among  the  indestructible  ultimate  atoms ;  and  the  chemist 
can  offer  all  the  elements  again,  mixed  or  separate  as  desired,  for  any  of 
the  useful  purposes  to  which  they  are  severally  applicable.  When  the  funeral 
piles  of  the  ancients,  with  their  charge  of  human  remains,  appeared  to  be 
wholly  consumed,  and  left  the  idea  with  survivors  that  no  base  use  could  be 
made,  in  after  time,  of  what  had  been  the  material  dwelling  of  a  noble  or 
beloved  spirit,  the  flames  had  only,  as  it  were,  scattered  the  enduring  blocks 
of  which  a  former  edifice  had  been  constructed,  but  which  were  soon  to  serve 
again  in  new  combinations. 

Facts  to  be  stated  under  the  heads  of  "  chemical  composition"  and  "  crys- 
tal," will  prove,  that  the  ultimate  particles  of  any  substance  must  be,  among 
themselves,  perfectly  similar. 

"  Minute."  (Read  the  Analysis,  page  22.) 

The  following  are  interesting  particulars  in  the  arts  or  in  nature,  helping  the 
mind  to  conceive  how  minute  the  ultimate  atoms  of  matter  must  be. 

Goldbeaters,  by  hammering,  reduce  gold  to  leaves  so  thin,  that  360,000 
must  be  laid  upon  one  another  to  produce  the  thickness  of  an  inch.  They 
are  so  thin,  that  if  formed  into  a  book,  1,800  would  occupy  only  the  space 
of  a  single  leaf  of  common  paper ;  and  an  octavo  volume  an  inch  thick  would 
have  as  many  pages  as  the  books  of  a  well-stocked  ordinary  library  contain- 
ing 1,800  volumes  of  400  pages  each ;  yet  those  leaves  are  perfect,  or  free 
from  holes,  so  that  one  of  them  laid  upon  any  surface,  as  in  gilding,  gives 
the  appearance  of  solid  gold. 

Still  thinner  than  this  is  the  coating  of  gold,  uponithe  silver  wire  of  what 
is  called  gold  lace ;  and  we  know  not  that  such  coating  is  of  only  one  atom 
thick.  If  we  place  a  piece  of  this  wire  in  nitric  acid,  so  as  to  dissolve  the 
silver  within,  the  gold  coating  remains  as  a  metallic  tube  of  exquisite 
tenuity. 

Platinum  can  be  drawn  into  wire  much  finer  than  human  hair. 

A  grain  of  blue  vitriol  or  carmine,  will  tinge  a  gallon  of  water,  so  that  in 
every  drop  the  colour  may  be  perceived. 

A  grain  of  musk  will  scent  a  room  for  twenty  years,  and  will  have  lost 
but  little  of  its  weight. 

The  carrion  crow  seems  to  smell  its  food  at  a  distance  of  many  miles. 

The  thread  of  the  silk  worm  is  so  small,  that  many  folds  have  to  be 
twisted  together  to  form  our  finest  sewing  thread;  but  that  of  the  spider  is 
smaller  still,  for  two  drachms  of  it  by  weight  would  reach  from  London  to  - 
Edinburgh,  or  400  miles. 

In  the  milk  of  a  cod-fish,  or  in  water  in  which  certain  vegetables  have 


24  CONSTITUTION    OF    MASSES. 

been  infused,  the  microscope  discovers  animalcules,  of  which  many  thou- 
sands together  do  not  equal  in  bulk  a  grain  of  sand ;  yet  these  have  their 
blood  and  other  subordinate  parts  like  larger  animals ;  and,  indeed,  nature, 
with  a  singular  prodigality,  has  supplied  many  of  them  with  organs  as  com- 
plex as  those  of  the  whale  or  elephant.  Now  the  body  of  an  animalcule 
consists  of  the  same  elementary  substances,  or  ultimate  atoms,  as  the  body 
of  man  himself.  In  a  single  pound  of  matter,  it  thus  appears,  that  there 
may  be  more  living  creatures  than  of  human  beings  on  the  face  of  this  globe, 
What  scenes  has  the  microscope  laid  open  to  the  admiration  of  the  philoso- 
phic inquirer. 

Water,  mercury,  sulphur,  or,  in  general,  any  substance,  when  sufficiently 
heated,  rises  as  invisible  vapour  or  gas ;  in  other  words,  is  made  to  assume 
the  aeriform  state.  Great  heat,  therefore,  would  cause  the  whole  of  the 
material  universe  to  disappear,  the  previously  most  solid  bodies'  becoming  as 
invisible  and  impalpable  as  the  air  we  breathe.  Utter  annihilation  would 
seem  but  one  stage  beyond  this. 

"MaMtrS* 

The  inconceivable  minuteness  of  ultimate  atoms,  as  shown  above,  has  led 
some  inquirers  to  doubt  whether  there  really  be  matter ;  that  is  to  say, 
whether  what  we  call  substance  or  matter  have  existence  or  not.  In  answer 
to  this  it  has  been  usual  to  adduce,  besides  the  weights  of  the  substances, 
and  the  proofs  of  indestructibility  already  mentioned,  which  seems  conclu- 
sive, the  fact  that  every  kind  or  portion  of  matter  obstinately  occupies  some 
space  to  the  exclusion  of  all  other  matter  from  that  particular  space.  This 
occupancy  of  space  is  the  simplest  and  most  complete  idea  which  we  have 
of  material  existence.  The  awkward  word  impenetrability  has  been  used  to 
express  it,  with  reference  of  course  to  the  individual  atoms.  The  following 
are  elucidations  : 

We  cannot  push  one  billiard-ball  into  the  substance  of  another,  and  then 
a  second,  and  then  a  third,  and  so  on ;  or  the  material  of  the  universe  might 
be  absorbed  in  a  point. 

A  mass  of  iron  on  a  support  will  resist  the  weight  of  thousands  of  pounds 
laid  upon  it  and  pressing  to  descend  into  its  place  ;  and  although  a  very 
great  weight  might  crush  or  break  it  into  pieces,  still  one  particle  would  not 
be  annihilated.  In  a  forcing  pump,  or  in  Braham's  water-press,  millions  of 
pounds  can  not  push  the  piston  down,  unless  the  water  below  it  be  allowed 
to  escape.  • 

A  weight  laid  upon  bladders  full  of  air,  or  on  the  piston  handle  of  a  closed 
air-pump,  is  supported  in  the  same  manner. 

A  quantity  of  air  escaping  from  a  vessel  under  water  ascends  through  the 
water  as  a  bubble  displacing  its  bulk  of  water  in  its  way. 

A  glass  tube,  left  open  at  bottom,  while  the  thumb  closes  the  top,  if 
pressed  from  air  into  water,  is  not  filled  with  water,  because  the  air  contained 
in  it  resists ;  but  if  the  air  be  allowed  to  escape  by  removing  the  thumb 
from  the  top,  the  tube  becomes  filled  immediately  to  the  level  of  the  water 
around  it.  In  a  goblet  or  basin  pushed  into  water,  with  the  mouth  down- 
wards, the  entrance  of  water  is  resisted  for  the  like  reason  ;  and  if  the  goblet 
be  inverted  over  a  floating  lighted  taper,  this  will  continue  to  float  under  it, 
and  to  burn  in  the  contained  air,  however  deep  in  the  water  it  may  be  car- 
ried— exhibiting  the  curious  phenomenon  of  light  below  water,  and  being 
an  emblem  of  the  living  inmate  of  a  diving  bell,  which  is  merely  a  larger 
goblet  holding. a  man  instead  of  a  candle. 


GENERAL    ATTRACTION.  25 


"Mutual  attraction"  (See  the  Analysis,  page  22.) 

Any  visible  mass  of  matter,  then,  as  of  metal,  salt,  sulphur,  &c.,  we 
know  to  be  really  a  collection  of  dust,  or -minute  atoms,  by  some  cause  made 
to  cohere  or  cling  together;  yet  there  are  no  hooks  connecting  them,  nor 
nails,  nor  glue ;  and  the  connection  may  be  broken  a  thousand  times,  by 
processes  of  nature  or  art,  but  is  always  ready  to  take  place  again  ;  the  cause 
being  no  more  destroyed  in  any  case  by  interruption,  than  the  weight  of  a 
thing  is  destroyed  by  frequent  lifting  from  the  ground.  Now  the  cause  we 
know  not,  but  we  call  it  attraction.  The  phenomena  of  attraction  and  its 
contrary,  repulsion,  particularly  when  occurring  between  bodies  at  consider- 
able distances  from  each  other,  are  as  inexplicable  as  any  subjects  which 
the  human  mind  has  to  contemplate;  but  the  manner  or  laws  of  the  pheno- 
mena are  now  well  understood.  The  general  nature  and  extensive  influence 
of  attraction  may  be  judged  of  from  the  following  facts  : 

Logs  of  wood  floating  in  a  pond,  or  ships  in  calm  water,  approach  each 
other,  and  afterwards  remain  in  contact.  When  the  floating  bodies  are  very 
small,  or  can  approach  very  near  to  each  other  at  the  water's  edge — as  glass 
bulbs  in  a  tea-cup — an  additional  force  is  called  into  play,  as  will  be  ex- 
plained unjer  the  head  of  "  capillary  attraction." 

l"he  wreck  of  a  ship,  in  a  smooth  sea  after  a  storm,  is  often  seen  gathered 
into  heaps. 

Two  bullets  or  plummets  suspended  by  strings  near  to  each  other,  are 
found  by  the  delicate  test  of  the  torsion  balance  (which  will  be  described 
afterwards)  to  attract  each  other,  and  therefore  not  to  hang  quite  perpen- 
dicularly. 

A  plummet  suspended  near  the  side  of  a  mountain  inclines  towards  it,  in 
a  degree  proportioned  to  its  magnitude ;  as  was  ascertained  by  the  well- 
known  trials  of  Dr.  Maskelyne  near  the  mountain  Schehallion,  in  Scotland. 

And  the  reason  why  the  plummet  in  such  a  case  tends  much  more  strongly 
towards  the  earth  than  towards  the  hill,  is  only  that  the  earth  is  larger  than 
the  hill 

At  New  South  Wales,  which  is  situated  on  our  globe  nearly  opposite  to 
England,  plummets  hang  and  fall  towards  the  centre  of  the  globe,  as  they 
do  here ;  so  that  in  respect  to  England,  they  are  hanging  and  falling  upwards, 
and  the  people  there,  like  flies  on  the  opposite  side  of  a  pane  of  glass,  are 
standing  with  their  feet  towards  us, — hence  called  our  antipodes.  Weight, 
therefore,  is  merely  general  attraction  acting  everywhere. 

But  it  is  owing  to  this  general  attraction  that  our  earth  itself  is  a  globe  : — 
all  its  parts  being  drawn  towards  each  other,  that  is,  towards  a  common 
centre,  the  mass  assumes  the  spherical  or  rounded  form. 

And  the  moon  also  is  round,  and  all  the  planets ;  nay,  the  glorious  sun, 
too,  so  much  larger  than  these,  is  round ; — suggesting  the  inference  that  all 
must  at  one  time  have  been  a  certain  degree  fluid,  and  that  all  are  subject 
to  the  same  law. 

Descending  again  to  the  earth  and  observing  minuter  masses,  we  have 
many  interesting  instances  of  roundness  from  the  same  cause ;  as — the  par- 
ticles of  a  mist  or  fog  floating  in  air — these,  mutually  attracting  and  coalescing 
into  larger  drops,  and  so  forming  rain — dew-drops — water  trickling  on  a 
duck's  wing — the  tear  dropping  from  the  cheek — drops  of  laudanum — glob- 
ules of  mercury,  like  pure  silver  beads,  coalescing  when  near,  and  forming 
larger  ones — melted  lead  allowed  to  rain  down  from  an  elevated  sieve,  and  by 


26 


CONSTITUTION    OP    MASSES. 


cooling  as  it  descends  so  as  to  retain  the  form  of  its  liquid  drops,  becoming 
the  spherical  shot-lead  of  the  sportsman,  &c. 

The  cause  of  this  extraordinary  phenomenon  which  we  call  attraction,  acts 
at  all  distances. — The  moon,  though  240,000  miles  from  the  earth,  by  her 
attraction,  raises  the  water  of  our  ocean  under  her,  and  forms  what  we  call 
the  tide. — The  sun,  still  farther  off,  has  a  similar  influence ;  and  when  the 
sun  and  moon  act  in  the  same  direction,  we  have  the  spring  tides. — The 
planets,  so  distant  that  they  appear  to  us  little  wandering  points  in  the 
heavens,  yet,  by  their  attraction,  affect  the  motion  of  our  earth  in  her  orbit, 
quickening  it  when  she  is  approaching  them,  retarding  it  when  she  is  re- 
ceding. 

The  attraction  is  greater  the  nearer  the  bodies  are  to  each  other  •  as  the  light 
of  a  taper  is  more  intense  near  to  the  taper  than  at  a  distance. 

A  board  of  a  foot  square,  represented  in  fig.  1  by  A  B  at  a  certain  distance 
from  a  light  supposed  at  C,  just  shadows  a  board  of  two  feet  square,  as  E  D, 
at  double  distance ;  but  a  board  with  a  side  of  two  feet  has  four  times  as 
much  surface  as  a  board  with  a  side  of  one  foot,  for  it  is  not  only  twice  as 
high  or  long,  which  would  make  it  double,  but  twice  as  broad  also,  which 

Fig.  1. 

E  * 


D 

makes  it  quadruple — as  a  globe  of  two  feet  in  diameter  requires  *just  four 
times  as  much  paper  to  cover  it  as  a  globe  of  one  foot, — and  the  corner,  or 
fourth  part  E  F,  of  the  larger  square  here  shown  is  just  equal  to  the  whole 
of  the  smaller  square  A  B.  Light,  therefore,  at  double  distance  from  its 
source,  being  spread  over  four  times  the  space,  has  only  one-fourth  of  the 
intensity;  and  for  a  similar  reason,  at  thrice  the  distance  it  has  only  a  ninth 
part,  at  four  times  a  sixteenth  part,  and  so  on.  Now  light,  heat,  attraction, 
sound,  and  indeed  every  influence  from  a  central  point  are  found  to  decrease 
in  the  proportion  here  illustrated,  viz.,  as  the  surface  of  squares  which  shadow 
one  another  increases.  The  technical  expression  is,  "  the  intensity  is  in- 
versely as  the  squares  of  the  distance ;  (the  distances  being  estimated  from 
the  centres  of  attraction  or  radiation)  or  one-fourth  part  as  strong  at  double 
distance,  four  times  as  strong  at  half  distance,  and  in  a  corresponding  man- 
ner for  all  other  distances. 

Accordingly,  what  weighs  1,000  Ibs.  at  the  sea-shore,  weighs  five  Iba.  less 
at  the  top  of  a  mountain  of  a  certain  height,  or  when  raised  in  a  balloon — as 
is  proved  experimentally  by  a  spring  balance,  or  other  means ; — and  at  the 
distance  of  the  moon,  the  weight,  or  force  towards  the  earth,  of  1,000  Ibs., 
is  diminished  to  five  ounces,  as  is  proved  by  astronomical  test. 

ATTRACTION  has  received  different  names  as  it  is  found  acting  under  differ- 
ent circumstances.  The  chief  distinctions  are  Gravitation,  Cohesion, 
Capillary  and  Chemical  attraction. 

Gravitation  is  the  name  given  to  it  when  acting  at  sensible  distance,  as  in 
the  cases  of  the  moon  lifting  the  tides — the  sun  and  earth  attracting  each 


COHESIVE    ATTRACTION.  27 

other — a  stone  falling,  &c.     Most  of  the  facts  enumerated  at  page  25, 
belong  to  this  head. 

Cohesion  is  the  name  given  when  it  is  acting  at  very  short  distances,  as  in 
keeping  the  atoms  of  a  mass  together., 

It  might  appear,  at  first  sight,  that  it  cannot  be  the  same  cause  which 
draws  a  piece  of  iron  to  the  earth  with  the  moderate  force  called  its  weight, 
and  which  contains  the  constituent  atoms  of  the  iron  in  such  strong  cohesion ; 
but  when  we  recollect  that  attraction  is  stronger  as  the  substances  are  nearer 
to  each  other,  the  difficulty  is  met.  Atoms  very  nearly  in  contact  may  be 
a  million  times  nearer  to  each  other  than  when  only  a  quarter  of  an  inch 
apart,  aud  therefore,  when  the  heat  among  the  atoms  of  any  mass  allows 
them  to  approach  very  near,  they  should  attract  mutually  with  great  force. 

If,  then,  the  surfaces  of  the  bodies  were  not  in  general  so  very  rough  and 
irregular,  that,  when  applied  to  each  other,  they  can  touch  only  in  a  few 
points  of  the  million,  perhaps,  which  each  surface  contains,  bodies  would 
be  invariably  sticking  together  or  cohering  by  any  accidental  contact.  The 
effect  of  artificially  smoothing  the  touching  surfaces  is  seen  in  the  following 
examples  : — we  may  remark,  however,  that  besides  irregularity  of  surface, 
there  is  another  ,reason,  explained  a  little  farther  on,  which  prevents  the 
cohesion. 

Similar  portions  being  cut  off  with  a  clean  knife  from  two  leaden  bullets, 
and  the  fresh  surfaces  being  brought  in  contact  with  a  slight  turning  pres- 
sure, the  bullets  cohere,  almost  as  if  they  had  been  originally  cast  in»one 
piece. 

Fresh-cut  surfaces  of  India-rubber  or  caoutchouc,  cohere  in  a  similar  way. 
We  may  hence  make  elastic  air-tight  tubes,  by  cutting  off  the  edges  of  a 
strip  of  India-rubber  and  bringing  the  cut  surfaces  into  contact  by  winding 
the  strip  spirally  round  any  small  rod  or  cylinder,  and  fixing  it  there  for  a 
time  by  tape  or  cord. 

Two  pieces  of  perfectly  smooth  plate  glass  or  marble,  laid  upon  each  other, 
adhere  with  great  force  :  and  so,  indeed,  do  most  well-polished  flat  surfaces. 

Cohesion  between  a  solid  and  liquid,  and  between  the  particles  of  a  liquid 
among  themselves,  is  seen  in  the  following  instances  : 

A  flat  piece  of  glass,  balanced  at  the  end  of  a  weighing  beam,  and  then 
allowed  to  come  into  contact  with  water,  adheres  to  the  water,  and  with 
much  more  force  than  the  weight  of  water  remaining  upon  it  when  again 
forcibly  raised  !  If  there  were  not  cohesion  or  attraction  of  the  water  par- 
ticles among  themselves,  as  well  as  to  the  glass,  the  latter  could  only  be 
held  down  by  the  weight  of  the  water  which  directly  adhered  to  it.  In 
pouring  water  from  a  mug  or  bottle-lip,  the  water  does  not  at  once  fall  per- 
pendicular, but  runs  down  along  the  inclined  outside  of  the  vessel ;  chiefly 
in  consequence  of  the  attraction  between  this  and  the  water;  hence  the  dif- 
ficulty of  pouring  from  a  vessel  which  has  not  a  projecting  lip. 

The  particles  of  water  cohere  among  themselves  in  a  degree  which  causes 
small  needles  gently  laid  on  the  surface  to  float : — the  weight  of  the  needles 
is  not  sufficient  to  overcome  the  cohesion  of  the  water  surface. 

For  the  same  reason,  many  light  insects  can  walk  upon  the  surface  of 
water  without  being  wetted. 

It  is  chiefly  the  different  force  of  the  attraction  of  cohesion  in  different 


28  CONSTITUTION    OF    MASSES. 

liquids  that  causes  their  drops  or  gutts  from  the  lip  of  a  phial  to  be  of  differ- 
ent magnitude.  Sixty  drops  of  water  fill  the  same  measure  as  10.0  drops 
of  laudanum  from  a  lip  of  the  same  size. 

In  a  larger  mass  of  liquid,  the  attraction  which,  if  acting  alone,  would 
draw  the  particles  into  the  form  of  a  distinct  globe,  yields  to  that  which 
draws  them  towards  the  centre  of  the  earth,  and  therefore  the  liquid  assumes 
more  or  less  completely,  what  is  called  the  level  surface,  that  is  to  say,  a 
surface  corresponding  with  the  general  surface  of  the  earth. 

Attraction  is  called  capillary  when  it  acts  between  a  liquid  and  the  interior 
of  a  solid,  which  is  tubular  or  porous. 

When  an  open  glass  tube  is  partially  immersed  in  water,  the  water  within 
it  stands  above  the  level  of  that  on  the  outside ;  and  the  difference  of  level 
is  greater  as  the  tube  is  less,  because  in  small  tubes,  the  glass  all  round 
being  nearer  to  the  raised  water,  attracts  it  more  powerfully. 

Between  the  two  plates  of  glass  standing  near  to  each  other,  with  their 
lower  edges  in  water,  a  similar  rising  of  water  will  occur ;  and  if  they  are 
closer  at  one  perpenificular  edge  than  at  the  other,  the  surface  of  the  sus- 
pended water  will  be  higher  there.  The  two  plates  of  glass  in  such  a  case 
are  found  to  be  drawn  towards  each  other  by  the  interposed  waters  with  a 
certain  force,  as  happens  also  to  glass  beads,  or  other  small  bodies,  floating 
in  water  with  their  surfaces  so  near  to  each  other  at  the  water's  edge,  that 
the  water  may  rise  between  them, — and  the  nearer  they  approach,  the 
higher  the  water  rises,  and  the  more  strongly  it  attracts. 

Water,  ink,  or  oil,  coming  in  contact  with  the  edge  of  a  book,  is  rapidly 
absorbed  far  inwards  among  the  leaves. 

A  piece  of  sponge  or  a  lump  of  sugar  touching  water  by  its  lowest  corner, 
soon  becomes  moistened  throughout. 

The  wick  of  a  lamp  lifts  the  oil  to  supply  the  flame,  from  two  or  three 
inches  below  it. 

A  mass  of  cotton  thread  hanging  over  the  e.dge  of  a  glass  from  the  water 
within  it,  will  empty  it  as  a  syphon  would.  A  towel  will  empty  a  basin 
of  water  in  the  same  way. 

Dry  wedges  of  wood  driven  into  a  groove  formed  round  a  pillar  of  stone, 
on  being  moistened,  will  swell  so  as  to  rive  off  the  portion  from  the  block. 
In  some  portions  of  Germany,  mill-stones  are  thus  cut  from  the  rock. 

An  immense  weight  or  mass  suspended  by  a  dry  rope  may  be  raised  a 
little  way,  by  merely  wetting  the  rope; — the  moisture  imbibed  by  capillary 
attraction  in  the  substance  of  the  rope  causes  it  to  swell  laterally,  and  to 
become  shorter. 

At  one  time,  the  small  vessels  of  vegetables  were  supposed  to  raise  the 
sap  from  the  roots,  by  capillary  attraction  j  but  this  is  known  now  to  be 
chiefly  an  action  of  vegetable  life. 

Attraction  has  received  the  name  of  chemical  attraction,  or  affinity,  when 
it  unites  the  atoms  of  two  or  more  distinct  substances  into  one  perfect 
compound. 

There  are  about  fifty  substances  in  nature  which  appear,  in  the  present 
state  of  science,  distinct  from  each  other,  and  are  therefore  called  kinds  of 
matter  ;  such  as  the  various  metals,  sulphur,  phosphorus,  &c.;  but  whether 
these  are  in  truth,  originally  and  essentially  different,  or  only  one  simple 


CAPILLARY    ATTRACTION.  29 

primordial  matter,  modified  by  circumstances  as  yet  unknown  to  us,  we 
cannot  at  present  positively  determine.  Diamond  and  pure  black  carbon 
are  the  same  substance  only  with  different  arrangement  of  atoms;  and 
steel,  which  in  the  soft  state  the  graver  cuts  as  it  would  copper  or  silver,  is 
exactly  the  same  substance  as  when,  after  being  tempered  by  heating  and 
sudden  cooling,  it  has  become  as  hard  nearly  as  diamond  itself.  Yet  these 
differences  are  more  striking  than  appear  between  some  substances,  which 
we  now  account  essentially  distinct. 

It  is  found,  however,  that  the  atoms  of  what  we  call  different  substances 
will  not  cohere  and  unite  indifferently,  to  form  masses,  as  atoms  of  the  same 
kind  do, — there  being  singular  preferences  and  dislikes  among  them,  if  it 
may  be  so  expressed,  or  affinities,  as  the  chemists  term  it :  and  when  atoms 
of  two  kinds  do  combine,  the  resulting  compound  generally  loses  all  resem- 
blance to  either  of  the  elements.— Thus  : 

Sulphuric  acid  will  unite  with  copper  and  form  a  beautiful  transcendent 
blue  salt ;  with  iron  it  will  form  a  green  salt ;  and  if  a  piece  of  iron  be  thrown 
into  a  solution  of  the  copper  salt,  the  acid  will  immediately  let  fall  the  cop- 
per, and  take  up  or  dissolve  the  iron. — Sulphuric  acid  will  not  unite  with  or 
dissolve  gold  at  all. — Quicksilver  and  sulphur  unite  in  certain  proportions 
and  form  the  paint  called  vermillion ;  in  other  proportions  they  form  the 
black  mass  called  Ethiops  Mineral, — Lead,  with  oxygen  absorbed  from  the 
atmosphere  or  other  source,  forms  what  is  called  red  lead,  used  by  painters. 
— Sea-sand,  or  flint  and  the  substance  called  soda,  when  heated  together, 
unite  and  form  that  most  useful  substance  called  glass. — Certain  proportions 
of  sulphur  and  of  iron  combine  and  produce  those  beautiful  cubes  of  pyrites 
or  gold-like  metal  which  are  seen  in  slate.  Chemical  attraction  operating 
thus,  does  not,  in  the  slightest  degree,  interfere  with  general  attraction  or 
gravity,  for  every  chemical  compound  weighs  just  as  much  as  its  elements 
taken  separately. 

The  history  and  classification  of  such  facts  connected  with  the  combina- 
tions and  analysis  of  different  substances,  constitute  the  science  of  chemistry, 
so  attractive  and  so  useful.  It  explains  how  the  fifty  kinds  of  matter  above 
alluded  to,  by  variously  combining,  form  the  endless  diversity  of  bodies 
which  constitute,  as  far  as  it  has  yet  been  explored,  the  mass  of  our  ^lobe. 
The  reasons  of  these  various  modifications  of  attraction  are  yet  much  hidden 
from  us. 

It  is  a  remarkable  truth,  that  when  different  substances  combine  in  the 
way  now  described,  the  proportions  of  the  ingredients  are  always  uniform, 
and  such  as  to  lead  to  the  conclusion,  that  for  every  atom  present,  of  one 
substance,  there  is  exactly  one,  or  two,  or  three,  &c.,  of  the  other;  so  that, 
if  there  be  ten  atoms  of  one  substance,  there  are  exactly  ten,  or  twenty,  &c., 
of  the  other,  but  never  an  intermediate  number,  as  13  or  23  to  10,  for  then 
a  particle  of  the  compound  would  consist  of  one  atom  of  the  first,  and  of 
one  and  three-tenths,  or  two  and  three-tenths,  &c.,  of  the  second  substance, 
an  absurdity  if  the  atom  be  indivisible.  For  instance,  a  certain  number  of 
atoms  of  quicksilver,  which  weigh  twenty-five  grains,  combine  with  a  certain 
number  of  atoms  of  sulphur,  weighing  two  grains,  and  form  a  black  com- 
pound called  Ethiops  Mineral,  or  black  sulphur  of  mercury ;  and  if  a  little 
more  of  either  ingredients  be  added,  it  lies  as  a  foreign  mixture  in  the  sul- 
phuret  of  mercury;  but  if  just  as  much  more  sulphur  be  added  as  at  first, 
so  that  there  may  be  two  atoms  of  it,  instead  of  one,  in  every  particle  of 
the  compound,  a  perfect  combination  of  the  whole  will  take  place,  and  a  new 
substance  will  appear,  which  we  call  vermilion.  Many  elementary  substances 


30  CONSTITUTION    OF    MASSES. 

will  only  unite  with  each  other  in  one  proportion,  so  that  any  two  such  sub- 
stances form  only  one  compound ;  but  others  unite  in  several  proportions, 
so  that  several  distinct  compounds  arise  out  of  the  same  two  elements. 

It  thus  appears,  that  although  we  do  not  know  the  exact  number  of  atoms 
in  a  given  quantity  of  any  substance, — whether,  for  instance,  a  grain  of 
sulphuret  of  mercury  has  more  or  less  than  a  million  of  them ;  still,  as. we 
know  that  in  that  grain  there  are  just  as  many  atoms  of  sulphur  as  of  mer- 
cury, and  that  the  weight  of  the  whole  sulphur  to  that  of  the  whole  mercury 
is  as  two  to  twenty-five,  we  know  that  the  single  atoms  must  have  the  same 
relation,  or  that  the  atom  of  mercury  is  12  £  times  as  heavy  as  that  of 
sulphur. 

Tables  have  been  formed  exhibiting  the  relative  weights  of  the  atoms  of 
different  substances ;  and  the  number  standing  opposite  to  each  substance 
is  called  its  chemical  equivalent, — that  is  to  say,  the  weight  of  its  atom  in 
relation  to  the  weight  of  the  atom  of  some  other  substance  chosen  as  a 
standard.  The  equivalent  of  a  compound  substance  depends,  of  course, 
both  on  the  equivalents  of  the  ingredients,  and  on  the  number  of  atoms 
existing  in  one  integrant  particle  of  the  compound. 

There  is  no  such  thing  as  an  atom  of  vermilion,  or  of  any  other  compound, 
for  the  ultimate  molecule  or  particle  must  contain  at  least  one  atom  of  the 
respective  ingredients. 

The  facts  of  the  peculiarities  and  constancy  of  chemical  unions  are  among 
the  strongest  arguments  for  the  existence  of  similar  ultimate  atoms. 

Besides  the  simple  cases  of  attraction  now  explained,  there  are  two  curious 
modifications,  called  electrical  and  magnetical  attractions,  which  from  their 
peculiarities  are  reserved  for  consideration  in  a  future  division  of  this  work. 

"  Atoms  are  more  or  less  close,  according  to  the  quantity  or  REPULSION  of 
heat  among  them;  hence  the  forms  of  solid,  fluid,  air,  &c."  (Read  the 
Analysis,  p.  22.) 

Were  there  in  the  universe  only  atoms  and  attraction,  as  hitherto  ex- 
plained, the  whole  material  of  creation  would  rush  into  close  contact,  forming 
one  huge  solid  mass  of  stillness  and  death.  But  there  is  also  heat  or  caloric, 
which  counteracts  attraction,  and  singularly  modifies  the  results.  It  has 
been  described  by  some  as-  a  most  subtle  fluid,  pervading  all  things,  some- 
what as  water  pervades  a  sponge  :  others  have  accounted  it  merely  a  vibra- 
tion among  the  atoms.  The  truth  is,  that  we  know  little  more  of  heat  as 
a  cause  of  repulsion  than  of  gravity  as  a  cause  of  attraction ;  but  we  can 
study  and  classify  most  accurately  the  phenomena  of  both. 

When  a  continued  addition  of  heat  is  made  to  any  body,  it  gradually 
increases  the  mutual  distance  of  the  constituent  atoms,  or  dilates  the.  body. 
A  solid  thus  is  first  enlarged  and  softened ;  then  melted  or  fused,  that  is  to 
say,  reduced  to  the  state  of  liquid,  as  the  cohesive  attraction  is  overcome; 
and  lastly,  the  atoms  are  repelled  to  still  greater  distances,  so  that  the  sub- 
stance is  converted  into  elastic  fluid  or  air.  Abstraction  of  heat  from  such 
air  causes  return  of  states  in  the  reverse  order. 

Thus  ice,  when  heated  becomes  water,  and  the  water  when  farther  heated 
becomes  steam ;  the  steam  when  cooled  again  becomes  water  as  before,  and 
the  water  when  cooled  becomes  ice.  Ice,  water  and  steam,  therefore,  are 
three  forms  or  states  of  the  same  substance — one  of  the  most  common  in 
nature,  being  the  material  of  the  ocean. 


LIQUID    AND    AIR.  31 

Other  substances  are  similarly  affected  by  heat,  but  as  all  have  different 
relations  to  it,  some  requiring  much  for  liquefaction,  and  some  very  little, 
we  have  that  beautiful  variety  of  solids,  liquids  and  air,  which  constitutes 
four  external  nature. 

Dilitation. — A  rod  of  iron,  which,  when  cold,  will  pass  through  a  certain 
opening,  and  will  lie  lengthwise  between  two  fixed  points,  when  heated,  be- 
comes too  thick  and  to  long  to  do  either. — For  accurate  mensuration,  there- 
fore, rods  or  chains  used  as  the  measure,  must  either  be  at  a  given  tempera- 
ture, or  due  allowance  must  be  made  for  the  difference. 

The  walls  of  a  building,  under  the  pressure  of  a  heavy  roof,  had  begun 
to  bulge  out  so  as  to  threaten  its  stability.  No  force  tried  was  sufficient  to 
restore  them  to  perpendicularity,  until  the  idea  occurred  of  using  the  con- 
tracting force  of  cooling  iron.  The  opposite  walls  were  then  connected  by 
a  number  of  iron  bars,  passing  through  both,  and  having  nuts  to  screw  close 
to  the  wall  upon  their  projecting  ends,  of  which  bars  one-half  were  heated 
at  a  time,  viz.,  every  second  or  alternate  bar,  by  lamps  placed  under  them, 
and  while  lengthened  in  consequence,  and  projecting  farther  beyond  the 
wall,  their  nuts  were  again  screwed  close  up ;  so  that  on  cooling  and  con- 
tracting, they  pulled  the  walls  in  a  degree  back  to  its  place.  The  nuts  of 
the  second  set  of  bars  being  then  screwed  home,  the  others  were  again  heated, 
and  advanced  the  object  as  much  as  the  first;  and  so  on,  until  the  object 
was  accomplished. 

The  iron  rim  of  a  coach  wheel,  when  heated,  goes  on  loosely  and  easily, 
but  when  afterwards  cooled,  it  binds  the  wheel  most  tightly,  giving  remark- 
able firmness  and  strength. 

Iron  hoops  on  masts  and  casks,  are  made  to  bind  in  .a  similar  manner. 

The  common  thermometer  for  measuring  degrees  of  heat,  is  a  glass  bulb, 
filled  with  mercury  or  other  fluid,  and  having  a  narrow  tube  rising  from  it, 
into  which  the  fluid,  on  being  expanded  by  heat,  ascends,  and  so  marks  the 
degree. 

A  bladder  not  quite  full  of  cold  air,  on  being  heated,  becomes  tense,  and 
if  weak,  may  even  be  burst. 

Liquid  and  Air. — A  piece  of  gold,  lead,  pitch,  ice,  sulphur,  or  of  other 
thing,  if  sufficiently  heated,  melts  or  becomes  liquid  ;  each  substance,  how- 
ever, requiring  a  different  degree  of  heat— gold  requires  5,000  degrees,  lead 
600,  ice,  32,  and  so  forth ;  and  if  the  heating  be  afterwards  continued,  most 
things  at  certain  higher  temperatures  suddenly  expand  again  to  many  time 
the  liquid  volume,  and  becomes  aeriform  fluids. 

The  conversion  of  water  into  steam  is  familiarly  known  to  all.  One  pint 
of  water  driven  off  as  steam  from  the  boiler  of  a  low-pressure  steam-engine, 
fills  a  space  of  nearly  2,000  pints,  and  raises  the  piston  through  this,  with 
a  force  of  many  thousands  of  pounds :  it  immediately  afterwards  appears 
again  in  the  cold  condenser  as  a  pint  of  water. 

Six  times  as  much  heat  is  required  to  convert  a  pint  of  water  into  steam, 
as  to  raise  it  from  an  ordinary  temperature  to  that  of  boiling  but  the  steam, 
by  occupying  nearly  2,000  times  the  space  -of  the  water,  proves  that  heat 
merely  produces  a  revulsion  among  the  particles,  and  by  no  means  fills  up 
the  interstices.  The  steam  rising  from  boiling  water  does  not  appear  to  the 
thermometer  hotter  than  the  water  itself;  and  hence  it  was  that  Dr.  Black, 
whose  genius  shed  so  much  light  on  this  part  of  knowledge,  gave  the  excess 
of  heat  the  name  of  latent  heat. 

The  latent  heat  of  common  air  is  made  sensible  in  the  matcJi  syringe.  In 
this,  which  is  close  at  the  bottom,  the  piston  Is  driven  down  quickly  and 


32  CONSTITUTION    OF    MASSES. 

strongly,  so  as  to  compress  very  much  the  air  which  is  underneath  it,  and 
the  heat  then  condensed  with  the  air  is  sufficiently  intense  to  light  a  small 
piece  of  tinder  attached  to  the  bottom  of  the  piston. 

Not  only  are  spirits,  aethers,  oils,  &c.,  convertible,  as  water  is  into  aerifon* 
fluid,  but  also  sulphur,  phosphorus,  mercury,  and,  indeed,  all  the  metals  and 
elementary  substances ; — some  of  them,  however,  requiring  heats  of  great 
intensity. 

The  varieties  of  form,  then,  in  the  bodies  on  the  face  of  this  earth,  may 
be  considered  accidental,  as  dependent  on  the  temperature  of  the  earth,  and 
do  not  mark  the  permanent  nature  ef  the  substances. 

In  the  planet  Mercury,  which  is  near  the  sun,  resin,  tallow,  wax  and  many 
vegetable  substances  deemed  by  us  naturally  solid,  would  all  be  liquid,  as  oil 
is  with  us ;  and  a  certain  mixture  of  tin,  zinc  and  lead,  which  with  us  is  solid 
at  common  temperatures,  but  melts  in  boiling  water,  would  there  bo  always 
liquid  like  our  quicksilver.  Our  water,  oils,  and  spirits,  would  there  be  in  a 
state  of  steam  or  air,  and  could  not  be  known  as  liquids,  except  by  cooling 
processes  and  compression,  such  as  we  have  lately  teamed  to  use  for  reducing 
our  different  airs  to  the  form  of  liquids. 

Again,  in  the  cold  planet  Herschel,  which  is  nineteen  times  farther  from 
the  sun  than  our  earth  is,  water,  if  it  exist,  can  be  known  only  as  rock  crys- 
tal, which  fire  would  have  to  melt  as  it  does  glass  with  us  :  our  oils  would 
be  as  .butter  or  resins,  and  quicksilver  might  be  hammered  as  lead  or  silver 
is  with  us. 

On  our  own  earth,  near  the  equator,  common  sealing-wax  will  not  retain 
impressions ;  butter  is  oil  in  the  day,  and  a  soft  solid  at  night ;  and  tallow 
candles  cannot  be  used.  And  near  our  pole,  in  winter,  the  quicksilver  from 
a  broken  thermometer  is  solid  metal ;  water  must  be  melted  by  fire  for  use ; 
oils  are  solid,  &c. 

To  judge,  then,  of  the  constitution  of  nature  aright,  we  must  always  take 
extended  surveys,  and  not  allow  prejudice  to  mislead  us,  as  it  did  that  Eastern 
potentate,  who  put  a  traveller  to  death  for  saying  he  had  visited  remote 
northern  countries,  where  water  was  sometimes  to  be  seen  solid  like  crystal, 
and  sometimes  white  and  fleecy,  like  feathers. — The  ancients  believed  that 
there  were  just  four  elements  concerned  informing  our  globe,  with  all  upon 
it,  viz.,  earth,  water  y  air  and  fire.  What  a  contrast  between  former  and 
present  knowledge ! 

Repulsion  without  sensible  Seat. 

As  we  stated  in  a  former  paragraph  that  besides  general  attraction,  under 
names  gravitation,  cohesion,  capillary  and  chemical  attraction,  there  are 
modifications  which  have  the  names  of  electrical  and  magnetical  attractions  ; 
so  we  nave  now  to  remark,  that,  besides  the  general  repulsion  of  heat  just 
described,  there  are  peculiarities  which  we  call  electrical  and  magnetical  re- 
pulsions. Whether  these  depend  altogether  on  different  causes,  or  are  only 
modifications  of  effect  from  the-  same  cause,  we  cannot  yet  positively  decide. 

And  it  is  a  curious  fact  connected  with  the  subject,  that  there  seems  to  be 
a  film  of  repulsion,  so  to  express  it,  covering  the  general  surfaces  of  all  bodies, 
and  preventing  their  meeting  in  absolute  contact,  even  when  they  appear  to 
the  human  eye  so  to  meet.  Were  it  not  for  this,  things  would  be  constantly 
approaching  so  closely  to  each  other,  that  they  would  stick  or  cohere,  in  a 
way  to  disturb  the  common  operations  of  nature.  The  following  facts  illus- 
trate this  superficial  repulsion,  and  the  means  which  art  uses  to  overcome  it 
for  particular  purposes. 


REPULSION  OP  SURFACES.  33 

Newton  found  that  a  ball  of  glass,  or  a  watch-glass,  laid  upon  a  flat  surface 
of  glass  does  not  really  touch  it  and  cannot  be  made  to  touch  it  by  a  force 
of  even  1,000  pounds  to  the  inch. 

In  like  manner,  when  glass,  stone,  porcelain,  or  indeed  almost  any  body 
is  broken,  we  cannot  make  the  parts  cohere  again  by  simply  pushing  them 
together  in  their  former  position.  Where  a  union,  therefore,  between  sepa- 
rate masses  is  desired,  we  are  compelled  to  have  recourse  to  various  artifices 

A  few  cases  in  which  cohesion  is  easily  affected,  were  enumerated  at  page 
27  :  the  following  are  other  instances  of  a  different  kind. 

Gold  leaf  laid  upon  clean  steel,  and  then  forciby  struck  by  a  hammer, 
coheres  to  the  steel,  and  gilds  it  permanently. 

But  iron  can  be  made  to  cohere  to  iron,  only  by  rendering  both  pieces  red 
hot  before  hammering : — the  process  is  called  welding.  Iron  and  platinum 
are  the  only  metals  that  can  be  welded. 

Tin  and  lead,  in  sheets,  pressed  together  between  the  strong  rollers  of  a 
flatting-mill,  cohere. 

The  other  metals  require  to  be  melted  before  the  superficial  repulsion 
gives  way  so  as  to  allow  separate  quantities  to  cohere  or  run  into  one  mass. 
It  is  thus,  for  instance,  that  gold,  silver,  lead,  &c.,  are  treated. 

In  many  cases  the  substances  are  not  such  as  can  be  melted,  (wood  or 
marble,  for  instance,)  and  then  it  is  necessary  to  use  some  soft  glue  or  cement. 
Cements  must  have  strong  attraction  for  both  substances,  and,  when  dry  or 
cool,  must  be  tenacious  in  themselves ;  solder,  paste,  common  glue,  motar, 
&c.,  are  the  principal  substances  of  this  kind. 

"  Certain  modifications  of  attraction  produce  the  subordinate  states,  called 
crystal,  porous,  dense,  &c"     (Read  the  Analysis,  page  22.) 

"  It  is  a  remarkable  circumstance,  that  attraction,  in  causing  the  atoms  to 
cohere  so  as  to  form  solid  masses,  seems  not  to  act  equally  all  around  each 
atom,  but  between  certain  sides  or  parts  of  one,  and  corresponding  parts  of 
the  adjoining  one;  so  that  when  atoms  are  allowed  to  cohere  according  to 
their  natural  tendencies,  they  always  assume  a  certain  regular  arrangement 
and  form,  which  we  call  crystalline.  Because  in  this  circumstance  they 
seem  to  resemble  magnets,  which  attract  each  other  only  by  their  poles,  the 
fact  has  been  called  the  polarity  of  atoms.  It  is  the  cause  of  several  of  the 
peculiarities  above  enumerated,  as  elasticity,  &c. 

"  Crystallization"  is  exemplified  in  the  following  particulars  : 

Water  beginning  to  freeze,  shoots  delicate  needles  across  the  surface ; 
these  thicken  and  interweave  until  the  whole  mass  has  become  solid,  but 
the  crystalline  arrangement  always  remains.  In  most  substances,  this 
arrangement  is  remarkably  proved,  by  the  forms  of  the  surfaces  left,  when 
the  mass  is  broken. 

Moisture,  freezing  on  the  window-pane  in  winter,  exhibits  a  beautiful 
variety  of  arborescence. 

A  flake  of  snow  viewed  in  the  microscope,  is  seen  to  be  as  symmetrically 
formed  as  a  fern-leaf  or  a  swan's  feather. 

If  a  piece  of  copper  be  thrown  into  a  solution  of  silver  in  nitric  acid,  it  is 
preferred  by  the  acid  to  silver,  and  is  dissolved  accordingly  :  the  silver  in 
the  mean  time,  during  its  precipitation  or  separation,  assumes  the  form  of 
a  singularly  beautiful  shrub  or  tree,  resting  on  the  remaining  copper  as  its 
root.  This  appearance  is  call  the  arbor  Diande. 

3 


34  CONSTITUTION    OF    MASSES. 

Any  metal  which  has  been  melted,  when  allowed  to  cool  again,  slowly 
and  at  rest,  becomes  solid  first  on  the  outside  of  the  mass.  If,  before  the 
cooling  be  completed,  the  remaining  liquid  be  poured  from  within,  a  curious 
internal  crystalline  structure,  like  grotto  work,  is  seen.  What  is  called  the 
grain  of  a  metal  is  the  result  of  this  crystallization. 

Saltpetre,  glaubler  salt,  copperas  (to  use  popular  names,)  or  any  other  of 
the  many  neutral  salts,  being  dissolved  in  water,  and  the  water  being  then 
allowed  slowly  to  evaporate,  reappears  in  beautiful  regular  crystals,  each 
salt  having  its  peculiar  forms,  bounded  by  perfectly  plane  and  polished 
surfaces.  If  any  such  crystal  be  broken  in  any  part,  the  broken  surface 
appears  to  the  microscope  as  if  regular  layers  of  particles  had  been  disturbed, 
(as  we  see  on  a  larger  scale  in  a  broken  stack  of  bricks,  or  broken  pile  of 
shot  in  a  battery  yard,)  and  the  defect  of  the  crystal  will  be  exactly  filled 
up  by  replacing  it  in  the  evaporating  solution — proving  that  the  ultimate 
particles  are  all  of  the  same  size. 

All  the  precious  stones  are  crystals,  and  can  be  well  cut  only  parallel  to 
their  natural  surfaces. 

The  basaltic  pillars  of  the  Giant's  Causeway  in  Ireland,  and  of  the  Isle 
of  Staffa,  which  appears  like  a  garden  supported  on  magnificent  columns  in 
the  midst  of  the  ocean,  are  natural  crystalline  arrangements  of  particles, 
equalling  in  regularity  and  beauty  any  human  work,  and  in  granduer  so  far 
surpassing  even  the  Egyptian  pyramaids,  that  superstitious  conjecture 
naturally  supposed  them  the  work  of  giant  architects. 

It  would  be  endless  to  go  on  enumerating  crystalline  masses,  for  nature's 
forms  generally,  in  the  inanimate  creation,  as  well  as  in  organized  bodies, 
are  regular  and  symmetrical ;  and  what  we  see  on  earth  of  broken  conti- 
nents, and  islands,  and  rocks,  and  wild  Alpine  scenery,  are  the  effects  of 
subsequent  convulsions,  which  have  deranged  a  primitive  and  natural  order. 

Much  ingenuity  has  been  employed  to  account  for  the  specific  forms 
which  different  crystalline  bodies  assume;  but  the  subject  is  not  yet  reduced 
to  a  state  fitting  it  to  be  a  part  of  this  elementary  study.  A  familiarity 
with  the  various  figures  which  the  exact  science  of  measures  treats  of,  is 
required  in  the  person  who  expects  to  pursue  it  with  pleasure  or  advantage. 
The  facts  are  extremely  curious,  and  the  scientific  investigation  of  them 
may  ultimately  give  important  information  respecting  the  intimate  consti- 
tution of  material  nature. 

"  Porous." — The  crossing  of  the  constituent  crystalline  needles  or  plates 
in  bodies,  causes  them  to  be  porous  or  full  of  small  vacant  spaces.  In  some 
cases  these  are  visible  to  the  eye,  in  many  more  cases,  they  are  visible  to  the 
microscope,  and  in  all,  they  are  to  be  proved  in  some  way. 

Owing  to  the  porosity  arising  from  the  new  arrangement  of  atoms  of 
solidifying,  water  and  a  very  few  other  substances  become  more  bulky  in 
the  change  from  the  liquid  to  the  solid  state.  Water  then  dilates  with 
such  force  as  to  burst  the  strongest  vessels  which  art  can  provide,  and  in 
winter  to  split  even  rocks,  where  it  has  been  retained  in  their  crevices ; — 
freezing  water  thus  curiously  producing  effects  which  surpass  those  of 
exploding  gunpowder.  This  agency  of  water  contributes  to  the  gradual 
breaking  down  of  our  Alpine  summits,  and  the  falling  of  their  destructive 
fragments  into  the  valleys. 

The  stone  called  hydrophane  (agate)  is  opaque,  until  dipped  into  water, 
when  it  absorbs  into  its  pores  one-sixth  of  its  weight  of  the  water,  and  after- 
wards gives  passage  to  light. 

Into  crystallized  sugar,  and  various  stones,  much  water  will  enter  without 
increasing  the  bulk. 


DENSITY.  35 

*  A  kind  of  sandstone,  suitably  shaped,  forms  an  excellent  filter  or  strainer 
for  water. 

Pressure  will  force  water  through  the  pores  of  the  most  solid  gold  : — as 
was  seen  in  the  famous  Florentine  experiment,  where  a  hollow,  thick,  golden 
ball,  being  filled  with  water  and  squeezed^  to  try  the  compressibility  of  water, 
was  found  to  perspire  all  over. 

The  examples  of  porosity  in  animal  and  vegetable  bodies,  are,  however,  the 
most  remarkable. 

Bone  is  a  tissue  of  cells  and  partitions,  as  little  solid  as  a  heap  of  empty 
packing-boxes. 

Wood  is  a  congeries  of  parallel  tubes,  like  bundles  of  organ  pipes.  It 
has  lately  been  proposed  to  prepare  wood  for  certain  purposes,  as  for  making 
the  great  wooden  pins  or  nails  used  in  ship-building,  by  squeezing  it  to  half 
its  lateral  bulk  between  very  strong  rollers,  and  thus  making  its  density 
approach  to  that  of  metal. 

A  piece  of  wood  sunk  to  a  great  depth  in  the  ocean,  and  exposed  to  the 
pressure  there,  has  its  pores  soon  filled  with  water,  and  becomes  nearly  as 
heavy  as  stone.  Thus  it  was  with  the  boat  of  a  whale-fishing  ship,  which  had 
been  dragged  far  under  water  by  a  whale,  and  which,  on  being  afterwards 
drawn  up,  was  supposed  by  the  crew  to  be  bringing  a  piece  of  rock  with  it. 

A  piece  of  cork  in  a  strong,  close  glass  vessel,  nearly  full  of  water,  may 
be  seen  floating  at  the  top ;  but  if  more  water  be  then  forcibly  pumped  into 
the  vessel,  the  cork  will  be  squeezed  and  reduced  in  size,  until  at  last  it  be- 
comes heavier  than  water,  and  sinks.  On  water  being  afterwards  allowed 
to  escape,  the  cork  will  resume  its  bulk  and  will  rise.  A  cork  sunk  200 
feet  under  water  will  never  rise  again  of  itself. 

A  bottle  of  fresh  water,  corked  and  let  down  thirty  or  forty  feet  into  the 
sea,  often  comes  up  again  with  the  water  saltish,  although  the  cork  be  still 
in  its  place:  the  explanation  being,  that  the  cork,  when  far  down,  is" so 
squeezed  as  to  allow  the  water  to  pass  in  or  out  by  its  sides,  but  on  rising, 
resumes  its  former  size. 

"  Density"  or  the  quantity  of  atoms  which  exist  in  a  given  space,  is  very 
different  in  different  substances. 

A  cubic  inch  of  lead  is  forty  times  heavier  than  the  same  bulk  of  cork 
Mercury  is  nearly  fourteen  times  heavier  than  an  equal  bulk  of  water. 

The  density  must  depend  on,  first,  the  size  or  weight  of  the  individual 
atoms ;  secondly,  the  degree  of  porosity  just  now  explained;  and  thirdly,  the 
proximity  of  the  atoms  in  the  more  solid  parts  which  stand  between  the  pores. 

From  many  circumstances  it  appears,  that  the  atoms  even  of  the  most 
solid  bodies  are  nowhere  in  actual  contact,  but  are  retained  in  their  places 
by  a  balance  between  attraction  and  repulsion — thus, 

A  body  dilates  or  contracts,  according  as  heat  is  added  or  taken  away 
from  it. 

A  weight  placed  on  any  upright  rod  or  pillar,  shortens  it  and  lessens  its 
bulk,  and  if  suspended  from  the  bottom,  lengthens  it  and  increases  its  bulk, 
— the  .rod  in  both  cases  returning  to  its  former  dimensions  when  the  weight 
is  removed. 

When  a  plank  or  rod  is  bent,  the  atoms  on  the  concave  side  are,  for  the 
time,  approximated,  and  those  on  the  convex  side  are  drawn  more  apart. 
It  is  remarkable  in  solid  bodies,  not  only  how  precisely  the  balance  between 


36  CONSTITUTION    OF    MASSES. 

attraction  and  repulsion  determines  the  relative  position  of  the  particles,  but 
also  how  strongly ;  for  any  farther  separation  of  the  particles  is  resisted  by 
all  the  force  which  we  call  the  tenacity  or  cohesion  of  the  substance,  and 
any  nearer  approach  by  all  the  force  which  we  call  the  hardness  or  incom- 
pressibility. 

Tin  and  copper,  when  melted  together,  to  form  bronze,  occupy  less  space 
by  one-fifteenth  than  when  separate  :  proving  that  the  atoms  of  the  one  are 
partially  received  into  what  were  vacant  spaces  in  the  other.  A  similar  con- 
densation is  observed  in  many  other  mixtures.  A  pound  of  water  and  a 
pound  of  salt,  when  mixed,  form  two  pounds  of  brine,  but  which  has  much 
less  bulk  than  the  ingredients  apart.  So  also  of  a  pound  of  sugar  dissolved 
in  a  pound  of  water. 

Water  and  liquids  generally  resist  compression  very  powerfully,  but  yield 
enough  to  show  that  the  particles  are  not  in  contact.  It  is  found  that  at 
1,000  fathoms  down  in  the  sea  the  water  is  compressed  by  the  superincum- 
bent water  so  as  to  have  bulk  about  a  hundredth  part  less  than  it  would  have 
at  the  surface. 

In  aeriform  masses  the  atoms  are  very  distant,  and  hence  the  masses  are 
more  easily  compressed.  A  pint  of  water,  on  assuming  the  aeriform  state, 
in  which  it  is  called  steam,  under  ordinary  pressure,  acquires  nearly  2,000 
times  its  former  bulk.  A  hundred  pints  of  common  air  may  be  compressed 
into  a  pint  vessel,  as  in  the  chamber  of  an  air-gun  ;  and  if  the  pressure  be 
much  farther  increased,  the  atoms  will  at  last  collapse  and  form  a  liquid. 
The  heat  which  was  contained  in  such  air,  and  gave  it  its  form,  is  squeezed 
out  in  this  operation,  and  becomes  sensible  all  around. 

From  these  proofs  of  the  non-contact  of  the  atoms,  even  in  the  most  solid 
parts  of  bodies ;  from  the  very  great  space  obviously  occupied  by  pores — 
the  mass  often  having  no  more  solidity  than  a  heap  of  empty  boxes,  of  which 
the  apparently  solid  parts  may  still  be  as  porous  in  a  second  degree,  and  so 
on ;  and  from  the  great  readiness  with  which  light  passes  in  all  directions 
through  dense  bodies  like  glass,  rock  crystal,  diamond,  &c.,  it  has  been 
argued  that  there  is  so  exceedingly  little  of  really  solid  matter,  even  in  the 
densest  mass,  that  the  whole  world,  if  the  atoms  could  be  brought  into 
absolute  contact,  might  be  received  into  a  nut-shell.  We  have  as  yet  no 
means  of  determining  exactly  what  relation  this  idea  has  to  truth. 

The  comparative  weights  of  equal  l>ulk$  of  different  bodies  are  called  their 
specific  gravities. 

In  thus  comparing  bodies,  it  was  necessary  to  choqse  a  standard ;  and 
water,  as  being  the  substance  most  easily  procurable  at  all  times  and  in  all 
places,  has  been  generally  adopted. 

The  metal  called  platinum,  the  heaviest  of  known  substances,  is  about 
twenty-two  times  as  heavy  as  an  equal  bulk  of  water,  and  is  therefore  said 
to  have  specific  gravity  of  22 — gold  is  nineteen  times  as  heavy—  mercury 
thirteen  and  a  half — lead  eleven — iron  eight  and  a  half — copper  eight — com- 
mon stones  about  two  and  a  half — woods  from  half  to  one  and  a  half — cork 
one-quarter,  &c. 

"  Hardness"  is  not  proportioned,  as  might  be  expected,  to  the  density  of  the 
different  bodies,  but  to  the  polarity  of  the  atoms  in  them,  that  is,  to  the 
force  with  which  the  atoms  hold  their  places  in  some  particular  arrangement. 

Hardness  is  measured  generally  by  the  circumstance  of  one  body  being 


DENSITY.  37 

capable  of  scratching  another.  It  is  here  worthy  of  notice,  however,  that 
the  powder  or  dust  of  a  softer  body  will  often,  through  an  effect  of  motion 
to  be  described  below,  aid  in  wearing  down  or  polishing  one  that  is  harder. 

Gold,  though  soft,  is  four  times  heavier  than  the  hard  diamond;  and 
mercury,  which  is  fluid,  is  nearly  twice  as  dense  as  the  hardest  steel. 

Diamond  is  the  hardest  of  known  substances.  It  cuts  or  scratches  every 
other  body,  and  is  generally  polished  by  means  of  its  own  dust. 

Glass-cutters  use  a  point  of  diamond  as  a  glass-knife,  for  dividing  and 
shaping  their  panes. 

Common  flint  also  cuts  glass,  as  is  proved  by  the  frequent  scribblings  on 
windows. 

It  is  remarkable,  that  the  preparation  of  iron,  called  steel,  may  either  be 
soft  like  pure  iron,  or  from  being  heated  and  suddenly  cooled,  in  the  process 
called  tempering,  may  become  nearly  as  hard  as  diamond.  The  discovery 
of  this  fact  is,  perhaps,  second  in  importance  to  few  discoveries  which  man 
has  made ;  for  it  has  given  him  all  the  edge  tools  and  cutting  instruments 
by  which  he  now  moulds  every  other  substance  to  his  wishes.  A  savage 
will  work  for  twelve  months,  with  fire  and  sharp  stones,  to  fell  a  great  tree, 
and  to  give  it  the  shape  of  a  canoe ;  where  a  modern  carpenter,  with  his 
tools,  could  accomplish  the  object  in  a  day  or  two. 

The  project  has  lately  been  realized,  of  engraving  on  plates  of  soft  steel, 
instead  of  copper,  and  afterwards  tempering  the  steel  to  such  hardness,  that 
it  may  be  used  as  a  type  or  die  to  make  its  impression,  not  on  paper,  but  on 
other  plates  of  soft  steel  or  of  copper ;  each  of  which  then  is  equal  in  value 
to  an  original  and  distinct  engraving.  By  this  means  the  beautiful  produc- 
tions of  art,  instead  of  being  limited  to  a  comparatively  small  number  of 
copies  and  of  persons,  may  be  multiplied  almost  to  infinity,  becoming  the 
cheap  delight  of  all. 

"  Elasticity"  is  present  in  a  mass  when  the  atoms,  cohering  in  a  particular 
arrangement  only,  yield,  however,  to  a  certain  extent,  when  force  is  applied, 
but  move  back  or  regain  their  natural  positions  on  the  force  being  with- 
drawn. 

Elastic  bodies  vary  much  as  to  the  extent  to  which  they  yield  without 
breaking,  and  as  to  the  degree  of  perfection  with  which,  after  the  bending, 
or  displacement  of  atoms,  they  regain  their  former  state.  India  rubber  is 
extensively  elastic,  for  it  yields  far ;  but  it  is  not  perfectly  elastic,  for  when 
stretched  much  or  often,  it  becomes  perfectly  elongated.  Glass,  again,  is 
perfectly  elastic,  for  it  will  retain  no  permanent  bend;  but,  unless  in  very 
thin  plates  indeed,  or  in  fine  threads,  it  will  not  bend  far  without  breaking. 

All  hard  bodies  are  elastic,  as  steel,  glass,  ivory,  &c.,  and  many  soft  ones, 
as  caoutchouc,  silk,  a  harp  string,  &c.  The  aeriform  bodies  are  all  per- 
fectly elastic,  as  is  rudely  seen  in  a  bladder  filled  with  air,  when  squeezed, 
and  allowed  to  expand  again ;  and  they  will  change  volume  to  a  very  great 
extent.  Liquids  also  are  perfectly  elastic,  but  to  a  small  extent. 

A  good  steel  sword  may  be  bent  until  its  ends  meet,  and  yet,  when 
allowed,  will  return  to  perfect  straightness. 

A  rod  of  bad  steel,  or  of  other  metal,  will  be  broken  in  bending,  or  will 
retain  a  bend. 

An  ivory  ball,  let  fall  on  a  marble  slab,  rebounds,  owing  to  the  great 
elasticity  of  both  bodies,  nearly  to  the  height  from  which  it  fell,  and  no 
mark  is  left  on  either.  If  the  slab  be  wet,  it  is  seen  that  the  ivory  or  mar- 


38  CONSTITUTION     OF     MASSES. 

ble,  or  both,  had  yielded  considerably  at  the  point  of  contact,  for  a  circular 
surface  of  some  extent  on  the  slab  is  found  dried  by  the  blow.  The  sudden 
expulsion  of  air  from  between  the  meeting  surfaces  might  contribute  to  the 
effect,  but  the  result  is  very  nearly  the  same  when  the  experiment  is  made 
in  a  vacuum.  Billiard  balls  scarcely  lose  even  their  polish  by  long  wear, 
although  the  touching  parts  yield  at  every  stroke. 

A  marble  chimney-piece,  long  supported  by  its  ends,  is  found  at  last  to 
be  bent  downwards  in  the  middle ;  and  the  bend  is  permanent. 

A  steel  watch-spring,  although  so  much  and  so  constantly  bent,  resumes 
its  original  form  when  freed  at  the  end  of  a  century ;  but  occasionally, 
without  evident  cause,  while  in  action,  it  will  suddenly  give  way. 

Elasticity  is  a  property  of  bodies  of  great  utility  to  man,  as  in  his  time- 
pieces, carriage-springs,  gun-locks,  &c.,  &c. 

"  Britileness"  designates  that  constitution  of  a  body  where,  with  hardness, 
and  elasticity  perfect  as  far  as  it  goes,  the  cohesion  among  the  atoms  exists 
within  such  narrow  limits  that  a  very  slight  change  of  position  or  increase 
of  distance  among  them  is  sufficient  to  produce  a  rupture.  A  compara- 
tively slight  force,  therefore,  if  sudden,  breaks  them.  It  belongs  to  most 
very  hard  bodies. 

Glass  scratches  an  iron  hammer,  proving  that  it  is  harder  than  iron — yet 
glass  is  the  very  type  of  fragility  j  yielding  to  the  stroke  of  soft  wood,  or, 
indeed,  of  almost  any  thing  which  can  give  a  blow. 

Steel,  when  tempered  so  as  to  be  very  hard,  becomes  brittle  also.  The 
steel  chisels  and  tools  with  which  artificers  now  shape  the  stones  and  metals 
as  they  formerly  did  wood,  require,  of  course,  to  be  exceedingly  hard ;  but 
they  thereby  lose  in  regard  to  the  extent  of  their  elasticity,  and  hence  are 
frequently  broken.  Cast  iron,  which  is  much  harder  than  malleable  or 
wrought  iron,  is  very  brittle,  while  soft  iron  and  steel  are  the  toughest  things 
in  nature. 

" Malleable"  or  reducible  into  thin  plates  or  leaves  by  hammering.  This 
property,  in  opposition  to  elasticity  and  brittleness,  belongs  to  bodies 
whose  atoms  cohere  equally  in  whatever  relative  situations  they  happen 
to  be,  and  therefore  yield  to  force,  and  shift  about  among  each  other,  with- 
out fracture  or  change  of  property,  almost  like  the  atoms  of  a  fluid. 

Gold  is  remarkably  malleable,  for  it  may  be  reduced  to  leaves  of  the  thin- 
ness of  360,000  to  the  inch,  or  of  1,800  to  a  sheet  of  common  paper.  For 
gold-beaters  the  metal  is  first  formed  into  rods,  these  are  ^afterwards  rolled  or 
battened  into  ribbons ;  the  ribbon  is  cut  into  portions,  which  are  extended, 
by  hammering,  to  great  breadth  and  thinness,  and  which  being  again  divi- 
ded into  portions,  are  hammered  and  extended  to  the  thinness  described. 

Silver,  copper  and  tin  may  also  be  hammered  until  very  thin.  Most  other 
metals  crack  or  are  torn  before  the  operation  is  carried  far ;  and  some,  on 
being  struck,  are  broken  at  once,  almost  like  glass. 

tl  Ductile"  or  susceptible  of  being  drawn  into  wire.  One  might  expect 
malleability  and  ductility  to  belong  to  the  same  substances  and  in  the 
same  degrees — but  they  do  not.  In  ductile  substances,  as  in  malleable, 
the  atoms  seem  to  have  no  more  fixed  relation  of  position  than  in  a  liquid, 
but  yet  they  cohere  very  strongly. 

One  end  of  a  rod  of  iron,  or  other  ductile  metal,  being  reduced  in  size  so 


DENSITY.  39 

as  to  pass  through  an  opening  in  a  plate  of  steel,  is  seized  by  strong  nippers 
on  the  other  side  of  the  plate,  and  the  whole  rod  is  drawn  through.  It  is 
thus  reduced,  of  course,  to  the  size  of  the  opening,  and  is  lengthened  in  a 
like  proportion.  By  repeating  the  operation  through  smaller  holes  succes- 
sively, a  wire  may  at  last  be  obtained  to  the  size  of  a  hair. 

Dr.  Wollaston's  ingenuity  produced  platinum  wire  finer  than  spider's 
thread.  He  filled  a  space  in  the  axis  of  a  silver  wire  with  small  platinum 
wire.  He  then  drew  or  reduced  the  compound  piece  to  the  smallest  wire 
possible,  and  on  dissolving  the  silver  from  the  outside,  he  exposed  to  view 
the  delicate  filament  of  platinum. 

The  order  in  which  metals  may  be  ranged  according  to  their  ductility  is, 
platinum,  silver,  iron,  copper,  gold,  &c. 

Melted  glass  has  great  ductility.  The  workers  draw  or  spin  it  into  threads 
by  merely  attaching  a  point,  pulled  out  from  the  mass,  to  the  circumference 
of  a  turning-wheel.  A  uniform  thread  then  continues  to  be  drawn  out  and 
wound  upon  the  wheel,  at  a  rate  of  1,000  yards  or  more  per  hour.  This 
glass  thread,  when  lying  together  in  quantities,  resembles  beautiful  white 
hair,  and  when  cut  in  bunches,  it  serves  as  an  ornament  to  the  female  head, 
waving  in  the  air  like  the  delicate  plume  of  a  bird  of  paradise. 

"Pliant."  In  bodies  distinguished  by  this  title,  the  cohesion  is  not  des- 
troyed by  considerable  change  of  direction  among  the  particles,  but  there 
is  little  elasticity,  and  unlike  what  happens  in  a  ductile  mass,  the  same 
atoms  always  remain  together. 

Of  all  pliant  things,  the  chief  are  animal  and  vegetable  fibres  and  mem- 
branes— as  silk,  bladder,  lint,  hemp,  &c.,  &c. 

"  Tenacity"  means  the  force  of  cohesion  among  the  atoms  of  any  mass. 
It  belongs  more  or  less  to  all  solids,  and  even  to  liquids. 

This  property  varies  much  in  different  substances.  Iron  and  its  modifica- 
tion called  steel  possess  it  in  the  most  remarkable  degree. 

The  following  table  shows  the  comparative  tenacity,  or  strength  to  resist 
pulling  of  certain  metals  and  woods.  Supposing  similar  wires  or  rods  of 
each  to  be  used,  and  of  such  a  size  that  the  surface  of  a  broken  end  or  cross- 
section  would  be  the  one-thousandth  of  a  square  inch,  the  weights  supported 
would  be  nearly  as  follows : 

METALS. 

Cast  Steel    .         .         .134  Ibs. 
Best  wrought  iron          .       70 
Cast  Iron      .       .......      .       19 

Copper  .",.,-  .,-:  .1-.^  19 
Platinum  .  .»  -^  ,  16 
Silver  .  ,;  .  ^  11 

Gold        .     .  ;•;:; "   9 

Tin  .       :,./.,.       5 

Lead  .      :V.^>:      2 

WOODS. 

Teak  .        .      ',13 

oak        .     .-;'  .:.'  12 

Beech  .         .         .       12* 

Ash         .     .  :$  .    14 

Deal  S.       11 


40  CONSTITUTION    OF     MASSES. 

Iron  compared  in  this  way,  is  five  or  six  times  stronger  than  oak. 

Steel  wire  will  support  about  39,000  feet,  that  is,  7?  miles  of  its  own 
length. 

Certain  animal  substances  have  great  tenacity;  as — the  silk-worm's  thread, 
which  is  our  strongest  connecting  or  sewing  material,  and  has  such  flexibility 
united  with  its  strength — the  ligaments  and  tendons  of  the  animal  body,  pos- 
sessing at  once  such  admirable  strength,  elasticity  and  pliancy  :  these  when 
dried,  and  otherwise  prepared,  constituted  the  tough  bow-strings  of  our  re- 
mote forefathers — the  hair  or  wool  of  animals  twisted  into  threads,  and 
worked  into  strong  and  beautiful  textures  of  the  loom — strips  of  animal  in- 
testines prepared  and  twisted,  forming  the  cords  of  harp  and  violin,  and  in 
strength  and  uniformity  rivaling  the  steel  wires  of  keyed  instruments. 

The  gradual  discovery  of  substances  possessed  of  strong  tenacity  and  which 
man  could  yet  easily  mould  to  his  purposes,  has  been  of  great  importance  to 
his  progress  in  the  arts  of  life.  The  place  of  the  hempen  cordage  of  Euro- 
pean navies  is  still  held  in  China  by  twisted  canes  and  strips  of  bamboo ; 
and  even  the  hempen  cable  of  Europe,  so  great  an  improvement  on  former 
usage  is  now  rapidly  giving  way  to  the  more  complete  and  commodious  secu- 
rity of  the  iron  chain — of  which  the  material  to  our  remote  ancestors  existed 
only  as  a  useless  stone  or  earth.  And  what  a  magnificent  spectacle  is  it,  at 
the  present  day,  to  behold  chains  of  tough  iron  stretched  high  across  a  chan- 
nel of  the  ocean,  as  at  the  Menai  Strait,  between  Anglesea  and  England, 
and  supporting  there  an  admirable  bridge-road  of  safety  along  which  crowded 
processions  may  pour,  regardless  of  the  deep  below,  or  of  the  storm ;  while 
under  it,  ships  with  full  sails  spread  pursue  their  course,  uninolesting  and 
unmolested ! 


APPENDIX 

TO   PART   I.— SECTION    I 

BY  THE  AMERICAN  EDITOR. 


Ir  the  reader  has  studied  the  preceding  section  with  attention  he  is  prepared 
to  understand  the  following  propositions. 

Prop.  1 — Matter  is  endowed  with  properties. 

Prop.  2. — The  properties  of  matter  are  distinguishable  into  two  classes, 
first,  those  which  are  general  or  belong  to  all  kinds  of  matter,  and  second, 
those  which  tare  peculiar  or  belong  only  to  particular  kinds  of  matter. 

Prop.  3. — The  general  properties  of  matter  are,  indestructibility  (p.  22;) 
extension  or  the  property  of  occupying  a  portion  of  space  (jp.  24; )  divisi- 
bility (p.  23 ;)  impenetrability  (p.  24;  )  and  inertia,  (p.  42.  ) 

Prop.  4. — Every  particle  of  matter,  and  also  all  masses,  have  a  mutual 
attraction  for  one  another,  or  endeavor  to  get  near  each  other ;  and  this 
attraction  is  inversely  as  the  squares  of  the  distances. 

Attractions  may  be  primarily  distributed  into  two  classes  :  one  consisting 
of  those  which  exist  between  the  molecules  or  constituent  parts  of  bodies, 
and  the  other  between  the  bodies  themselves.  The  former  are  called  mole- 
cular or  atomic  attractions,  the  latter  gravitation  (p.  26  : )  of  the  former 
there  are  several  varieties,  1st,  cohesion  (p.  27;)  when  this  variety  of 
molecular  attraction  is  exhibited  by  liquids  pervading  the  interstices  of  porous 
bodies,  ascending  in  crevices  or  in  the  pores  of  small  tubes,  it  is  called  ca- 
pilliary  attraction  (p.  28. )  The  other  varieties  of  molecular  attractions 
are  affinity  or  chemical  attraction  (p.  28, )  and  electric  and  magnetic  attrac- 
tion, (p.  30. ) 

Prop.  5. — Attraction  of  gravitation,  or  that  force  by  which  all  the  masses 
of  matter  tend  towards  each  other,  is  exerted  at  all  distances. 

Prop.  6. — Attraction  of  cohesion  acts  only  within  certain  limits,  and  where 
its  sphere  of  attraction  ends,  a  repulsive  force  begins. 

Prop.  7.— Repulsion,  except  when  dependent  on  electricity  or  magnetism, 
is  owing  to  the  presence  of  heat,  which  latter  pervades  all  matter. 

Prop.  8. — The  particles  of  matter  are  more  or  less  close,  according  to  the 
quantity  of  heat  among  them ;  but  they  are  never  in  actual  contact  (p.  30- 
31,)  and  hence  porosity  is  usually  considered  as  one  of  the  properties  of 
matter. 

Prop.  9. — The  peculiar  properties  of  matter  are  density  (p.  35, )  hardness 
(p.  36, )  elasticity  (p.  37, )  brittleness  (p.  38,  )  malleability  (p.  38, )  duc- 
tility (p.  38, )  pliability  (p.  39, )  tenacity,  (p.  39, )  &c. 


42  MOTIONS    AND    FORCES. 


SECTION  II.— THE  MOTION  OR  PHENOMENA  OF  THE 
UNIVERSE.* 

ANALYSIS  OF  THE  SECTION. 

The  bodies  or  masses  composing  the  universe  may  be  at  rest  or  in  motion, 
and  to  change  any  present  state,  force  proportioned  to  the  quantity  of 
the  body  and  to  the  degree  of  change,  is  equally  required,  whether  to  give 
motion,  to  take  it  away,  or  to  bend  it : — a  truth  expressed  by  saying  that 
matter  has  INERTIA,  or  figuratively,  a  stubbornness.  Uniform  straight 
motion,  then,  is  as  naturally  permanent  as  rest.  And  the  motion  in  any 
body,  measured  by  its  velocity,  quantity  of  matter  and  direction,  is  the 
measure  of  the  amount  and  direction  of  any  single  force  or  of  any  com- 
bination of  forces,  which  has  produced  it,  as  also  of  the  force  or  momen- 
tum which  the  body  can  exhibit  again  when  opposed  or  made  to  act  itself 
as  a  cause  of  some  new  motion. 

The  great  forces  of  nature,  referred  to  by  the  two  words  ATTRACTION  and 
REPULSION,  acting  upon  INERT  matter,  produce  the  equable,  accelerated, 
retarded  and  bent  motions  which  constitute  the  great  phenomena  of  the 
universe. —  Tides,  currents,  winds,  falling  bodies,  &c.,  exemplify  attrac- 
tion.— Explosion,  steam  collision,  &c.,  exemplify  repulsion.  And  as  in 
every  case  of  attraction  or  repulsion  two  masses  at  least  must  be  con- 
cerned, there  is  no  motion  or  action  in  the  universe,  without  an  equal  and 
opposite  motion  or  re-action. 


«  Motion" 

Is  the  term  applied  to  the  phenomenon  of  the  changing  of  place  among 
bodies. 

Were  there  no  motion  in  the  universe  it  would  be  dead.  It  would  be 
without  the  rising  or  setting  sun,  or  river  flow,  or  moving  winds,  or  sound, 
or  light,  or  animal  existence. 

To  understand  the  nature  and  laws  of  the  motions  or  changes  which  are 
going  on  around  him,  is  to  man  of  the  greatest  importance,  as  it  enables  him 
to  adapt  his  actions  to  what  is  coming  in  futurity,  and  often  to  interfere  so 
as  to  control  futurity  for  his  special  purposes. 

Motion,  in  any  particular  case,  is  described  by  referring  to  certain  objects 
to  mark  place,  and  to  some  other  motion  chosen  as  the  standard  of  velocity. 
— A  man  sitting  on  the  deck  of  a  sailing  ship,  has  common  motion  with 
the  ship  :  if  walking  on  the  deck,  he  has  relative  motion  to  the  ship  :  but 
if  he  be  walking  towards  the  stern,  just  as  fast  as  the  ship  advances,  he 
is  at  rest  relatively  to  the  bottom  or  shore.  A  ship  sailing  against  the  tide, 
just  as  fast  as  the  tide  runs,  is  as  much  at  rest  relatively  both  to  the  earth 
and  water  as  if  she  were  at  anchor.  Absolute  motion  is  that  which  is  rela- 
tive to  the  whole  universe,  or  rather  to  the  space  in  which  the  universe  ex- 
ists. We  have  no  means  of  ascertaining  such :  for  although  we  know  how 
fast  our  globe  whirls  upon  its  axis  and  wheels  round  the  sun,  we  have  no 
measure  of  the  motion  of  the  sun  himself — revolving  possibly  round  some 

*  The  reader  should  here  re-peruse  the  title  and  Analysis  at  page  22. 


MOTION.  43 

more  distant  centre,  but  almost  certainly  having  a  progress  in  space,  and 
carrying  all  the  planets  along  with  him. 

Motion  is  called  rapid,  as  that  of  lightning — slow,  as  that  of  the  sun-dial 
shadow ;  both  terms  have  reference  to  the  ordinary  intermediate  velocities 
observed  upon  earth.  It  is  called  straight  or  rectilineal,  in  the  apparent 
path  of  a  failing  body — bent,  or  curvilinear,  in  the  track  of  a  body  thrown 
obliquely — accelerated,  in  a  stone  falling  to  the  earth — retarded,  in  a  stone 
thrown  upwards  while  rising  to  the  point  where  it  stops  before  again 
descending. 

"  Owing  to  the  INERTIA  of  bodies,  force  is  equally  required  to  impart  motion 
and  to  take  it  away."  (Read  again  the  last  Analysis.) 

If  a  man  put  his  hand  to  the  crank  of  a  heavy  fly-wheel  or  grindstone,  to 
turn  it,  he  experiences  a  certain  resistance,  which,  however,  gradually  yields 
to  his  effort,  and  he  leaves  the  wheel  whirling  with  velocity  proportioned  to 
the  effort.  If  he  then  puts  out  his  hand  again  to  stop  the  wheel,  he  experi- 
ences an  opposite  but  similar  resistance,  which  however,  as  before,  gradually 
yields,  and  he  brings  the  wheel  to  rest.  In  the  second  case  the  effort  re- 
quired of  him  is  less  than  the  first,  by  reason  of  the  friction  of  the  turning 
axle,  and  the  resistance  of  the  air  in  which  the  wheel  moves, — obstructions 
which,  when  he  was  giving  motion,  opposed  him,  but  when  taking  it  away 
assisted  him.  That  these  obstructions  caused  the  whole  difference  in  such  a 
case,  and  that  they  are  the  great  reasons  why  all  ordinary  motions  on  earth 
seem  to  tend  of  themselves  to  cease,  will  be  shown  in  subsequent  pages.  It 
is  the  resistance  overcome  in  moving  the  wheel  or  in  stopping  it,  and  occa- 
sioning an  expenditure  of  force  proportioned  to  the  mass  and  to  the  degree 
of  change  of  state,  which  is  called  the  INERTIA  of  the  mass,  or  the  vis  iner- 
tias, and  sometimes,  to  help  the  conception  of  the  student,  the  stubbornness, 
sluggishness,  or  inactivity;  but  none  of  these  words  can  originally  suggest 
to  the  mind  all  that  is  intended  to  be  conveyed. 

An  exact  measure  of  the  amount  of  inertia  is  contained  in  the  familiar  fact 
that  a  body  let  fall  near  the  surface  of  the  earth,  falls  rather  more  than  16 
feet  in  the  first  second  of  time, — the  well-known  weight  of  the  body,  or 
force  of  terrestrial  attraction  acting  upon  it  for  one  second,  being  just  suffi- 
cient to  overcome  its  inertia  to  the  extent  stated.  Were  the  inertia  of  matter 
only  half  of  what  it  is,  a  body  near  the  earth  would  fall  32  feet  in  the  second, 
instead  of  16,  as  it  equally  would,  if,  with  present  inertia,  the  attraction  of 
the  earth  were  doubled.  And  were  there  no  inertia,  it  would  fall  or  pass 
through  any  height,  however  great,  in  one  instant.  As  the  amount  of  inertia 
thus  determines  the  amount  of  other  force  required  to  give  motion  to  a 
mass,  so  does  it  determine  the  amount  of  force  required  to  destroy  motion 
in  a  mass.  A  heavy  cannon-ball,  if  wanting  inertia,  might  be  dispatched 
with  the  speed  of  lightning  by  the  slightest  force,  but  then  the  stiffness  of  a 
stalk  of  corn  would  suffice  to  arrest  it ;  and  while  the  ball,  with  the  inertia 
now  existing,  takes  the  force  of  pounds  of  gunpowder  to  give  it  its  usual 
motion,  it  may  not  be  stopped,  even  by  the  cohesion  of  a  block  of  granite, 
which  accordingly  it  shivers  to  pieces.  The  numerous  examples  now  to 
follow  will  prove  the  immense  importance  of  inertia  in  the  general  opera- 
tions of  nature. 

When  the  sails  of  a  ship  are  first  spread  to  receive  the  force  or  impulse  of 
the  wind,  the  vessel  does  not  acquire  her  full  speed  at  once,  but  slowly,  as 
the  continuing  force  gradually  overcomes  the  inertia  of  her  mass.  When  the 


44  MOTIONS    AND    FORCES. 

sails  are  afterwards  taken  in,  she  does  not  loose  her  motion  at  once,  but 
slowly  again,  as  the  continued  resisting  force  of  the  water  destroys  it. 

Horses  must  make  a  greater  effort  at  first  to  put  a  carriage  in  motion  than 
to  maintain  the  motion  afterwards.  And  a  strong  effort  is  required  to  stop 
a  moving  carriage.  When  a  carriage,  of  which  the  body  hangs  from  springs, 
is  first  moved,  the  body  appears  to  fall  back,  and  a  person  within  seems  to 
be  suddenly  forced  against  the  back  cushion.  When  the  carriage  is  stopped 
again  the  body  swings  forward,  and  if  the  stoppage  be  very  sudden,  a  care- 
less passenger  may  unwittingly  pop  his  head  through  a  front  glass.  These 
particulars  prove  the  inertia,  first  of  rest,  and  secondly  of  motion. 

A  man  standing  carelessly  at  the  stern  of  a  boat,  when  the  boat  begins  to 
move,  falls  into  the  water  behind  ;  because  his  feet  are  pulled  forward  while 
the  inertia  of  his  body  keeps  it  where  it  was,  and  therefore  behind  its  sup- 
port. The  stopping  of  a  boat,  again,  illustrates  the  opposite  inertia  of  motion, 
by  the  man's  falling  forward. 

An  awkward  rider  on  horsback  may  be  left  behind,  when  his  horse  starts 
forward  suddenly :  or  may  be  thrown  off  on  one  side  by  the  horse  starting 
to  the  other.  A  horse  at  speed,  stopping  suddenly,  often  sends  his  cavalier 
over  his  ears — as  was  mortifyingly  experienced  by  a  coxcomb,  who,  on  an  old 
cavalry  horse,  chose  to  canter  along  a  foot-path,  to  the  annoyance  of  the 
company,  and  whose  horse  on  hearing  the  word  halt  loudly  addressed  to  it 
by  a  waggish  officer  of  the  regiment,  who  happened  to  be  there  and  to  recog- 
nize it,  suddenly  stood  and  got  rid  of  its  load.  The  mind  or  will  of  the  beau 
had  sinned  against  the  law  of  propriety,  but  his  body  very  perfectly  obeyed 
the  laws  of  inertia  and  gravity,  by  shooting  forward  in  a  parabolic  curve  to 
the  earth. 

A  young  man  not  yet  accustomed  to  the  whip,  drove  his  phaeton  against 
a  heavy  coach  on  the  road,  and  then  to  his  father  foolishly  excused  his  awk- 
wardness, in  a  way  which  led  to  the  prosecution  of  the  coachman  for  furious 
driving.  At  the  trial,  the  youth  and  the  servant  both  deposed  that  the 
shock  of  the  coach  was  such  as  to  throw  them  over  their  horses'  heads,  and 
thus  lost  the  case,  by  unconsciously  proving,  that  the  faulty  velocity  was 
their  own. 

A  man  jumping  from  a  carriage  at  speed  is  in  great  danger  of  falling  for- 
ward, when  his  feet  reach  the  ground ;  for  his  body  has  as  much  forward 
velocity  as  if  he  bad  been  running  with  the  speed  of  the  carriage  and  unless 
he  advance  his  feet  like  a  running  man,  to  support  his  advancing  body,  he 
must  as  certainly  be  dashed  to  the  ground,  as  a  runner  whose  feet  are  sud- 
denly arrested.  A  man  racing  who  receives  a  signal  to  stop,  and  a  man 
jumping  from  a  flying  vehicle,  must  check  their  motion^  nearly  in  the  same 
way. 

A  person  wishing  to  leap  over'a  ditch  or  chasm,  first  makes  a  run,  that 
the  motion  thereby  acquired  may  help  him  over.  A  standing  leap  falls  much 
short  of  a  running  one. 

An  African  traveller  saw  himself  pursued  by  a  tiger,  from  which  he  could 
not  escape  by  running ;  but  perceiving  that  the  animal  was  watching  an 
opportunity  to  seize  him  by  its  usual  spring  or  leap,  he  artfully  led  it  to 
where  the  plain  terminated  in  a  precipice  hidden  by  brush-wood,  and  he  had 
just  time  to  transfer  his  hat  and  cloak  to  a  bush,  and  to  retreat  a  few  paces 
when  the  tiger  sprung  upon  the  bush,  and  by  the  moral  inertia  of  its  body, 
was  carried  over  the  precipice  and  destroyed. 

From  a  glass  of  water  suddenly  pushed  forward  on  the  table,  the  water  is 
spilt  or  left  behind  \  but  if  the  glass  be  already  in  motion,  as  when  carried 


MOTION.  45 

by  a  person  Balking,  and  if  it  then  be  suddenly  stopped  by  coming  against 
an  impediment,  the  water  is  thrown  or  spilt  forward. 

A  servant  carrying  a  tray  of  glasses  or  china  in  the  dark,  and  coming  sud- 
denly against  an  obstacle,  hears  all  his  freight  slipping  forward  and  crashing 
at  his  feet :  and  a  too  hurried  departure  with  such  a  load  causes  equal  des- 
truction, on  the  opposite  side. 

The  actions  of  beating  a  coat  or  carpet  with  a  cane,  to  expel  the  dust ;  of 
shaking  the  snow  from  one's  shoes,  by  kicking  against  a  door-post ;  of  clean- 
ing a  dusty  book  by  knocking  it  against  a  table,  or  shutting  it  violently — all 
illustrate  the  same  principle. 

If  a  guinea  be  laid  on  a  card  which  is  already  balanced  on  the  point  of  the 
finger,  a  small  fillip  or  blow  to  the  edge  of  the  card  will  cause  it  to  dart  off, 
but  the  guinea,  owing  to  its  inertia,  will  remain  resting  on  the  finger, — its 
inertia  being  greater  than  the  friction  on  it  of  the  card  passing  from  under- 
neath it. 

When  we  desire  a  person,  with  suspected  disease  of  the  brain,  to  shake 
his  head  and  tell  whether  and  where  he  feels  pain,  we  are  doing  nearly  as 
if  we  touched  the  naked  brain  with  the  finger  to  find  the  tender  part ;  for 
the  inertia  of  the  brain,  when  the  skull  is  moved,  causes  a  momentary  pres- 
sure between  it  and  the  skull,  almost  equivalent,  for  the  purpose  desired,  to 
such  a  touch. 

This  kind  of  pressure  is  sufficient  to  break  and  destroy  tender  wares — as 
glass  or  eggs — in  packages  which  are  too  suddenly  moved  or  stopped. 

A  weight  suspended  by  a  spring  on  ship-board  is  seen  vibrating  up  and 
down  as  the  ship  pitches  with  the  waves.  It  seems  to  fall  as  the  ship  rises, 
and  to  rise  as  the  ship  falls :  but  the  motion  is  really  in  the  ship,  and  the 
comparative  rest  is  in  the  weight.  A  heavy  weight  so  supported,  and  con- 
nected with  a  pump-rod,  would  work  the  pump. 

Like  the  weight  last  mentioned,  the  mercury  of»a  common  barometer  on 
ship-board  is  seen  rising  and  falling  in  the  tube;  and  until  the  important 
improvement  was  lately  made,  of  narrowing  one  part  of  the  tube  to  prevent 
this,  the  mercurial  Barometer  was  useless  at  sea.  The  explanation  is,  that 
the  tube  rises  and  falls  with  the  ship,  from  being  connected  with  it ;  but  the 
mercury,  which  plays  freely  in  the  tube,  and  is  supported  by  the  atmospheric 
pressure,  tends,  by  its  inertia,  to  remain  at  rest,  and  thus  makes  the  motion 
of  the  ship  apparent. 

What  happens  to  the  mercury  in  the  barometer-tube  on  ship-board,  indi- 
cates what  happens  to  the  blood  in  the  vessels  of  animals  under  similar  cir- 
cumstances. In  any  long  vein  below  the  heart,  when  the  body  falls,  the 
blood,  by  its  inertia  and  the  supporting  action  of  the  vessels,  does  not  fall  so 
fast,  and  therefore  really  rises  in  the  vein :  and  as  there  are  valves  in  the 
veins  preventing  return,  the  circulation  is  thus  quickened  without  any  mus- 
cular exhaustion  on  the  part  of  the  individual.  This  helps  to  explain  the 
effect  of  the  movement  of  carriages,  of  vessels  at  sea,  of  swings,  &c.,  and  of 
passive  exercise  generally,  on  the  circulation,  and  leaves  it  less  a  mystery 
why  these  means  are  often  so  useful  in  certain  states  of  weak  health. 

If  a  cannon  ball  were  to  break  to  pieces  in  its  flight,  its  parts  would  still 
advance  with  the  previous  velocity.  And  thus,  in  the  deadly  contrivance  of 
the  Shrapnell-shell,  which  is  in  a  case  containing  hundreds  of  musket  bul- 
lets, when  these  are  scattered  at  the  desired  distance  from  the  devoted  body 
of  men,  they  retain  the  forward  velocity  of  the  shell,  and  spread  death 
around  like  the  near  discharge  of  a  whole  battalion  of  musketry. 

On  the  awful  occasion  of  a  ship  in  rapid  motion  being  suddenly  arrested 


46  MOTIONS    AND    FORCES. 

by  a  sunken  rock,  all  things  on  board,  men,  guns,  and  furniture,  start  from 
their  places  and  dash  forwards ;  while  the  onward  inertia  or  moral  obstinacy 
of  the  hinder  parts  of  the  ship,  suffices  to  crush  her  bow  against  the  rock. 

"  Motion  as  naturally  permanent  as  rest." 

From  the  instances  now  given,  it  is  seen  that  a  body  at  rest  would  never 
move  if  force  were  not  applied,  and  that  a  body  put  in  motion  retains  mo- 
tion, at  least  for  a  time,  after  the  force  has  ceased;  but  there  is  a  feeling 
from  common  experience,  that  motion  is  an  unnatural  or  forced  state  of 
bodies,  and  that  all  moving  things,  if  left  to  themselves,  would  gradually 
come  to  rest.  It  is  recollected  that  a  stone  projected  comes  to  rest,  or  a 
wheel  left  moving,  or  a  bowl  rolled  on  the  green,  or  the  waves  heaving 
after  "a  storm — and  in  a  word,  that  there  is  no  perpetual  motion  on  the  earth. 

On  more  attentive  consideration,  however,  it  may  be  perceived  that  there 
are  prodigious  differences  in  the  duration  of  motions,  and  that  the  differences 
are  always  exactly  proportioned  to  evident  causes  of  retardation,  and  chiefly 
to  friction  and  the  resistance  of  the  air. 

Friction  is  the  resistance  which  bodies  experience  when  rubbing  or  sliding 
upon  each  other;  and  however  much  it  may  be  diminished  by  art,  it  can  in 
no  case  be  annihilated.  Air-resistance,  again,  to  motions  going  on  in  air,  is 
of  the  same  nature  as  water-resistance  to  motions  going  on  in  water,  only 
less  in  degree  :  and  as  advancing  science  has  shown  the  true  nature  of  our 
atmosphere,  the  amount  of  this  resistance  is  perfectly  ascertained. 

A  smooth  ball  rolled  on  the  grass  soon  stops — if  rolled  on  a  green  cloth 
over  a  smooth  plank  it  goes  longer — on  the  bare  plank,  longer  still — on  a 
smooth  and  level  sheet  of  ice,  it  hardly  suffers  retardation  from  friction,  and, 
if  the  air  be  moving  with  it,  will  reach  a  distant  shore. 

Two  little  wind-mill  wheels  set  in  motion  together  with  equal  velocity,  but 
of  which  one  has  the  flat  sides  of  the  vanes  turned  to  their  course,  and  the 
other  the  edges,  if  moving  in  the  air,  will  stop  at  very  different  times,  but  if 
tried  in  a  vessel  from  which  the  air  has  been  removed,  they  will  both  go 
much  longer,  and  will  then  stop  exactly  together. 

As  it  is  to  facilitate  the  motion  of  fishes  in  the  water,  that  they  are  of 
sharp  form  before  and  behind ;  so  it  is  to  facilitate  the  motion  of  birds  in 
the  air  that  they  have  somewhat  of  a  similar  form. 

A  large  spinning-top,  with  a  fine  hard  point,  set  in  motion  in  a  vacuum, 
and  on  a  hard,  smooth  surface,  will  continue  turning  for  hours. 

A  pendulum  moving  in  a  vacuum  has  only  to  overcome  slight  friction  at 
its  point  of  suspension,  and,  therefore,  if  once  put  in  motion,  will  vibrate  for 
a  day  or  more. 

But  it  is  in  the  celestial  spaces  that  we  see  motions  completely  freed  from 
the  obstacles  of  air  and  friction — and  there  they  seem  eternal. 

Had  the  human  eye,  unassisted,  been  able  to  descry  the  four  beautiful 
moons  of  Jupiter,  wheeling  around  him  for  these  thousands  of  years,  with 
such  unabated  regularity,  and  which  now  form,  to  the  telescope  of  the  astro- 
nomer, a  perfect  and  magnificent  time-piece  in  the  sky,  or  had  science  long 
proved  that  the  velocity  imparted  to  our  globe,  when  first  launched  into  its 
present  orbit,  still  wheels  it  along  as  swiftly  as  in  the  days  of  the  first  man, 
this  error  or  prejudice,  that  motion  is  always  tending  to  rest,  would  never 
have  arisen. 

Indeed,  had  these  or  other  such  truths,  been  long  familiar  to  the  common 


MOTION    UNIFORM.  47 

mind,  the  opposite  prejudice  might  as  well  have  obtained,  that  motion  is  the 
natural  state,  and  rest  a  forced  or  unknown  state.  We  know  of  nothing 
which  is  absolutely  at  rest.  The  earth  is  whirling  round  its  axis  and  round 
the  sun ;  the  sun  is  moving  round  its  axis  and  round  the  centre  of  gravity 
of  the  solar  system,  and  possibly,  round  some  more  remote  centre  in  the 
great  universe,  carrying  all  its  planets  and  comets  about  his  path. 

If  there  were  any  natural  tendency  in  moving  bodies  to  stop,  a  thing  float- 
ing in  a  trough  of  water,  on  board  a  sailing  ship,  should  always  be  found  at 
the  end  of  the  trough  nearest  the  stern ;  and  in  all  the  seas  and  lakes  of  the 
earth,  the  floating  things  should  be  accumulated  on  the  western  shores, 
because  the  surface  of  the  earth  is  always  turning  towards  the  east.  We 
know  that  neither  of  these  suppositions  is  truth.  A  man  on  board  a  mov- 
ing ship  can  throw  any  body  just  as  far  towards  the  bow  as  towards  the 
stern ;  although  in  the  two  cases  the  velocity,  as  regards  the  earth  is  so 
different. 

Ignorance  of  the  law  of  moral  inertia  led  a  story-telling  sailor  to  assert,  as 
a  proof  of  the  speed  of  his  favourite  ship,  that  when  a  man  one  day  fell  from 
the  mast-head,  the  ship  had  passed  from  under  him  before  he  reached  the 
deck :  the  fact  in  such  a  case,  being,  that  he  must  have  fallen  on  the  same 
part  of  the  deck,  whether  the  ship  were  in  motion  or  at  rest,  because  his 
body  had  just  the  motion  or  rest  which  belonged  to  the  ship. 

Another  equally  sapient  man,  reflecting  that  the  earth  turned  round  once 
in  twenty-four  hours,  proposed  rising  in  a  balloon,  and  waiting  aloft,  until 
the  country  which  he  desired  to  reach  should  be  passing  under  him. 

"Motion  naturally  uniform"  (See  the  Analysis.) 

It  is  only  repeating  that  a  body  can  neither  acquire  motion  nor  lose  motion 
without  a  cause,  to  say  that  free  motion  must  be  uniform. 

The  perfect  uniformity  of  undisturbed  motion  is  proved  by  every  fact 
observed  in  the  universe.  If  any  continued  motion,  as  of  a  planet,  for  in- 
stance, be  found  at  one  time  to  have  certain  relative  velocity  to  some  other 
continued  motion,  the  same  relation  is  found  always  to  hold :  or  deviations 
from  perfect  uniformity  are  exactly  proportioned  to  the  disturbing  causes. 
Thus  we  can  foretell  the  exact  time  of  an  eclipse,  a  thousand  years  before 
its  occurrence. 

Had  motion  not  been  in  its  nature  uniform,  a  man  could  have  formed  no 
rational  conjecture  or  anticipation  as  to  future  events;  for  it  is  by  assuming, 
for  instance,  that  the  earth  will  continue  to  turn  uniformly  on  its  axis,  that 
he  speaks  of  to-morrow  and  of  next  week,  &c.,  and  that  he  makes  all  his 
arrangements  for  future  emergencies :  and  were  the  coming  day,  or  season, 
or  year,  to  arrive  sooner  or  later  than  such  anticipation,  it  would  throw  such 
confusion  in  all  his  affairs,  that  the  world  would  soon  be  desolate. 

To  calculate  futurities,  then,  or  to  speak  of  past  events,  is  merely  to  take 
some  great  uniform  motion  as  a  standard  with  which  to  compare  all  others ; 
and  then  to  say  of  the  remote  event,  that  it  coincided  or  will  coincide  with 
some  described  state  of  the  standard  motion.  The  most  obvious  and  best 
standards  are  the  whirling  of  the  earth  about  its  axis,  and  its  great  revolution 
round  the  sun.  The  first  is  rendered  very  sensible  to  man  by  his  alternately 
seeing  and  not  seeing  the  sun,  and  it  is  called  a  day  ;  the  second  is  marked 
by  the  succession  of  the  seasons,  and  it  is  called  a  year.  The  earth  turns 
upon  its  axes  nearly  365  times  while  it  is  performing  one  circuit  round  the 
sun,  and  thus  divides  the  year  into  so  many  smaller  parts,  and  the  day  is 


48  MOTIONS     AND     FORCES. 

divided  into  smaller  parts,  by  the  progress  of  the  earth's  whirling  being  so 
distinctly  marked,  in  the  constantly  varying  direction  of  the  sun,  as  viewed 
from  any  given  spot  on  the  face  of  the  earth.  When  advancing  civilization 
made  it  of  importance  for  men  to  be  able  to  ascertain  with  precision  the 
very  instant  of  the  earth's  revolution,  connected  with  any  event,  various  con- 
trivances were  introduced  for  the  purpose ;  as, — sun-dials,  where  the  shadow 
travels  progressively  round  the  divided  circle ; — the  uniform  flux  of  water 
through  a  prepared  opening — the  flux  of  sand  in  the  common  hour-glass,  &c. 
But  the  great  triumphs  of  modern  ingenuity  are  those  astronomical  clocks 
and  watches,  in  which  the  counted  equal  vibrations  of  the  pendulum,  or 
balance-wheel,  have  detected  periodical  inequalities  even  in  the  motion  of 
the  earth  itself,  and  have  directed  attention  to  unsuspected  disturbing  causes, 
important  to  be  known. 

It  is  the  natural  uniformity  of  undisturbed  motion  which  causes  any  num- 
ber of  bodies  moving  together,  as  the  furniture  of  a  sailing  ship,  to  appear 
among  themselves  as  if  at  rest, — no  one  tending  to  pass  before,  or  to  fall 
behind,  or  to  move  to  one  side  or  another.  For  the  same  reason  a  person 
who  is  moving  with  such  bodies  is  absolutely  insensible  of  his  uniform  pro- 
gression, and  knows  it  only  by  reasoning  from  such  facts  as  the  changing 
appearance  of  other  objects  around  which  do  not  share  the  motion,  the  rush- 
ing of  the  waves  or  wind,  &c.  When  a  ship  is  becalmed  at  sea,  she  may, 
as  numberless  sad  accidents  have  proved,  be  carried  by  rapid  currents  in  any 
direction,  without  one  of  the  crew  suspecting  that  she  has  motion  at  all ;  and 
if  the  suspicion  do  arise,  the  truth  can  be  come  at  only  by  such  means  as  the 
sounding  line,  where  the  bottom  can  be  reached,  or  careful  observation  of  the 
heavenly  bodies  where  it  cannot.  A  man  in  the  hold  of  a  ship  in  a  river  or 
tides-way  cannot  say  whether  the  rushing  of  water,  which  he  hears  from 
without,  be  a  rapid  tide  passing  the  ship  at  anchor,  or  the  effect  of  the  ship's 
advance  in  the  river.  A  man  in  a  balloon  going  80  miles  an  hour,  knows 
not  in  what  direction  he  is  moving,  nor  indeed  that  he  is  moving  at  all,  but 
by  observing  the  objects  below. 

This  explains  why  men  are  not  sensible  of  the  motion  of  the  earth  itself, 
which  they  know,  however,  to  be  turning  around  its  axis  once  in  twenty -four 
hours,  and  therefore  to  have  its  surface  near  the  equator  moving  with  a  speed 
of  more  than  1,000  feet  per  second ;  and  as  in  the  case  of  a  ship  or  balloon, 
there  will  be  no  difference  of  sensation  whether  the  speed  were  of  one  mile 
per  hour  or  of  10  or  100,  so  in  the  case  of  the  earth,  there  would  be  none 
whether  it  turned  as  now,  once  in  twenty-four  hours ;  or,  like  the  planet 
Jupiter,  once  in  ten.  A  hunter  among  the  hills,  who  during  the  heat  of 
noon,  rests  and  contemplates  around  him  a  sublime  scfene  of  solitude  and 
silence,  may  little  think  that  if,  amidst  that  apparent  repose  of  nature,  he 
were  for  a  moment  lifted  up  from  the  earth  and  held  at  rest  above  its  sur- 
face, he  would  see  its  face  of  hill  and  dale  sweeping  past  beneath  him  at 
the  prodigious  rate  of  1,000  miles  an  hour,  on  account  solely  of  the  whirling 
of  the  earth. 

The  fact  that  a  cannon-ball  can  be  shot  just  as  far  upon  the  surface  of  the 
earth,  eastward,  in  the  direction  of  the  earth's  motion,  as  westward,  against 
it,  illustrates  the  truth,  that  whatever  common  motion  objects  may  have,  it 
does  not  interfere  with  the  effect  of  a  force  producing  any  new  relative 
motion  among  them.  All  the  motions  seen  on  earth  are  really  only  slight 
differences  among  the  common  motions :  as  in  a  fleet  of  sailing  ships,  the 
apparent  changes  of  place  among  them  are  in  reality  only  slight  alterations 
of  speed  or  direction,  in  their  individual  courses. 


MOTION     STRAIGHT.  49 

A  man  continuing  to  throw  upwards  a  ball  or  orange,  or  several  of  them 
at  once,  and  to  catch  and  return  them  alternately,  uses  no  difference  of  art 
as  regards  them,  whether  he  be  standing  on  the  earth  and  whirling  with  it, 
or  on  a  sailing  ship's  deck,  or  in  a  moving  carriage,  or  on  a  galloping  horse's 
back.  He  and  the  oranges  have  always  the  same  forward  common  motion. 
And  when  a  man,  standing  on  a  galloping  horse,  leaps  through  a  hoop  held 
across  his  course,  he  does  not  leap  forward — for  this  would  throw  him  over 
the  horse's  ears — but  merely  jumps  up  and  allows  his  moral  inertia  to  carry 
him  through. 

The  reason  why  a  lofty  spire  or  obelisk  stands  more  securely  on  the  earth, 
than  even  a  short  pillar  stands  on  the  bottom  of  a  moving  wagon,  is,  not  that 
the  earth  is  more  at  rest  than  the  wagon,  but  that  its  motion  is  uniform. — 
Were  the  present  rotation  of  our  globe  to  be  arrested  but  for  a  moment, 
imperial  London,  with  its  thousand  spires  and  turrets,  would,  by  the  moral 
inertia,  be  swept  from  its  valley  towards  the  eastern  ocean,  just  as  loose 
snow  is  swept  away  by  a  gust  of  wind. 

"  Force  is  required  to  bend  motion" 

If  a  body  moving  freely  cannot  vary  its  velocity  without  a  cause,  neither 
can  it  vary  its  course  without  a  cause ;  and  free  motion,  therefore,  is 
straight  as  well  as  uniform. 

A  ball  shot  directly  up  or  down  gives  men  their  simplest  idea  of  straight 
motion. 

A  bullet  or  arrow,  projected  horizontally,  is  gradually  drawn  downwards 
by  the  attraction  of  the  earth,  but  it  deviates  neither  to  the  right  nor  to 
the  left. 

William  Tell,  trusting  to  the  natural  straightness  of  motion,  obeyed  the 
tyrant's  order,  and  shot  an  apple  placed  on  his  child's  head. 

And  the  right  eye  of  Philip  of  Macedon  is  said  to  have  been  destroyed 
by  an  arrow  which* brought  a  label  on  it,  telling  its  destination. 

Riflemen  shooting  at  a  target,  hit  the  very  spot  they  choose  to  aim  at. 

A  stone  in  a  sling,  the  moment  it  is  set  at  liberty,  darts  off  as  straightly  as 
an  arrow  from  the  bow-string  or  a  bullet  from  a  gun-barrel,  and  it  is  only 
because  the  point  of  its  circle,  from  which  it  should  depart,  cannot  in  prac- 
tice be  accurately  determined,  that  the  same  sure  aim  cannot  be  taken  with  it. 

A  body  moving  in  a  circle,  then,  or  curve,  is  constrained  to  do  what  is 
contrary  to  its  inertia.  A  person  on  first  approaching  this  subject,  might 
suppose  that  a  body,  which  for  a  time  has  been  constrained  to  move  in  a 
circle,  should  naturally  continue  to  do  so  when  set  at  liberty.  But  on 
reflecting  that  a  circle  is  as  if  made  up  of  an  infinite  number  of  little  straight 
lines,  and  that  the  body  moving  in  it  has  its  motion  bent  at' every  step  of  the 
progress,  the  reason  is  seen  why  constant  force  becomes  necessary  to  keep 
it  there,  and  force  just  equal  to  the  inertia  with  which 
the  body  tends,  at  every  point  of  the  circle,  rather  to  Fig.  2. 

pursue  the  straight  line,  called  a  tangent,  of  which  that 
point,  as  seen  in  Fig.  2,  is  the  commencement,  than  the 
circle  itself.  The  force  required  to  keep  the  body  in  the 
bent  course,  is  called  centripetal  or  centre-seeking  force  ; 
while  the  inertia  of  the  body  tending  outwards,  that  is, 
to  move  in  a  straight  line  rather  then  in  a  curve,  is  called 
the  centrifugal  or  centre-flying  force ;  and  the  term  cen- 
tral forces  is  applied  to  both. 


50  MOTIONS    AND    FORCES. 

A  sling-cord  is  always  tight  while  the  cord  is  whirling  :,and  its  tension  is 
of  course  the  measure  both  of  the  centripetal  and  centrifugal  force.  A  means, 
then,  of  measuring  the  tension  of  a  sling-cord  would  experimentally  demon- 
strate the  amount  of  centrifugal  force ;  and  such  a  means  we  possess  in  the 
contrivance  called  the  "  whirling  table/'  upon  which  is  a  leading  sling,  or  any 
mass  with  a  string  attached  to  it,  may  be  placed  to  revolve,  at  any  desired 
distance  from  the  centre,  and  with  any  desired  velocity,  while  the  string 
passing  over  a  pulley  at  the  centre,  is  made  to  lift  weights  proportioned  to 
the  outward  dragging  of  the  revolving  mass.  By  this  apparatus  it  is  found, 
as  would  be  expected,  that  centrifugal  force — in  other  words  the  force  with 
which  the  inertia  of  moving  matter  resists  the  bending  of  its  course  from 
straight  to  circular,  is  proportioned,  first,  to  the  quantity  of  matter  moved—- 
every separate  particle  having  its  own  inertia ;  second,  to  the  size  of  the 
circle  or  orbit  described  in  the  same  time — a  body  moving  in  a  circle  of 
double  diameter  for  instance,  having  to  be  forced  inwards  from  the  tangent, 
at  every  departure,  twice  as  far  in  a  given  time ;  third,  that  with  a  double 
revolution  in  the  same  time,  the  centrifugal  force  is  not  double  but  quadruple 
(a  corresponding  proportion  existing  for  other  velocities,)  because,  not  only 
are  there  twice  as  many  bindings  or  angular  departures  from  the  tangent  for 
the  two  circles  as  for  one,  requiring,  as  may  be  said,  twice  as  many  tugs  or 
impulses  of  the  centripetal  force,  but  every  impulse  must  be  made  with 
double  energy,  for  it  has  to  drive  the  mass  inwards  through  the  required  dis- 
tance in  half  the  time ;  and  twice  as  many  impulses,  every  one.  being  twice 
as  strong,  make  a  quadruple  amount  of  force  on  the  whole ;  fourthly  and 
lastly,  it  is  found,  agreeing  with  the  relation  between  inertia  and  terrestrial 
gravity  described  at  page  43,  that  a  body  revolving,  for  instance,  in  a  circle 
of  four  feet  diameter,  that  it  may  have  centrifugal  force  just  equal  to  its 
weight,  required  to  complete  its  revolution  in  one  second  and  a  half  of  time. 
This  and  similar  facts  will  be  more  particularly  considered  when  we  come  to 
treat  of  the  motions  of  the  planets  round  the  sun.  This  analysis  of  central 
forces  will  suffice  to  excite  in  the  student  a  due  interest  touching  the  kindred 
phenomena  now  to  be  described. 

Bodies  laid  on  a  whirling  horizontal  wheel,  are  readily  thrown  off. 

In  a  corn-mill,  the  grain,  after  being  admitted  between  the  stones  through 
an  opening  in  the  centre  of  the  upper  stone,  is  then  kept  turning  round 
between  them,  and  is,  by  its  centrifugal  force,  always  tending  and  travelling 
outwards  until  it  escapes  as  flour  from  the  circumference. 

A  man,  if  he  lie  down  on  a  turning  millstone  with  his  head  near  the  edge, 
falls  asleep,  or  dies  of  apoplexy,  from  the  new  pressure  of  blood  on  the 
brain. 

A  wet  mop,  or  bottle-brush,  made  to  turn  quickly  on  its  handle  as  an  axis, 
throws  the  water  off  in  all  directions,  and  soon  dries  itself. 

Sheep,  in  wet  weather,  thus  discharge  the  water  from  their  fleeces,  by  a 
semi-rotatory  shake  of  the  skin.  Water-dogs,  on  coming  to  land,  dry  them- 
selves by  the  same  action. 

A  tumbler  of  water  placed  in  a  sling,  may  be  made  to  vibrate  like  a  pen- 
dulum with  gradually  increasing  oscillation,  and  at  last  to  describe  the  whole 
circle,  and  continue  revolving  about  the  hand,  without  spilling  a  drop : — the 
water,  by  its  inertia  of  straightness,  or  centrifugal  force,  tending  more  away 
from  the  centre  of  motion  towards  the  bottom  of  the  tumbler,  even  when 
that  is  uppermost,  than  towards  the  earth  by  gravity. 

As  solid  bodies  laid  on  a  whirling  table  are  thrown  off,  so  water  in  a  ves- 
sel caused  to  spin  round  in  any  way,  as  on  the  centre  of  a  horizontal  wheel, 


CENTRIFUGAL    FORCE.  51 

instead  of  lying  at  the  bottom,  is  raised  up  all  round,  against  the  sides  of 
the  vessel. 

"Water,  poured  obliquely  into  a  funnel,  runs  round  the  interior  of  it,  and 
often  leaves  an  open  passage  of  air  all  the  way  down  through  it,  as  if  there 
were  merely  a  lining  of  water  to  the  funnel.  The  centrifugal  force  of  the 
turning  water  is  a  chief  reason  of  this  phenomenon  :  another  reason  will  be 
considered  farther  on,  under  the  head  of  atmospheric  pressure. 

Great  whirlpools  at  sea,  and  smaller  ones,  or  eddies  in  rivers,  occur  when- 
ever a  current  is  obliged  suddenly  to  bend,  as  in  rounding  a  point  of  land  or 
a  rock,  or  in  meeting  and  mingling  with  a  contrary  current.  The  water,  by 
tending  to  continue  its  straight  motion,  falls  in  behind  the  obstruction,  re- 
luctantly as  it  were,  and  leaves  there  a  pit  surrounded  by  a  liquid  revolving 
ridge.  Charybdis,  in  the  Mediterranean,  and  the  great  whirlpool  off  the 
Norwegian  coast,  are  noted  examples. 

It  is  owing  to  the  centrifugal  force  in  any  bending  part  of  a  stream  of 
water,  that  is  to  say,  the  tendency  away  from  the  centre  of  the  curvature, 
that  when  a  bend  has  once  commenced,  it  increases,  and  is  soon  followed  by 
others,  until  that  complete«flserpentine  winding  is  produced,  which  charac- 
terizes most  rivers  in  their  course  across  extended  plains.  The  water  being 
thrown  by  any  cause  to  the  left  side,  for  instance,  wears  that  into  a  curve  or 
elbow,  and,  by  its  centrifugal  force,  acts  constantly  on  the  outside  of  the  bend, 
until  rock  or  higher  laud  resists  the  gradual  progress;  from  this  limit  being 
thrown  back  again,  it  wears  a  similar  bend  to  the  right  hand,  and  after  that, 
another  to  the  left,  and  so  on. 

Carriages  are  often  overturned  in  quickly  rounding  corners.  The  inertia 
carries  the  body  of  the  vehicle  in  the  former  direction,  while  the  wheels  are 
suddenly  pulled  round  by  the  horses  into  a  new  one.  A  loaded  stage-coach 
running  south,  and  turning  suddenly  to  the  east  or  west,  strews  its  passen- 
gers on  the  south  side  of  the  road.  Where  a  sharp  turning  in  a  carriage-road 
is  unavoidable,  the  road  towards  the  outside  of  the  bend  should  always  be 
made  higher  than  at  the  inside,  to  prevent  such  accidents. 

A  man  or  a  horse  turning  a  corner  at  speed,  leans  much  inward,  or  to- 
wards the  corner,  to  counteract  the  centrifugal  force,  that  would  throw  him 
away  from  it. 

In  skating  with  great  velocity,  this  leaning  inwards  at  the  turnings  be- 
comes very  remarkable,  and  gives  occasion  to  the  fine  variety  of  attitudes 
displayed  by  the  expert ;  and  if  a  skater,  in  running,  finds  his  body  inclined 
to  one  side  and  in  danger  of  falling,  he  merely  makes  his  skate  describe  a 
slight  curve  towards  that  side,  when  the  tendency  of  his  body  to  move 
fttraightly,  or  its  centrifugal  force,  refusing  to  follow  in  the  curve,  allows  the 
foot  to  push  itself  again  under  the  body,  and  to  restore  the  perpendicularity. 
Skating  becomes  to  the  intelligent  man  an  intellectual  as  well  as  a  sensitive 
or  bodily  treat,  from  its  exemplifying  so  pleasingly  the  law  of  motion. 

The  last  example  explains,  also,  why  a  hoop  rolled  along  the  ground  goes 
so  long  without  falling :  if  it  incline  to  one  side,  threatening  to  fall,  by  that 
very  circumstance,  the  part  touching  the  ground  is  made  to  bend  its  course 
to  that  side,  and  as  in  the  case  of  the  skater  who  turns  his  foot,  the  sup- 
porting base  is  again  forced  directly  under  the  mass  of  the  body. 

A  coin  dropped  on  the  table  or  floor  often  exhibits  the  same  phenomenon. 
It  is  said  to  run  and  hide  itself  in  the  corner.  Just  before  falling,  if  not 
obstructed,  it  describes  several  turns  of  a  decreasing  spiral,  the  minute  ex- 
amination of  which  is  a  pleasing  mathematical  exercise. 

The  reason  also  why  a  spinning  top  stands,  will*;be  understood  here. 


52  MOTIONS    AND     FORCES. 

While  the  top  is  quite  upright,  the  extremity  of  its  peg,  being  directly  under 
its  centre,  supports  it  steadily,  and  although  turning  so  rapidly,  and  with 
much  friction,  has  no  tendency  to  move  from  the  place  :  but  if  the  top  in- 
cline at  all,  the  edge  or  side  of  the  peg,  instead  of  its  very  point,  is  in  con- 
tact with  the  floor,  and  the  peg  then  becoming  as  a  turning  little  roller, 
advances  quickly,  and  describes  a  curve  somewhat  as  a  skater's  foot  does, 
until  it  come  directly  under  the  body  of  the  top  as  before.  It  thus  appears 
that  the  very  fact  of  the  top  inclining,  causes  the  point  to  shift  its  place, 
and  to  continue  moving  until  it  comes  again  directly  under  the  centre  of  the 
top.  It  is  remarkable  that  even  in  philosophical  treatises  of  authority  the 
standing  of  a  top  is  still  vaguely  attributed  to  centrifugal  force.  And  some 
persons  believe  that  a  top  spinning  in  a  weighing  scale,  would  be  found 
lighter  than  when  at  rest;  and  others  most  erroneously  hold  that  the  centri- 
fugal force  of  the  whirling,  which  of  course  acts  directly  away  from  the 
axis,  and  quite  equally  in  all  directions,  yet  becomes,  when  the  top  inclines, 
greater  upwards  than  downwards,  so  as  to  counteract  the  gravity  of  the 
top.  The  way  in  which  centrifugal  force  really  helps  to  maintain  the 
spinning  of  atop  is,  that  when  the  body  inclines.or  begins  to  fall  in  one  direc- 
tion, its  motion  in  that  direction  continues  until  the  point  describing  its 
curve,  like  the  foot  of  a  skater,  has  forced  itself  under  the  body,  again. 

By  reason  of  centrifugal  force  also,  it  is  easier  to  do  feats  of  horsemanship 
in  a  small  ring  as  at  our  theatres,  than  if  the  animal  were  running  on  a 
straight  road.  We  see  the  man  and  the  horse  always  inclining  inwards  to 
counteract  centrifugal  force,  and  if  the  rider  tend  to  fall  inwards,  he  -has 
merely  to  quicken  the  pace ;  if  to  fall  outwards,  he  has  to  slacken  it,  and  all 
is  right  again. 

If  a  pair  of  common  fire-tongs,  suspended  by  a  cord  from  the  top,  be 
made  to  turn  by  the  twisting  or  untwisting  of  the  cord,  the  legs  will  separate 
from  each  other  with  force  dependent  on  the  speed  of  rotation,  and  will 
again  collapse  when  the  turning  ceases.  Mr.  Watt  adapted  this  fact  most 
ingeniously  to  the  regulation  of  the  speed  of  his  steam-engine.  His  steam- 
gouernor  may  in  truth  be  described  as  a  pair  of  tongs  with  heavy  balls  at 
the  ends,  to  make  their  opening  more  energetic,  attached  to  some  turning 
part  of  the  machine.  If  the  engine  move  with  more  than  the  assigned 
speed  the  balls  open  or  fly  asunder  beyond  their  middle  station,  and  by  a 
simple  contrivance  are  then  made  to  act  on  a  valve  which  contracts  the  steam 
tube ;  on  the  contrary,  with  too  slow  a  motion,  they  collapse  and  open  the 
valve. 

A  half- formed  vessel  of  soft  clay,  placed  in  the  centre  of  the  potter's 
table, — which  is  made  to  whirl  and  is  called  his  wheel, — opens  out  or 
widens  merely  by  the  force  of  its  sides  and  thus  assists  the  worker  in  giving 
its  form. 

A  ball  of  soft  clay,  with  a  spindle  fixed  through  its  centre,  if  made  to  turn 
quickly,  soon  ceases  to  be  a  perfect  ball.  It  bulges  out  in  the  middle,  where 
the  centrifugal  force  is  great,  and  becomes  flattened  towards  the  ends,  or 
where  the  spindle  issues. 

This  change  of  form  is  exactly  what  has  happened  to  the  ball  of  our 
earth.  It  has  bulged  out  seventeen  miles  at  the  equator,  in  consequence  of 
its  daily  rotation,  and  is  flattened  at  the  poles  in  a  corresponding  degree. — 
A  mass  of  lead  that  weighs  one  thousand  pounds  at  our  pole,  weighs  about 
five  pounds  less  at  the  equator,  by  reason  of  the  centrifugal  force. 

In  the  planets  Jupiter  and  Saturn,  of  which  the  rotation  is  much  quicker 
than  of  our  earth,  the  middle  or  equator  bulges  out  still  more — even  so  as 
to  offend  an  eye  which  expects  a  perfect  sphere. 


QUANTITY    OF    MOTION.  53 

If  the  rotation  of  our  earth  were  seventeen  times  faster  than  it  is,  the 
bodies  or  matter  at  the  equator  would  have  centrifugal  force  equal  to  their 
gravity,  and  a  little  more  velocity  would  cause  them  to  fly  off  altogether,  or 
to  rise  and  form  a  ring  around  the  earth  like  that  which  surrounds  Saturn. 
Saturn's  double  ring  seems  to  have  been  formed  in  this  way,  and  is  now 
supported  chjefly  by  the  centrifugal  force  of  the  parts.  Were  it  to  crumble 
to  pieces,  the  pieces  might  still  revolve,  as  so  many  little  satellites.  His 
true  satellites  are  only  more  distant  masses  sustained  in  the  same  manner. 
And  our  earth  and  the  other  primary  planets  have  the  same  relation  to  the 
sun  that  these  satellites  have  to  Saturn — all  being  sustained  by  an  admira- 
ble balance  between  centrifugal  force  and  gravity. 

"  The  quantity  of  Motion  in  a  body  measured  by  the  velocity  and  quantity 

of  matter. 

If  a  single  atom  of  matter  were  moving  at  the  rate  of  one  foot  per  second 
it  would  have  a  definite  quantity  of  motion  expressed  by  these  words ;  and 
if  it  were  moving  ten  feet  per  second,  it  would  have  ten  times  the  quantity. 
Again,  in  a  mass  consisting  of  many  atoms,  the  quantity  of  motion  would  be 
still  as  much  greater  as  there  were  more  atoms  in  it  than  one. 

By  experiment  it  is  found,  that  if  a  ball  of  soft  clay  of  one  pound,  sus- 
pended by  a  cord  as  a  pendulum,  be  allowed  to  fall  with  a  velocity  of  ten  feet 
per  second,  against  u  ball  of  nine  pounds  suspended  in  the  same  way,  but  at 
rest,  the  two,  after  contact,  will  start  together  at  the  rate  of  one  foot  per 
second,  the  original  quantity  of  motion  being  then  diffused  through  ten 
times  the  quantity  of  matter,  and  therefore  exhibiting  only  one  tenth  of  the 
velocity. 

A  cannon  ball  of  a  thousand  ounces,  moving  one  foot  per  second,  has  thus 
the  same  quantity  of  motion  in  it  as  a  musket-ball  of  one  ounce,  leaving  the 
gun-barrel  with  a  velocity  of  a  thousand  feet  in  the  second. 

"  The  quantity  of  motion  in  a  body  is  the  measure  of  the  force  which  pro- 
duced it." 

The  experiment  of  the  balls  of  clay  mentioned  above  furnishes  one  instance 
of  this  truth.  Again,  a  body  falling  for  ten  seconds,  acquires  ten  times  as 
much  velocity  as  by  falling  for  one  second ;  its  motion  thus  measuring  the 
force  of  gravity  which  has  been  exerted  upon  it. 

When  a  large  body  or  mass  of  many  atoms  falls,  it  of  course  has  as  much 
more  motion  than  a  smaller  body,  as  there  are  more  atoms  in  it  than  in  the 
smaller ;  but  as  gravity  acts  equally  on  every  atom,  the  force  causing 
either  body  to  fall  is  still  exactly  indicated  by  the  quantity  of  motion  in  it. 

A  large  body  or  mass  of  many  atoms  falls  where  there  is  no  impediment, 
with  the  same  velocity  as  a  smaller  body  or  a  single  atom ;  for  gravity  pulls 
equally  at  each  atom,  and  must  overcome  its  inertia  equally,  whether  it  be 
alone  or  with  others. 

This  remark  contradicts  the  popular  opinion,  that  a  large  and  heavy  body 
should  fall  to  the  earth  much  faster  than  a  small  and  light  one ;  an  opinion 
which  has  arisen  from  our  constantly  seeing  such  contrasts,  as  the  rapid  fall 
of  a  gold  coin,  and  the  slow  descent  of  a  feather.  The  true  cause  of  the 
contrast  is,  that  the  atoms  of  the  feather  are  much  spread  out,  so  as  to  be 
more  resisted  by  the  air  than  those  of  the  gold.  If  the  two  be  let  fall  toge- 
ther in  a  vessel  from  which  the  air  has  been  extracted — as  in  the  common 
air-pump  experiment,  they  arrive  at  the  bottom  in  exactly  the  same  time; 


54  MOTIONS    AND     FORCES. 

and  even  in  the  air,  if  the  coin  be  hammered  out  into  gold  leaf,  it  will  fall 
still  more  slowly  than  the  feather.  One  brick  dropped  from  a  height,  be- 
cause its  motion  is  not  much  affected  by  the  air,  reaches  the  earth  very 
nearly  as  soon  as  ten  bricks  let  fall  near  it,  whether  they  be  connected  or 
separate — as  a  single  horse  may  reach  the  goal  as  soon  as  ten  horses  gallop- 
ing abreast. 

A  man's  force  will  move  a  small  skiff  quickly,  a  loaded  barge  very  slowly, 
and  a  large  ship  in  a  degree  scarcely  to  be  perceived.  In  each  case,  however, 
the  quantity  of  motion  may  be  the  same,  and  a  true  measure  of  the  force 
which  produced  it. 

A  ball  of  one  pound  weight,  impelled  by  a  given  force,  moves  twice  as 
fast  as  a  ball  of  two  Bounds  impelled  with  the  same;  yet,  although  the 
velocities  are  different,  the  quantities  of  motion,  as  ascertained  by  the  rule 
already  given,  are  equal,  and  indicate  an  equality  of  producing  force. 

"  The  quantity  of  motion  in  a  body  is  the  measure  also  of  the  force  or  mo- 
mentum which  it  can  exhibit  again"     (See  the  Analysis,  42. ) 

Bodies,  owing  to  their  inertia,  may  be  regarded  as  passive-  reservoirs  of 
force  or  motion,  always  ready  to  return  as  much  as  they  have  received.  Mo- 
mentum is  the  name  given  to  the  motion  in  a  body;  with  reference  to  the 
production  by  it  of  new  motions  or  the  overcoming  of  resistances,  and  is  but 
another  term  for  the  quantity  of  motion 

A  cannon  ball,  according  to  the  quantity  of  motion  in  it,  may  have  only 
the  force  or  momentum  that  will  bruise  a  plank,  or  it  may  have  enough  to 
penetrate  a  tree,  or  even  to  shoot  its  rapid  way  through  a  block  of  the  hardest 
stone. 

A  block  of  wood  floating  against  a  man's  leg  with  moderate  velocity, 
would  be  little  felt;  but  a  loaded  barge,  coming  at  the  same  rate  and  press- 
ing it  against  the  quay,  might  break  the  bones;  a  large  ship,  again,  although 
moving  no  faster,  would  crush  his  body  against  any  fixed  obstacle ;  and  an 
island  of  ice,  opposed  in  its  approach  to  another,  even  by  a  first-rate  man-of- 
war,  would  destroy  it,  as  meeting  barges  destroy  a  floating  egg-shell. 

A  hail-stone  falling,  strikes  rudely ;  a  stone  rolled  from  a  height,  as  of  old, 
by  the  besieged  against  besiegers,  may  carry  death  with  it  to  many ;  an 
avalanche,  breaking  from  its  hold  on  a  mountain  steep,  may  sweep  away  a 
village. 

To  meeting  bodies,  the  shock  is  the  same,  whether  the  motion  be  shared  be- 
tween them  or  be  all  in  one. 

If  a  running  man  come  against  a  man  who  is  standing,  both  receive  a  cer- 
tain shock.  If  both  be  running  at  the  same  rate  in  opposite  directions,  the 
shock  is  doubled.  In  some  such  cases,  as  where  swift  skaters  have  met,  the 
shock  has  proved  fatal. 

The  meeting  fists  of  boxers  not  unfrequently  dislocate  or  break  bones. 

A  man's  skull  is  fractured  as  certainly  by  its  being  dashed  against  a  tree 
or  beam,  while  he  is  on  a  galloping  horse,  as  by  a  blow  of  a  similar  beam 
coming  upon  him  with  the  velocity  of  the  horse. 

When  two  ships  in  opposite  courses  meet  at  sea,  although  each  may  be 
sailing  at  a  moderate  rate,  the  destruction  is  often  as  complete  to  both  as  if 
with  a  double  velocity  they  had  struck  on  a  rock.  Many  melancholy  in- 
stances of  this  kind  are  on  record.  In  the  darkness  of  night  a  large  ship  has 
met  one  smaller  and  weaker,  and  in  the  lapse  of  a  few  seconds,  have  followed 


DIRECTION    OF    FORCES. 


55 


the  shock  of  the  encounter,  the  scream  of  the  surprised  victims,  and  the  hor- 
ribly silence  when  the  waves  had  again  closed  over  them  and  their  vessel  for 
ever. — In  November,  1825,  on  the  coast  of  Scotland,  the  Comet  steamboat 
was  thus  destroyed,  and  carried  to  the  bottom  with  her  about  seventy  pas- 
sengers, into  whose  ears  the  drowning  water  rushed  before  the  sound  of  ar- 
rested music  and  joy  had  died  away, 

"  Direction  of  the  force  or  forces  producing  motion." 

When  only  one  force  acts  on  a  body,  the  body  obeys  in  the  exact  direction 
of  the  force. 

A  ball  floating  in  water,  or  lying  on  smooth  ice,  is  driven  exactly  south 
by  a  wind  blowing  to  the  south.  A  bullet  issues  from  the  mouth  of  a  cannon, 
in  the  direction  of  the  axis  of  the  cannon — which  is,  as  the  force  impels  it. 

"When  two  or  more  forces,  not  in  the  same  direction,  act  upon  a  body  at  the 
same  time,  as  it  cannot  move  two  ways  at  once,  it  holds  a  middle  course 
between  the  directions.  This  course  is  called  the  resulting  direction,  viz., 
resulting  from  the  composition  of  the  forces. 

A  ball  or  ship  moving  south  by  a  direct  wind,  may,  at  the  same  time,  be 
carried  east,  just  as  fast,  by  a  tide  or  current  moving  east ;  every  instant, 
therefore,  it  will  go  a  little  south  and  a  little  east,  and  really  will  describe 
a  middle  line  pointing  south- east. 

These  particulars  may  be  well  represented  on  paper,  as  by  fig.  3  :  where 
b  is  the  original  place  of  the  ball  or  ship,  e  the  east,  s  the 
south,  and  b  a  the  middle  line  pointing  to  the  south-east, 
and  showing  the  true  ceurse  of  the  vessel.  This  figure  is 
called  the  parallelogram  of  forces,  and  is  an  important  help 
to  the  understanding  of  many  facts  in  natural  philosophy. 
The  minute  investigation  of  the  subject  belongs  to  the 
science  of  measures,  or  technical  mathematics ;  but  the 
general  truths  are  quite  intelligble  to  common  sense,  or 
the  mathematics  of  common  experience. 

When  two  forces  act  upon  a  body,  like  the  wind  and 
tide  in  the  last  example,  the  result  is  the  same,  whether  they  act  together  or 
one  after  the  other.  For  instance,  if  the  wind  drive  a  vessel  one  mile  south, 
as  from  b  to  s,  fig.  3,  and  immediately  afterwards  the  tide  drive  it  one  mile 
east,  as  s  to  a,  the  vessel  will  be  in  the  same  place  at  last,  viz.,  at  a,  as  if  she 
had  been  driven  at  once  south-east,  in  the  line  b  a,  by  the  simultaneous 
action  of  the  two.  Therefore,  by  drawing  the  lines  b  s  and  b  e  to  represent 
the  force  and  direction  of  the  two  causes  of  motion,  and  by  then  adding  one 
of  them,  or  an  equivalent,  to  the  end  of  the  other  as  s  a  to  b  s,  or  e  a  to 
b  e,  the  square  or  parallelogram  is  sketched,  of  which  the  middle  line  or 
diagonal,  as  it  is  called,  shows  the  resultant  of  the  forces,  and  the  true 
course  of  the  body  obeying  them. 

What  is  thus  true  of  the  effect  of  continued  forces  like  wind  and  tide  is 


Fig.  3. 


Fig.  4. 


Fig.  5. 


Fig.  6. 


Fig.  7. 


56  "      MOTION  SAND    FORCES. 

true  also  of  momentary  impulses,  like  the  blows  of  clubs  simultaneously 
striking  a  ball,  or  of  two  billiard-balls  striking  a  third. 

When  the  forces  exactly  cross  each  other,  and  are  equal,  as  in  the  case  of 
the  ship  above  supposed,  the  figure  becomes  a  square,  at  fljg.  3  ;  but  if  one 
of  the  forces  be  greater  than  the  other,  the  figure  becomes  oblong,  as  at  fig. 
4 ;  if  the  forces  cross  obliquely,  the  fig.  becomes  as  at  fig.  5 ;  and  if  they 
cross  in  an  opposing  direction,  it  will  be  as  at  fig.  6.  In  all  the  cases, 
however,  the  diagonal  still  shows  the  result.  It  is  evident  that  the  same  line 
may  be  the  diagonal  of  many  figures,  as  seen  in  b  a  at  fig.  7  }  and  therefore, 
that  very  different  degrees  and  directions  of  combined  forces  may  produce 
the  same  result. 

Forces  crossing  each  other  so  obliquely  as  to  be  represented  by  lines  drawn 
in  almost  opposite  directions,  would  form  a  parallelogram  having  scarcely 
any  breath,  that  is  to  say,  the  diagonal  would  approach  to  nothing;  showing 
thus,  that  opposing  forces  neutralize  or  destroy  each  other.  In  fig.  6,  by 
reason  of  this  crossing,  the  resultant  is  less  than  either  of  the  constituents. 
And  for  the  same  reason,  when  forces  cross  so  acutely  as  to  advance  nearly 
parallel  to  each  other,  the  resultant  is  longer  than  either,  as  seen  in  fig.  5. 
Forces  directly  opposed,  or  entirely  agreeing  in  direction,  give  as  their  re- 
sultant their  difference  or  their  sum. 

Forces  crossing  each  other  directly,  or  at  right  angles,  as  is  true  of  the 
exactly  eastward  force  b  e,  and  the  exactly  southward  force  b  s,  in  figures  3 
and  4, — do  not  in  the  slightest  degree  neutralize  or  alter  each  other,  for  the 
body,  when  arrived  at  a,  is  just  as  far  east  as  it  would  be  at  e,  and  as  far 
south  as  it  would  be  at  s.  This  explains  why  the  progressive  motion  of  the 
planets  in  their  orbits  is  not  at  all  affected  by  the  directly  crossing  centri- 
petal force  of  gravity  which  keeps  them  at  their  due(  distances  from  the  sun. 
In  all  cases  where  the  two  crossing  forces  are  equal,  with  whatever  ob- 
liquity they  cross,  the  resulting  direction  must  be  midway  between  them. — 
Thus  a  boat  impelled  by  oars,  goes  straight,  although  the  direction  in  which 
the  oar  acts  is  constantly  changing ;  because  the  changing  obliquity  of  the 
force  is  always  the  same  on  both  sides. — This  explains  also  why  a  bird  fly- 
ing, or  a  man  swimming,  holds  a  perfectly  straight  course,  although  in  both 
cases  the  direction  of  the  impelling  forces  is  constantly  varying. — And  it 
explains  why  a  body  suspended,  as  a  plummet,  or  falling  to  the  earth  as  an 
apple  does  from  a  tree,  is  always  in  a  line  towards  the  centre  of  the  earth : 
for,  while  the  part  of  the  earth  immediately  under  the  body  is  pulling  it 
straight  down  to  the  centre,  the  action  of  parts  on  any  one 
side  of  the  perpendicular  is  exactly  counterbalanced  by  the 
action  of  corresponding  parts  on  the  opposite  side  ;  and  the 
perpendicular  is  still  the  diagonal  or  middle  line  of  every 
pair  of  attracting  parts.  In  fig.  8,  b  a  represents  the  common 
diagonal.  In  speaking  of  the  attraction  of  our  earth,  there- 
fore, which  really  is  the  united  attraction  of  all  the  individual 
atoms,  we  may  always  consider  it  as  a  single  force  acting  to- 
wards the  centre  of  the  earth. 
When  a  body  is  carried  below  the  surface  of  the  earth,  its  weight  becomes 
less,  because  the  matter  then  above  it  is  drawing  it  up,  instead  of  down,  as 
before.  A  descent  of  a  few  hundred  feet  makes  a  sensible  difference,  and 
at  the  centre  of  the  earth,  if  man  could  reach  it,  he  would  find  things  to 
have  no  weight  at  all ;  and  there  would  be  neither  up  nor  down,  because 
bodies  would  be  attracted  equally  in  all  directions. 

When  more  than  two  forces  act  on  a  body,  the  resulting  direction  may  be 


DIRECTION    OF    FORCES. 


57 


found,  first  of  two,  and  then  of  the  last  resultant  with  each  of  the  others 
successively  : — or  the  forces  may  be  represented  on  paper  by  lines  tacked 
together,  of  which  one  denotes  the  strength  and  direction  of  each  :  the  ex- 
tremity of  the  last  line  will  mark  the  place  of  the  body  after  being  acted 
upon  by  the  combined  forces.  A  sailor,  to  know  the  true  place  of  his  ship 
and  the  course  which  she  has  steered,  considers,  first,  the  forward  progress 
as  found  by  the  log,  then  the  leeway  or  sideward  motion  produced  by  a 
cross  wind,  and  then  the  effect  of  any  tide  or  current  in  which  he  may  be 
sailing. 

Resolution  of  Forces  is  a  phrase  pointing  to  another  important  use  of  such 
parallelograms  or  figures  as  have  just  been  described,  viz.,  the  enabling  us 
when  force  or  motion  is  given,  to  find  the  forces  or  motions  in  any  other 
directions  of  which  it  may  be  the  resultant,  and  those  into  which  it  may 
itself  be  resolved. 

Thus,  if  a  line  b  a  (in  any  of  the  preceding  figures  4,  5,  6,  &c.)  represent 
a  force  or  motion,  and  the  line  b  s  represent  one  of  two  elements  composing 
it,  we  have  but  to  complete  the  parallelogram  b  s  a  e  to  obtain  the  other 
line,  b  e  representing  the  only  other  force  or  motion  which,  combined  with 
the  first  element,  can  produce  the  given  resultant. — If  a  ship  pass  from  b  to 
a  (fig.  5)  while  sailing  through  the  water  eastward,  a  distance  expressed  by 
b  e,  she  must  at  the  same  time  have  been  carried  by  a  tide  current  to  the 
distance  and  in  the  direction  marked  by  the  line  b  s. 

Again,    if  a  line  be  given  representing  a  single 
force  or  motion  as  b  a,  and  if  it  be  desired  to  know  Fig.  9. 

how  much  there  is  in  this  capable  of  acting  in  another        jy e 

direction  as  b  d  ;  it  is  only  necessary  to  draw  a  line        f\  -   C 

in  the  direction,  as  b  d,  from  the  commencement  of 

b  a,  and  to  cut  such  line  by  another  drawn  directly 

upon  it — or  at  right  angles  to  it,  as  the  term  is,  from 

the  other  end  of  b  a :  the  length  of  b  d,  so  cut  off, 

viz.,  b  s,  shows  the  proportion  required. 

It  is  thus  that  a  sailor  who  knows  how  far  he  has 
sailed  in  an  oblique  direction  finds  out  how  much 
he  has  gone  north  and  east  or  south  and  west  j  in 
other  words,  finds  out  the  difference  in  latitude  and  longitude  between  his 
present  place  and  a  former  one.  In  the  above  figure,  b  a  may  represent  the 
course  and  distance  sailed,  b  s  the  difference  of  latitude,  and  b  e  the  differ- 
ence of  longitude. 

Thus  again,  if  a  ball  b  strike  a  table  a  c, 
•with  velocity  and  direction,  both  represented  Fig  10. 

by  the  line  be;  and  if  the  ball  be  supposed 
afterwards  with  the  same  velocity  to  approach 
the  table  in  the  oblique  directign  e  c,  it  will 
then  strike  with  as  much  less  force  than  before, 
as  the  line  e  a  is  shorter  than  e  c.  For  e  a  is 
found  according  to  the  rule  for  decomposing  a 
force,  given  above;  and,  to  common  sense,  it 
is  obvious,  that  if  the  whole  velocity  of  the 
ball  be  represented  by  e  c,  the  rate  of  approxi- 
mation towards  the  table,  or  merely  downward 

.  velocity  and  therefore  the  downward  force  is  marked  by  the  line  e  a.     The 
body  only  falls  through  the  distance  e  a  while  moving  all  the  way  from  e  to  c. 


O 


58  MOTIONS    AND    FORCES. 

Figure  10,  explains  the  important  cases  of  the  force  of  wind  upon  ships 
sails,  windmill  vanes,  &c.  ;  and  the  force  of  water  upon  float-boards,  water- 
wheels,  &c. ;  showing  that  the  moving  mass  exerts  force  upon  a  surface, 
not  in  proportion  to  the  speed  with  which  it  may  he  passing  along  or  near  the 
surface,  but  to  the  rate  of  perpendicular  approximation.  It  explains  also,  why 
the  slanting  blow  of  a  club  or  ball  is  so  light,  compared  with  the  direct  blow. 

"  The  two  great  forces  of  Nature  ore  Attraction  and  Repulsion."     (Read 

the  Analysis.) 

A  person,  on  first  approaching  this  subject,  is  far  from  supposing  that  the 
beautiful  and  almost  endless  variety  of  phenomena  exhibited  in  the  universe 
around,  all  are  referrable  to  the  two  principles,  attraction  and  repulsion^ 
examined  in  the  first  section : — but  such  is  the  truth. — It  will  first  be  shown 
here,  how  the  great  classes  of  accelerated,  retarded,  and  bent  motions  arise 
from  them. 

Attraction. — Until  Newton  said,  that  what  we  call  weight  of  bodies  is 
merely  an  instance  of  that  universal  attraction  of  matter  which  diminishes 
with  increasing  distance,  it  was  never  suspected  that  weight  was  less,  high 
up  in  the  air  than  on  the  ground ;  or  on  a  lofty  mountain  than  on  the  sea- 
shore. But  this  we  now  know  to  be  the  case.  However,  in  studying  what 
goes  on  in  obedience  to  gravity  near  the  surface  of  the  earth,  except  in  a  few 
very  nice  cases,  gravity  may  be  considered  as  a  uniform  power ;  for  man  has 
neither  approached  the  centre  of  the  earth  in  mines,  nor  receded  from  it  in 
balloons,  by  more  than  about  a  thousandth  part  of  his  distance  from  it;  and 
weight  has  relation  to  the  distance  from  the  centre,  not  to  the  distance  from 
the  surface. 

11  Accelerated  Motion  from  Gravity" 

Owing  to  the  inertia  of  matter,  any  force  continuing  to  act  on  a  mass 
which  is  free  to  obey  it,  produces  in  the  mass  a  quickening  or  accelerated 
motion ;  for  as  the  motion  given  in  the  first  instance,  continues  afterwards 
without  any  farther  force,  merely  on  account  of  the  inertia,  it  follows,  that 
as  much  more  motion  is  added  during  the  second  instant,  and  as  much  again 
during  the  third,  and  so  on.  A  falling  body,  therefore,  under  the  influence 
of  attraction,  is,  as  it  were,  a  reservoir,  receiving  every  instant  fresh  velocity 
and  momentum. 

It  is  said  that  Newton's  sublime  genius  read  the  nature  of  attraction  in 
the  simple  incident  of  an  apple  falling  before  him  from  a  lofty  branch  in 
his  garden. — The  eye  which  perceives  an  apple  beginning  to  fall,  can  follow 
it  for  a  time  and  mark  the  gradual  acceleration  of  its  descent,  but  soon  sees 
its  path  only  as  a  shadowy  line. 

A  boy  letting  a  ball  drop  from  his  hand,  can  catch  it  again  in  the  first 
instant,  but  after  a  litttle  delay  his  hand  pursues  it  in  vain. 

A  fragment  of  rock,  detached  from  thejwow  of  a  hill  by  the  lightning 
stroke,  begins  its  motion  slowly;  but  once  fairly  launched,  it  gathers  fresh 
speed  and*momentuni  with  every  instant,  and  bounds  from  steep  to  steep 
driving  every  obstacle  before  it. 

Any  liquid  falling  from  a  reservoir,  forms  a  descending  mass  or  stream,  of 
which  the  bulk  diminishes  from  above  downwards,  in  the  same  proportion  as 
the  velocity  of  the  particles  increases.  This  truth  is  well  exemplified  in  the 
pouring  out  of  molasses  or  thick  syrup;  if  the  height  of  the  fall  be  con- 
siderable, the  bulky  sluggish  mass,  wliich  first  escapes,  is  reduced,  before  it 
reaches  the  bottom,  to  a  small  thread;  but  the  thread  is  moving  proportion- 
ately faster,  and  fills  the  receiving  vessel  with  surprising  rapidity.  The  same 


MEASURE    OF    ATTRACTION.  59 

truth  is  exhibited  on  a  vast  scale  in  the  falls  of  Niagara;  where  the  broad 
river  is  seen  first  bending  over  the  precipice  a  deep  slow  moving  mass,  then 
becoming  a  thinner  and  a  thinner  sheet  as  it  descends,  until  at  last,  sur- 
rounded by  its  foam  or  mist,  it  flashes  into  the  deep  below,  apparently  with 
the  velocity  of  lightning. 

When  velocity  becomes  considerable  in  any  case  of  falling;  it  cannot  be 
measured  accurately  by  thereye,  but  its  effects  ascertain  it.  A  man  leaps 
from  a  chair  with  impunity,  from  a  table  with  a  shock,  from  a  high  window 
with  fracture  of  his  bones,  and  in  falling  from  a  balloon  his  body  is  literally 
dashed  to  pieces. 

The  force  of  gravity  or  general  attraction  is  such  at  the  surface  of  this 
earth,  that,  in  the  first  second  of  time,  it  gives  to  a  body  allowed  to  fall  a 
velocity  of  32  feet  nearly  per  second,  that  is,  a  velocity  which,  remaining 
uniform  from  the  end  of  the  second,  would  carry  it,  without  farther  action 
of  gravity,  through  32  feet  in  the  next  second  Yet  the  body  falls  only  16 
feet  in  the  first  second;  and  the  reason  is,  that  the  velocity  of  32  feet  pos- 
sessed at  the  end  of  the  second  is  gradually  acquired,  the  body  having  only 
half  of  it  at  the  half  second,  and  as  much  less  than  half  at  any  distance  be- 
fore that  time,  as  it  has  more  than  half  at  the  same  distance  afterwards;  and 
the  average,  therefore,  is  only  half  of  the  32,  or  16  feet  in  the  whole  second. 
In  the  next  second,  it  falls,  of  course,  through  the  whole  32  feet,  with  16 
additional,  from  the  new  action  of  gravity,  in  all  three  times  as  much  as  in 
the  first  second;  and  in  two  seconds,  therefore,  it  falls  altogether  four  times 
as  far  as  in  one  second.  At  the  end  of  two  seconds  the  velocity  is  doubled 
or  is  64  feet  per  second,  so  that  in  the  third  second  the  body  falls  64,  and 
other  new  16,  in  all  five  times  as  much  as  in  the  first  second ;  and  in  three 
seconds,  therefore,  it  has  descended  nine  times  as  far  as  in  one  second,  &c. 
Knowing  this  progress,  the  velocity  acquired  by  a  falling  body,  and  the  dis- 
tance through  which  it  falls,  in  any  given  time,  are  easily  calculated ;  and 
the  height  of  a  precipice,  or  the  depth  of  a  well,  may  be  ascertained  by 
marking  the  time  required  for  a  body  to  fall  through  the  space. 

The  doctrines  of  falling  bodies  are  of  such  importance  in  the  minute  ex- 
amination of  many  of  the  phenomena  of  nature,  that  much  attention  has  been 
bestowed  upon  them.  Mr.  Atwood's  ingenious  contrivance  by  which  the 
motion  of  falling  bodies  may  be  retarded  in  any  desired  degree,  without  the 
character  of  the  motion  being  otherwise  altered,  has  enabled  experimenters 
to  render  evident  to  the  senses  all  that  abstract  calculation  had  anticipated. 
A  pound  weight,  left  quite  free,  falls  towards  the  ground,  sixteen  feet  in  the 
first  second,  proving  that  attraction  of  one  pound  is  just  sufficient  to  over- 
come the  inertia  of  one  pound  at  that  rate.  But  if  the  inertia  were  doubled, 
or  tripled,  or  increased  in  any  other  degree,  the  fall  of  course  would  be  just  so 
much  slower.  Now  Mr.  Atwood's  machine  in  effect  increases 
it,  by  causing  falling  weights  to  overcome  not  only  their  own  Fig.  11. 
inertia,  but  also  that  of  other  weights;  fig.  11.  Thus  a  and  b, 
being  weights  of  two  pounds  each,  balancing  each  other  over 
the  very  easily  turned  pulley  c,  are  moved  by  a  weight  of  one 
pound  d,  hooked  to  one  of  them ;  and  gravity  in  pulling  this 
down,  with  force  of  one  pound,  has  to  overcome,  not  the  inertia 
of  one  nound,  but  of  five,  for  the  other  two  weights  must  move 
as  fast  as  the  one  pound  does ;  and  thus,  the  velocity  being 
reduced  to  one-fifth  of  what  is  natural  to  a  falling  body,  the  d~*  <i» 

descent  can  be  minutely  observed.     The  experiments  with  At-     ai    ]   1 Jb 

wood's  machine  may  be  varied  exceedingly,  and  they  are  most         Fj 
interesting. 


60 


MOTIONS    AND    FORCES. 


11  Retarded  Motion" from  gravity. 

What  has  been  said  of  the  changing  velocity  of  a  falling  body,  from  gra- 
vity, is  exactly  true,  in  a  reversed  way,  respecting  a  rising  body  exposed  to 
the  same  influence. 

A  bullet  shot  directly  upwards,  every  instant  loses  a  part  of  its  velocity, 
until  at  last  it  comes  to  rest  in  the  sky, — where  a  soaring  eagle  might  see 
the  messenger  of  death  motionless  and  harmless  for  a  moment  by  his  side : 
the  ball  then  descends  again,  and  so  that,  at  corresponding  points  of  the 
ascent  and  descent,  but  for  the  resistance  of  the  air,  the  velocities  would  be 
equal;  and,  on  reaching  the  ground,  it  would  have  acquired  exactly  the 
velocity  with  which  it  first  departed. 

It  is  explained  in  a  preceding  paragraph,  that  a  body  falls  four  times  as 
far  in  two  seconds  as  in  one,  although  the  velocity  at  the  end  of  two  seconds 
is  only  doubled.  For  the  same  reason,  a  body  shot  upwards  with  double 
velocity,  rises  four  times  as  far  as  if  shot  with  a  single  velocity;  if  shot  with 
triple  velocity,  it  rises  nine  times  as  far,  and  so  forth. 

In  aiming  for  amusement  at  bodies  thrown  up  into  the  air,  it  is  easy  to 
hit  them  near  their  point  of  turning,  and  more  difficult  always  as  they  are 
nearer  to  the  ground,  whether  rising  or  falling. 

An  upward  jet  of  water  is  small  below,  where  it  issues  from  the  pipe  with 
great  velocity,  but  it  becomes  more  bulky  as  the  water  loses  velocity  in 
ascending,  and  at  the  top  it  often  spreads  a  little  like  a  palm  tree,  and  any 
light  round  solid  will  continue  supported  and  playing  upon  its  summit. 

The  rise  of  a  pendulum  from  the  bottom  of  its  arc,  is  an  exact  copy,  re- 
versed, of  its  previous  descent  to  that  point. 

"  The  Pendulum" 

exemplifies  well  both  accelerated  and  retarded  motion.  The  name  is  appli- 
cable to  any  body  so  suspended,  that  it  may  swing  freely  backwards  and 
forwards.  When  such  a  body  is  made  of  certain  form  and  length,  although 
so  simple,  it  is  one  of  the  most  admirable  contrivances  of  man's  ingenuity. 

Galileo  having  observed  the  hanging  chandeliers  of  lofty  ceilings  to  con- 
tinue vibrating  long  and  with  singular  uniformity,  after  any  accidental  cause 
of  disturbance,  was  led  to  investigate  the  laws  of  the  phenomenon ;  and  out 
of  what,  in  some  shape  or  other,  had  been  before  men's  eyes,  but  uselessly, 
from  the  beginning  of  the  world,  his  powerful  genius  extracted  the  most 
important  results.  Independently  of  the  light  which  the  theory  of  the  pen- 
dulum has  thrown  on  various  branches  of  physics,  the  instrument  itself,  with 
a  few  wheels  attached,  to  record  its  vibrations,  has'  now  become  the  perfect 
time-keeper,  regulating  many  of  the  affairs  of  men. 

A  common  pendulum  consists  of  a  ball,  fig.  12,  as  a,  suspended  from  a  rod 

from  a  fixed  point  as  b,  and  made  to  swing 
backwards  and  forwards,  or  to  vibrite  un- 
der this  point.  Being  raised  to  c,  and  then 
set  at  liberty,  it  falls  back  to  a  with  an  ac- 
celerating motion,  like  a  ball  rolling  down 
a  slope,  and  when  arrived  there,  it  has  just 
acquired  momentum  enough  to  carry  it  to 
d,  at  an  elevation  on  the  other  side;  from 
this  it  falls  back  again,  again  to  rise ;  and 
would  so  go  on  for  ever,  but  for  the  impedi- 
ments of  air  and  friction.  The  pendulum 
is  strictly  an  object  of  mathematical  study; 


Fig.  12. 


PENDULUM.  61 

but  we  shall  give  a  general  idea  of  its  important  characteristics  in  common 
language. 

1.  The  times  of  the   vibrations  of  a  pendulum   are  very  neary  equal, 
whether  it  be  moving  much  or  little,  that  is  to  say,  whether  the  arc  described 
by  it  be  large  or  small.     This  remarkable  property  is  what  makes  a  time- 
keeper.    The  reason  that  a  large  vibration  is  performed  in  the  same  time  as 
a  small  one,  in  other  words,  that  the  pendulum  always  moves  faster  in  pro- 
portion as  its  journey  is  longer — is,  that  in  proportion  as  the  arc  described 
is  more  extended,  the  steeper  are  its  beginning  and  ending,  and  the  more 
rapidly,  therefore,  the  pendulum  falls  down  at  first,  sweeps  along  the  inter- 
mediate space,  and  stops  at  last.  *  It  is  evident,  for  instance,  that  the  portion 
c  e  of  the  arc  (fig.  13)  is  much  more  steep  than  the  equal  portion  e  c. — 
A  pendulum  made  to  vibrate  in  the  curve  called  a  cycloid,  which,  in  the 
central  part,  very  nearly  coincides  with  a  circular  arc,  but  towards  the  ex- 
tremity rises  a  little  more  steeply,  has  its  beats  perfectly  isochronous,  or  in 
equal  times,  whatever  their  extent. 

A  common  clock  is  merely  a  pendulum  with  wheel-work  attached  to  it,  to 
record  the  number  of  the  vibrations,  and  with  a  weight  or  spring  having  force 
enough  to  counteract  the  retarding  effects  of  friction  and  the  resistance  of  the 
air.  The  wheels  show  how  many  swings  or  beats  of  the  pendulum  have 
taken  place,  because  at  every  beat,  a  tooth  of  the  last  wheel  is  allowed  to 
pass.  Now  if  this  wheel  has  sixty  teeth,  as  is  common,  it  will  just  turn 
round  once  for  sixty  beats  of  the  pendulum,  or  seconds,  and  a  hand  fixed 
on  its  axis  projecting  through  the  dial-plate,  will  be  the  second  hand  of  the 
clock.  The  other  wheels  are  so  connected  with  the  first,  and  the  numbers 
of  teeth  on  them  so  proportioned,  that  one  turns  sixty  times  slower  than  the 
first,  to  fit  its  axis  to  carry  a  minute  hand,  and  another,  by  moving  twelve 
times  slower  still,  is  fitted  to  carry  an  hour  hand. 

2.  The  length  of  a  pendulum  influences  the  time  of  its  vibration. — Long 
pendulums  virbrate  more  slowly  than  short  ones,  be- 
cause, in  corresponding  arcs  or  paths,  the  hob  or  ball  Fig.  13. 

of  the  long  pendulum  has  a  greater  journey  to  per- 
form, without  having  a  steeper  line  of  descent.  If  a 
pendulum  b  a  be  twice  as  long  as  another  reaching 
from  b  to  e,  it  has  twice  as  much  to  fall  in  its  descend- 
ing arc  c  a,  as  the  other  jn  its  arc  d  e,  while  in  cor- 
responding parts  of  the  two  paths,  the  slope  or  incli- 
nation is  always  equal : — -the  ball  of  the  long  pendulum 
may  be  considered  as  having  rolled  twice  as  far  down 
a  given  slope  as  the  ball  of  the  short  pendulum.  Now 
as  a  body  falls  four  times  as  far,  either  directly  or  on 
any  uniform  slope,  in  two  seconds,  as  in  one,  a  pendu- 
lum must  be  four  times  as  long,  to  beat  once  in  two 
seconds,  as  to  beat  every  second.  A  pendulum  of  a  Jittle  more  than  39 
inches  beat  seconds ;  one  of  four  times  the  length  is  required  to  beat  double 
seconds,  and  one  of  one-fourth  the  length  to  beat  half  seconds. — As  a  pen- 
dulum to  answer  its  purpose  must  be  of  invariable  length,  one  which  beats 
seconds  constitutes  an  easily  found  standard  of  measure. 

Because  the  smallest  change  in  a  length  of  a  pendulum  alters  the  rate  of 
going  of  the  clock,  it  is  important  to  be  able  to  counteract  the  dilatation  or 
contraction  of  pendulums  caused  by  the  changing  heat  of  the  seasons ;  and 
for  this  purpose  various  ingenious  means  have  been  contrived.  One  of  the 
best  of  these  is  the  gridiron  pendulum,  as  it  is  called,  from  consisting  of  various 


62 


MOTIONS    AND    FORCE 


rods  of  metal.    It  renders  the  different  dilatability  by  heat  of  two 
Fig.  14.      metals  composing  it,  the  cause  of  unchanged  length  in  the  whole, 
o  The  adjoining  sketch  may  show  that  if  the  central  rod  of  brass 

represented  by  the  strong  line  from  b  to  c,  dilate  alone  just  as 
much  as  the  two  rods  of  steel,  represented  by  the  weaker  lines  on 
either  side  of  the  other,  dilate  together  (the  expansion  of  brass  by 
heat  is  about  double  that  of  steel,)  it  will  exactly  counteract  the 
lengthening  of  these,  and  will  keep  the  ball  d  always  at  the  same 
distance  from  the  point  of  suspension  a.  Some  astronomical 
clocks  in  the  present  day  are  so  perfect  that  they  do  not  err  one 
beat  of  the  pendulum  in  a  year.  Common  clocks  are  regulated 
by  a  screw  which  lifts  or  lets  down  the  ball  of  the  pendulum,  and 
so  changes  the  effective  length,  that  is,  the  distance  between  the 
point  of  suspension  and  what  is  called  the  centre  of  oscillation, 
treated  of  in  the  next  chapter. 
3.  The  force  of  gravity,  of  course,  is  what  determines  how  long  the  pen- 
dulum shall  be  in  falling  to  the  bottom  of  its  arc,  and  how  long  in  rising,  for 
the  ball  of  the  pendulum,  as  already  stated,  may  be  considered  as  a  body 
descending  by  its  weight  on  a  slope;  a  change  in  the  force  of  gravity,  there- 
fore, would  at  once  alter  the  rate  of  all  the  clocks  on  earth.  At  the  equator 
of  our  earth,  where  the  gravity  of  bodies  is  counteracted  in  a  small  degree 
by  the  centrifugal  force  arising  from  the  earth's  motion  (as  explained  at  page 
53,)  a  pendulum  vibrates  more  slowly  than  elsewhere,  and  must  therefore  be 
made  shorter  to  answer  the  same  purpose.  Corresponding  results  take  place 
when  a  pendulum  is  carried  to  a  mountain  top,  and  therefore  farther  away 
from  the  centre  of  the  earth,  which  is  the  centre  of  attraction — or  when  car- 
ried to  the  bottom  of  a  mine,  where  it  is  attracted  by  the  matter  above  it,  as 
well  as  by  the  matter  beneath. 

The  popular  prejudice  refuted  at  page  53,  that  a  large  or  heavy  body 
should  fall  to  the  earth,  even  in  'a  vacuum,  more  quickly  than  a  small  or 
light  body  attaches  itself  also  to  the  case  of  a  heavy  and  a  light  pendulum. 
Now  there  is'no  difference  for  pendulums  of  the  same  kngth,  whatever  their 
weight  or  material,  but  what  depends  on  the  resistance  of  the  air.  It  is  a 
very  remarkable  fact  thus  proved,  that  in  all  substances  the  gravity  and 
inertia  perfectly  agree. 

There  is  a  small  pendulum  called  a  metronome,  used 
Fig.  15.  by  musicians  for  marking  time ;  which,  although  very 

short,  may  still  be  made  to  beat  whole  seconds,  or  even 
longer  intervals.  The  reason  of  its  slow  motion  is,  that 
its  rod  is  prolonged  beyond  its  axis  of  support,  at  a,  up- 
wards, to  b}  and  has  a  ball  upon  the  top  at  b}  as  well  as 
on  the  bottom  at  c  ;  which  upper  ball  prevents  the  under 
one  from  moving  so  fast  as  it  otherwise  would,  just  as  a 
small  weight  attached  to  one  end  of  a  weighing-beam, 
prevents  a  greater  weight  attached  to  the  other  end  from 
falling  so  fast  as  it  would  if  there  were  no  counterpoise. 
The  rate  of  motion  changes  with  any  change  in  the  dis- 
tance of  the  ball  b  from  the  centre  of  motion  a  ;  and  to 
allow  of  such  change,  the  ball  is  made  to  slide. 

A  pocket-watch  differs  from  a  clock  in  having  a  vibrating  wheel  instead 
of  a  vibrating  pendulum  ;  and  as,  in  a  clock,  gravity  is  always  pulling  the 
pendulum  down  to  the  bottom  of  its  arc,  which  is  its  natural  place  of  rest, 
but  does  not  fix  it  there,  because  the  momentum  acquired  during  its  fall 


PENDULUM.  63 

from  one  side  is  just  sufficient  to  carry  it  up  to  an  equal  height  on  the  other 
— so  in  a  watch,  a  spring,  generally  spiral,  surrounding  the  axis  of  the  ba- 
lance-wheel is  always  forcing  this  towards  a  middle  position  of  rest,  but  does 
not  fix  it  there,  because  the  momentum  acquired  during  its  approach  from 
either  side  to  the  middle  position,  carries  it  .just  as  far  past  on  the  other  side, 
and  the  spring  has  to  begin  its  work  again.  The  balance-wheel  at  each 
vibration  allows  one  tooth  of  the  adjoining  wheel  to  pass,  as  the  pendulum 
does  in  a  clock,  and  the  record  of  the  beats  is  preserved  by  the  wheels  which 
follow,  as  already  explained  for  the  clock.  A  main-spring  is  used  to  keep 
up  the  motion  of  a  watch,  instead  of  the  weight  used  in  a  clock ;  and  as  a 
spring  acts  equally,  whatever  be  its  position,  a  watch  keeps  time  although 
carried  in  the  pocket  or  in  a  moving  ship. 

As  the  rate  of  a  clock  is  influenced  by  the  length  of  its  pendulum,  so  is 
the  rate  of  a  watch  by  the  size  or  diameter  of  its  balance-wheel ;  and  heat, 
which  retards  the  motion  of  a  common  clock  by  lengthening  the  pendulum, 
retards  the  motion  of  a  common  watch  by  dilating  the  balance-wheel.  Inge- 
nuity, however,  has  found  a  remedy  for  the  latter  case  as  for  the  former, 
viz.,  the  contrivance  called  the  expansion  balance-ivheel.  Of  this  the  cir- 
cumference, instead  of  being  a  continuous  ring,  is  made  up  of  two  half-rings, 
each  attached  by  one  end  only,  to  a  cross  bar,  and  which  half  rings  being  of 
brass  on  the  outside  and  of  steel  within,  bend  or  curl  inwards  by  heat — as 
a  sheet  of  damp  paper  bends  when  held  to  the  fire — and  thus  diminish  the 
size  of  the  wheel  at  their  loose  extremities,  so  as  just  to  counter  balance  its 
increase  by  the  expansion  of  the  cross  bar. 

As  the  motion  of  a  pendulum  has  relation  to  the  force  of  gravity,  so  has 
the  motion  of  a  balance-wheel  to  the  stiffness  of  the  balance-spring  ;  and  the 
regulator  of  a  watch  is  merely  a  pin  which  bears  against  the  balance-spring, 
and  by  sliding  backwards  or  forwards,  so  as  to  shorten  or  lengthen  the  part 
of  the  spring  left  free  to  act,  changes  the  degree  of  its  stiffness.  A  change 
produced  by  the  variation  of  temperature  is  compensated  for  by  the  expan- 
sion-wheel described  above. 

It  would  be  exceeding  the  limit  marked  out  for  this  general  work,  to  speak 
more  particularly  here  of  those  admirable  watches  which  have  been  produced 
within  the  last  thirty  years  under  the  name  of  chronometers,  for  the  pupose 
of  ascertaining  the  longitude  at  sea;  but  the  author  may  perhaps  be  excused 
for  mentioning  a  moment  of  surprise  and  delight  which  he  experienced,*  on 
first  seeing  their  singular  perfection  actually  proved.  After  months  spent 
in  a  passage  from  South  America  to  Asia,  his  pocket  chronometer,  with 
others  on  board,  announced  one  morning  that  a  certain  point  of  land  was 
then  bearing  east  from  the  ship  at  a  distance  of  fifty  miles ;  and  in  an  hour 
afterwards,  when  a  mist  had  cleared  away,  the  looker-out  on  the  mast  gave 
the  joyous  call  of  a  Land  a-head  !"  verifying  the  report  of  the  chronometers 
almost  to  a  mile  after  a  voyage  of  thousands.  It  is  natural,  at  such  a 
moment,  with  the  dangers  and  uncertainties  of  ancient  navigation  before  the 
mind,  to  exult  in  contemplating  what  man  has  now  achieved.  Had  the  rate 
of  the  wonderful  little  instrument  in  all  that  time  been  changed  even  a  little, 
its  announcement  would  have  been  worse  than  useless, — but  in  the  night 
and  in  the  day,  in  storm  and  in  calm,  in  heat  and  in  cold,  while  the  persons 
around  it  were  experiencing  every  vicissitude  of  mental  and  bodily  condition, 
its  steady  beat  went  on,  keeping  exact  account  of  the  rolling  of  the  earth 
and  of  the  stars ;  and  in  the  midst  of  the  trackless  waves  it  was  always  ready 
to  tell  its  magic  tale  of  the  very  spot  of  the  globe  over  which  it  had  arrived. 
The  mode  of  using  a  chronometer  for  so  valuable  a  purpose  will  be  explained 
in  the  section  on  ^astronomy. 


64:  MOTIONS    AND    FORCES. 

Bent  or  curvilinear  motion  from  attraction. — This  takes  place  whenever 
attraction  is  acting  across  the  path  of  an  existing  free  motion.  The  flying 
cannon-ball  or  stone,  drawn  down  by  gravity,  is  an  example,  for  the  pro- 
jectile force  ceases  with  the  first  impulse,  but  the  bending  force  is  acting 
every  instant  and  by  every  instant  producing  a  new  effect,  causes  a  cur- 
vilinear path. 

An  oblique  jet  of  water  is  to  the  eye  a  permanent  exhibition  of  the  curve 
described  by  a  body  thus  projected.  The  particles  of  the  liquid  move  in  the 
line  which  they  would  describe  if  projected  singly,  and  the  continued  suc- 
cession of  them  marks  the  line  of  situations  through  which  each  passes  in  its 
course  to  the  earth. 

A  cannon  or  musket-ball,  shot  quite  horizontally  over  a  level  plain,  will 
touch  the  ground  or  plain  just  as  soon  as  another  ball  dropped  at  the  same 
instant  directly  from  the  cannon's  mouth ;  for  the  forward  or  projectile  motion 
does  not,  in  such  case  at  all  interfere  with  the  action  of  gravity.  This  re- 
sult, which  most  persons,  before  consideration,  would  be  disposed  to  doubt, 
makes  strikingly  sensible  the  extraordinary  speed  of  the  cannon-ball;  viz., 
that  it  has  already  moved,  perhaps,  six  hundred  feet  forward,  during  the 
half  second  that  a  ball  dropped  from  the  hand  of  a  standing  person  requires 
to  reach  the  earth  only  four  feet  beneath.  This  fact  also  explains  why,  for 
a  long  range,  the  gun  must  be  pointed  more  or  less  upwards. 

A  dozen  marbles  swept  horizontally  from  off  a  table  by  a  stick,  all  reach 
the  floor  at  the  same  instant,  how  different  soever  the  distance  to  which 
they  may  respectively  be  driven. 

The  particular  study  of  the  subject  projectiles  is  very  important  to  mili- 
tary engineers ;  and  we  know  how  successfully  they  have  pursued  it  by  the 
precision  with  which  they  now  direct  their  shot  and  shells  to  objects  at  very 
great  distances. 

A  cannon-ball  shot  horizontally  from  the  top 

Fig.  16.  of  a  lofty  mountain,  would  go  three  or  four 

miles.  (The  mountain  is  here  represented  on 
an  enlarged  scale,  as  standing  on  the  globe  b,  c, 
d,  at  a.)  If  there  were  no  atmosphere  to  resist 
its  motion,  or  if  the  mountain  top  were  above 
the  surface  of  the  atmosphere,  the  same  original 
velocity  would  carry  it  thirty  or  forty  miles  be- 
fore it  fell,  as  to  b :  with  more  force  still,  it 
would  reach  to  c,  and  with  still  more  to  d.  And 
if  it  could  be  dispatched  with  about  ten  times 
the  velocity  of  a  common  cannon-shot,  it  would 
not  have  approached  nearer  to  the  earth  than  at 
first,  even  when  it  had  again  reached  round  e 

or  to  a ;  and  its  velocity  being  undiminished,  it  would  perform  a  second 
similar  tour,  and  then  a  third,  and  so  forth :  it  would,  in  fact,  have  become 
a  little  satellite,  or  planetary  body,  revolving  round  the  earth.  In  the  suc- 
cessive ranges  represented  in  the  figure,  it  is  seen  that  the  centrifugal  force 
of  the  ball,  or  its  tendency  to  move  in  a  straight  line  becomes  more  and 
more  nearly  a  counterbalance  to  gravity,  and  at  last  is  exactly  equal  to  it. 
If  the  force  given  to  the  ball  were  more  than  sufficient  to  bring  it  round 
again  to  the  level  of  a,  it  would  for  a  time  fly  off,  or  increase  its  distance 
from  the  earth,  acquiring  somewhat  the  eccentric  motion  of  a  comet.  There 
may  really  be  such  revolving  masses  above  our  atmosphere,  although  invisible 
to  us,  owing  to  their  srnallness.  It  has  been  supposed  by  some,  that  the 


PROJECTILES..  65 

meteoric  stones,  which  fall  to  the  earth  every  now  and  then,  came  from  such 
bodies,  or  are  the  entire  masses,  having  become  entangled  in  our  atmosphere, 
so  as  to  lose  their  forward  velocity.  The  four  little  planets  discovered  lately 
beyond  the  orbit  of  Mars,  are  not  larger  than  a  six-thousandth  part  of  our 
earth. 

REPULSION, — produces  accelerated,  retarded  and  lent  motions,  like  attrac- 
tion, but  it  acts  only  at  minute  distances,  while  attraction  draws  from  the 
sun,  or  from  the  very  limits  of  the  universe ;  repulsion  acts,  for  instance,  be- 
tween the  adjoining  atoms  of  an  elastic  fluid.  Yet  repulsion  plays  a  part  in 
the  economy  of  nature,  not  at  all  inferior  to  its  sister  attraction.  We  have 
already  seen,  when  considering  the  constitution  of  masses,  in  section  first, 
that  repulsion  prevents  or  modifies  the  contact  of  the  atoms  of  all  bodies  ; 
that  with  increase  of  temperature,  it  causes  these  atoms  to  separate,  and  of 
a  solid  forms  a  liquid,  or  even  an  air;  that  it  operates  around  all  masses  as 
if  it  were  a  film  or  covering,  preventing  their  mutual  cohesion,  &c.,  &c. 

Accelerated  motion  from  repulsion  is  seen  when  the  atoms  of  gunpowder 
explode  and  propel  the  bullet  from  the  bottom  of  a  piece  to  the  muzzle  with 
such  rapidly  increasing  velocity.  The  strength  of  this  repulsion  of  gun- 
powder is  so  much  greater  than  the  strength  of  gravity  or  common  attraction, 
that  its  action  on  a  bullet,  during  the  passage  along  a  barrel  of  five  or  six 
feet  in  length,  may  not  be  overcome  by  gravity,  during  an  ascent  of  a  mile 
or  more. 

A  visible  retarded  motion  from  repulsion  is  exemplified  by  a  moving  body 
coming  against  a  spring  or  a  bladder  full  of  air,  or  against  the  piston-handle 
of  an  air-syringe,  so  as  to  compress  the  air  beneath  it. 

Any  elastic  body  striking  against  another  body  and  recoiling,  exhibits  in 
conjunction  the  phenomena  of  retardation,  acceleration,  and  often  also  of 
bending,  chiefly  from  repulsion ;  for  instance  : 

An  ivory  ball,  driven  forcibly  "against  a  marble  slab,  does  not  stop  at  the 
instant  that  apparent  contact  takes  place,  but  still  advances  and  compresses 
that  part  of  the  substance  which  is  against  the  marble, — as  is  proved  by  the 
facts  mentioned  at  page  37.  While  this  compression  of  the  ivory  is  going 
on,  the  resistance  made  by  the  increasing  repulsion  of  the  particles  gradually 
retards,  and  ultimately  destroys  the  forward  motion  of  the  ball ;  and  at  the 
instant  of  its  final  arrest,  the  parts  in  contact,  both  of  the  ball  and  of  the  mar- 
ble, being  in  their  greatest  degree  of  compression,  act  on  the  ball,  and  repel 
it  again  with  gradually  accelerating  motion,  until  it  leaves  the  marble  with 
the  same  velocity  which  it  had  on  approaching.  The  retardation  and  accel- 
eration take  place  here  within  so  small  a  space,  and  in  so  short  a  time,  that 
they  are  not  apparent  to  sense,  but  the  mind  perceives  the  nature  of  the  phe- 
nomenon as  distinctly  as  if  the  ball  had  rolled  against  the  end  of  a  long  steel 
spring.  If  the  ball  strike  the  marble  obliquely,  as  from  a  to  c,  in  a  path  form- 
ing the  angle  acd,  with  a  perpendicular  line,  it  does  not  rebound  in  the  same 
line  by  which  it  approached,  but  just  as  obliquely  to- 
wards the  other  side,  viz.,  from  c  to  6;  and  it  then  exhibits 
a  bent  motion  from  repulsion.  This  case  illustrates  also 
the  "  resolution  of  motions,"  for  the  oblique  descent  a  c 
being  composed  of  a  direct  downward  motion  from  a  to 
the  table,  and  a  horizontal  or  forward  motion  from  a  to- 
wards the  perpendicular,  the  table  destroys  the  down- 
ward motion,  and  converts  it  into  an  opposite  directly 
upward  motion,  but  it  does  not  affect  the  forward  mo- 
tion,  which  immediately  combines  again  with  the  up- 

5 


66  MOT  I.ONS     AND     FORCES. 

ward,  and  carries  the  ball  as  far  beyond  the  perpendicular  at  b  as  it  was 
distant  from  it  at  a.  The  important  law  in  physics,  of  which  this  case  is 
an  example,  is  usually  expressed — '•'  The  angles  of  incidence  and  of  reflec- 
tion are  equal."  It  applies  to  all  reflected  bodies,  as  balls,  waves,  sound, 
light,  &c. 

If  the  ivory  ball  and  marble,  in  the  above  case,  were  supposed  to  be  both 
perfectly  hard,  and  without  elasticity,  still  the  repulsion  which  surrounds  all 
bodies,  as  a  thin  covering,  preventing  their  cohesion,  (see  page  32,)  would 
act  exactly  as  the  real  elasticity  of  the  ivory,  and  would  cause  a  retarded 
motion  until  perfect  rest  came,  and  then  an  accelerated  motion  back  again, 
until  the  ball  recovered  its  primitive  velocity. 

Collision  between  hard  bodies  always  exhibits  more  or  less  of  the  truth 
now  described ;  when  it  occurs  between  soft  bodies,  as  lumps  of  lead  or  of 
moist  clay,  the  approaching  parts  mutually  displace  each  other,  and  there  is 
no  recoil. 

When  a  straight  steel  plate,  of  which  the  end  is  fixed  in  a  block,  is  bent 
as  by  a  ball  rolling  against  it,  the  particles  on  the  side  which  becomes  con- 
cave are  made  to  approximate,  and  there  is  a  resistance  or  repulsion  gradu- 
ally increasing  among  them ;  the  particles  on  the  convex  side,  again,  are 
drawn  a  little  more  from  each  other,  and  are  therefore  exerting  attraction  to 
return  :  the  recoil  of  the  spring  is  thus  owing  to  both  forces  trying  to  replace 
the  particles  in  their  former  relative  situations. 

"  Tides,  Winds,  &c.,  exemplify  ATTRACTION."      (Read  the  Analysis, 

page  42.) 

Until  we  reflect  attentively  on  this  subject,  we  are  far  from  perceiving  that 
all  the  phenomena  of  nature  are  only  instances  of  attraction  and  repulsion, 
acting  under  a  variety  of  circumstances.  • 

ATTRACTION. —  Tides  are  raised  by  the  attraction  of  the  moon  and  sun, 
and  fall  again  by  the  general  attraction  of  the  earth;  producing  in  many  of 
the  shallower  parts  of  the  ocean  very  rapid  horizontal  currents.  They  do  a 
great  deal  of  work  for  man.  They  carry  his  ships  along  the  coasts,  and  up 
and  down  the  rivers;  they  turn  water-wheels  for  him;  they  fill  his  docks 
and  canals  at  convenient  times ;  they  rise  to  receive  his  ships,  launched  from 
elevated  building-yards,  &c.  What  a  busy  scene  is  a  great  sea-port  river, 
during  the  rising  and  falling  of  the  tide — with  the.  thousands  of  people 
along  its  bank,  borrowing  assistance  in  their  various  occupations  ! 

Winds  are  produced  chiefly  by  the  fluid  atmosphere  seeking  its  level,  in 
obedience  to  the  attraction  of  the  earth,  after  the  action  of  disturbing  causes, 
such  as  the  heat  of  the  sun,  &c.  They  help  man  in  the  important  business 
of  navigation  ;  they  turn  his  windmills,  &c. 

The  currents  of  rivers  are  water  constantly  descending  on  slopes,  that  is, 
regaining  its  level,  in  obedience  to  the  earth's  attraction.  Water-mills  and 
inland  navigation  are  among  the  advantages  which  they  afford  to  man. 

All  fatting  and  pressing  bodies  exhibit  attraction  in  its  simplest  form. 

REPULSION — is  instanced  in  explosion,  steam,  the  action  of  springs,  &c. 

Explosion  of  gunpowder  is  repulsion  among  the  particles  when  assuming 
the  form  of  air. 

Steam,  by  the  repulsion  among  its  particles,  moves  the  piston  of  the 
steam-engine.  In  our  days  it  performs  half  the  labour  of  society. 

Accidental  explosion  of  fire-damp,  or  hydrogen,  in  mines,  and  the  tre- 


PRODUCTION    OF    GREAT    VELOCITIES.  67 

mendous  evolutions  of  elastic  fluid  in  volcanoes  and  earthquakes,  are  other 
instances  of  the  same  class. 

Elasticity,  as  seen  in  springs,  collision,  &c.,  belongs  chiefly  to  repulsion  ; 
as  seen  in  India-rubber,  and  other  substances  resuming  their  usual  length 
after  extension,  it  belongs  chiefly  to  attraction. 

A  spring  is  often,  as  it  were,  a  reservior  of  force,  kept  ready  charged  for 
a  purpose ;  as  when  a  gunlock  is  cocked,  a  watch  wqund  up,  &c. 

It  will  be  remarked,  with  respect  to  many  of  the  phenomena  now  and  here- 
after to  be  mentioned,  that  it  is  not  the  original  Attraction  or  Repulsion 
which  man  uses  as  his  servant,  but  the  momentum  gradually  accumulated 
in  masses  by  the  exertion  of  such  attraction  or  repulsion ;  in  other  words, 
the  inertia  is  used  as  a  great  working  power  or  force. 

Electrical,  galvanic,  magnetical,  and  optical,  phenomena,  are  also  in  great 
part  peculiar  attractions  and  repulsions,  as  will  be  seen  in  the  chapters  devoted 
to  the  explanation  of  them.  And  even  the  actions  of  animals,  so  infinitely 
varied,  are  all  results  of  a  shortening  of  the  fleshy  threads  called  muscular 
fibres,  which  is  produced  by  the  mutual  attraction  of  their  component  parti- 
cles ; — just  as  the  varied  motion  of  a  telegraph,  or  of  a  ship's  yard,  are  pro- 
duced by  the  shortening  of  certain  ropes  of  connection. 

Howeyer  closely  allied  the  last-mentioned  particular  attractions  and  repul- 
sions may  be  to  the  general  attraction  and  repulsion  formerly  treated  of,  it 
is  found  convenient  to  consider  them  apart. 

In  the  remarkable  phenomena  of  nature  and  art,  all  the  motions  being  caused, 
as  now  shown,  by  Attraction  and  Repulsion,  these  forces  do  not  operate  by 
a  single  impulse,  but  through  a  repetition  of  impulses,  or  a  continued  action, 
of  which  the  effect  is  gradually  accumulated  in  the  inertia  of  matter.  Thus 
all  great  velocities  and  momenta  are  the  terminations  of  an  accelerated 
motion.  ^ 

Meteoric  stones  falling  from  great  heights,  bury  themselves  deep  in  the 
earth  by  the  force  of  their  gradually  acquired  velocity. 

When  the  wood-cutters  among  the  Alps  launch  an  enormous  tree  from  high 
on  the  mountain  side,  along  the  smooth  wooden  trough  or  channel  prepared 
for  it  and  in  fewer  minutes  than  it  traverses  miles,  it  is  seen  plunging  into 
the  lake  below;  it  acquires  its  frightful  velocity,  not  at  once,  but  through 
the  action  of  gravity  continued  during  the  whole  of  its  descent. 

The  shock  or  blow  of  the  ram  of  a  pile-engine,  is  not  the  effect  of  momen- 
tary attraction  between  it  and  the  earth,  but  of  that  attraction  accumulating 
motal  inertia  or  power,  during  the  descent  of  the  ram  through  a  space  of 
twenty  or  thirty  feet. 

A  common  hammer,  in  its  instantaneous  shock,  has  the  condensed  effect 
of  the  arm  and  of  gravity,  as  accumulated  through  its  whole  previous  course ; 
and  when  a  powerful  blow  is  intended,  the  hammer,  or  hatchet,  or  club,  or 
fist  in  boxing,  is  lifted  high,  or  carried  far  back,  that  there  may  be  time  and 
space  for  imparting  greater  power. 

The  inferior  animals,  by  many  of  their  actions,  illustrate  the  same  truth, 
and  prove  their  experimental  or  instinctive  acquaintance  with  it. 

Sea-birds  carry  shell-fish  up  into  the  air,  and  drop  them  on  smooth  stones 
to  break  them,  and  to  obtain  the  food.  It  is  related  in  Grecian  story,  that  a 
bird  once  mistook  the  venerable  bald  head  of  a  sage  meditating  on  the  sea- 


68  MOTIONS    AND    FORCES. 

shore  for  a  smooth  stone,  and  by  the  same  act  killed  an  oyster  and  the 
philosopher. 

There  are  some  long-necked  birds,  that  fight  and  kill  their  prey  by  a  blow 
of  their  beak.  They  draw  back  the  head,  bending  the  neck  like  a  swan  or 
serpent,  and  then  dart  it  forward,  with  a  continued  effort,  until  the  strong 
wedge-like  beak  reaches  its  destination,  almost  with  the  velocity  of  a  pistol 
bullet.  One  snake  in  darting  its  fangs  at  another  passing  swiftly  across  its 
coil,  has  been  known  to  miss  its  aim  and  inflict  a  mortal  wound  on  its  own 
flesh. 

Bulls,  rams  and  goats,  in  fighting,  alternately  recede  and  run  at  each 
other,  that  the  shock  may  be  great  when  their  foreheads  meet. 

A  horse  in  kicking,  from  the  great  length  of  his  leg,  and  the  consequent 
space  through  which  he  can  be  adding  velocity  to  his  foot,  dri¥es  at  last 
against  the  object  almost  like  a  cannon  shot. 

A  bow-string  propelling  an  arrow,  follows  it  through  a  considerable  space, 
and  so  gives  the  great  velocity  at  last  produced. 

A  sling  gives  to  the  hand  the  power  of  adding  velocity  to  the  stone  through 
a  long  path ;  for  the  hand  moves  in  a  small  circle  while  the  stone  moves  in 
a  larger,  and  the  hand  being  kept  always  in  somewhat  advance  of  the  stone, 
pulls  at  it  without  intermission,  until  the  moment  of  discharge. 

The  battering-rams  of  the  ancients  allowed  those  about  them  to*  accumu- 
late in  them  the  efforts  of  many  hands,  and  of  a  considerable  duration  of 
action,  so  as  to  give  at  last  one  great  and  sudden  shock. 

Even  the  gentle  action  of  the  human  breath,  exerted  for  a  time  on  a  pea 
or  small  hard  ball  of  clay  while  passing  through  a  long  smooth  tube,  gives 
a  velocity  which  will  inflict  a  sharp  and  painful  stroke  on  a  distant  animal. 
In  Borneo  and  other  of  the  Eastern  Islands,  poisoned  arrows  are  thrown  in 
this  way  with  great  force  and  precision. 

The  action  of  gunpowder  or  bullets,  although  appearing  so  sudden,  is  still 
not  an  instantaneous,  but  a  gradual,  and  therefore  accelerating  action ;  and 
accordingly  we  find  the  effect  to  depend  on  the  length  of  the  piece  along 
which  the  force  pursues  the  ball.  A  small  fast  sailing  vessel  with  a  single 
long  gun,  has  often  compelled  a  very  superior  vessel,  whose  guns  were 
shorter,  to  yield. 

For  the  same  reason  that  all  great  velocities  require  continued  action  or  re- 
peated impulse  to  produce  them,  so  do  they  also  destroy  them ;  the  in- 
ertia of  motion  and  of  rest  being  equal.  v 

A  vast  mass  of  rock  suspended  like  a  pendulum,  and  allowed  to  sweep 
down  its  curve  to  a  considerable  elevation,  would  arrive  at  the  bottom  like  a 
battering-ram,  with  force  sufficient  to  shake  a  thick  wall  or  rampart  to  its 
foundation.  The  continued  action  of  gravity  would  have  given  this  force, 
and  if,  instead  of  the  solid  resistance  supposed,  and  which  would  scarcely  be 
sufficient  to  take  the  whole  momentum  away,  the  mass  were  merely  allowed 
to  continue  its  course  as  a  pendulum,  and  to  ascend  on  the  other  side,  the 
continued  action  of  gravity  then  opposing  its  motion,  would  bring  it  to  pow- 
erless rest  again,  by  the  time  when  it  had  reached  an  elevation  equal  to  that 
from  which  it  fell. 

Soft  air  expanding  gives  gradually  the  death-carrying  velocity  to  the  can- 
non-ball ;  and  soft  air,  or  cotton,  or  wood,  resisting  in  a  close  strong  tube, — 
if  the  bullet  could  be  directed  exactly  into  it — would  again  gradually  anni- 
hilate the  motion.  Were  the  attempt  made,  however,  to  stop  the  ball  sud- 


PRODUCTION    OF    GREAT     VELOCITIES.  69 

denly,  by  a  block  of  the  hardest  granite,  the  block  would  instantly  be  riven 
by  its  force. 

Bales  of  cotton  or  thick  masses  of  cork,  attached  round  a  ship,  will  receive 
cannon-balls,  and  bring  them  to  rest,  without  themselves  suffering  much, 
while  the  naked  firmer  side  of  the  ship  would  be  penetrated.  The  cotton  or 
cork  offers  an  increasing  resistance  through  a  considerable  space,  while  the 
oak  opposes  its  hard  front  at  once,  and  must  instantly  suffice  or  be  destroyed. 
A  hard  body,  that  it  may  at  once  destroy  such  a  motion  as  we  are  supposing, 
must  be  able  to  oppose  as  much  force  in  perhaps  the  space  of  one-hundredth 
of  an  inch,  that  is,  in  the  extent  to  which  its*  elasticity  .will  let  it  yield  with- 
out breaking,  as  the  moving  cause  gave,  through  a  much  greater  space  ( a 
plate  of  steel  will  thus  oppose  a  pistol-bullet; )  and  when  it  cannot  do  this, 
it  must  be  broken  or  penetrated  by  the  moving  body.  It  is  to  be  remarked, 
however,  that  the  continued  opposition  of  a  thick  mass  of  wood,  stone,  or 
earth,  to  an  entered  bullet,  brings  it  to  rest  at  least  as  an  elastic  unbroken 
opposition  would.  Gunners  have  ascertained  the  exact  depth  in  each  sub- 
stance to  which  a  ball  will  penetrate ;  and  they  call  buildings  bomb-proof 
or  ball-proof,  which  have  a  thickness  or  depth  exceeding  that. 

A  hempen  or  silken  rope,  supporting  the  scale  of  a  weighing  beam,  would 
resist  a  greater  weight  falling  into  the  scale  than  would  be  resisted  by  an 
iron  chain  which  were  even  stronger  than  the  rope  for  the  purpose  of  bearing 
a  quiescent  weight;  because  the  hemp  or  silk  would  yield  by  its. elasticity, 
and  continue  its  resistance  through  a  considerable  space  and  time,  and  thus 
would  at  last  gradually  overcome  the  momentum ;  while  the  iron,  by  scarcely 
yielding  at  all,  would  require  to  be  strong  enough  to  stop  the  mass  suddenly 
or  would  break. 

Yet  for  the  same  reason  that  iron  is  weakest  in  such  a  case  as  the  last,  it 
is  stronger  than  hemp  or  rope  when  used  as  a  cable  for  a  ship,  to  withstand 
the  sudden  force  of  waves. 

This  will  be  understood  on  considering,  that  the  chain  by  its  weight  hangs 
as  *a  curve  or  inverted  arc  in  the  water,  while  the  rope,  being  nearly  of  the 
weight  of  water,  is  supported  in  it  almost  as  a  straight  line  from  the  anchor 
to  the  ship;  therefore,  when  a  great  wave  dashes  against  the  ship,  the  bent 
chain  will  yield  until  it  be  drawn  nearly  straight,  by  which  great  extent  of 
yielding,  and  consequent  length  of  resistance,  it  will  withstand  a  great 
shock ;  whereas,  the  straight  rope,  as  it  can  yield  only  .by  the  elasticity  of 
its  material,  and  comparatively,  therefore,  a  little  way,  will  resist  much 
less. 

A  heavy  ship  moving  quickly  with  the  tide  or  wind,  could  not  be  stopped 
instantly  by  a  short  rope  or  chain  of  any  magnitude :  if  the  attempt  were 
made  to  destroy  at  once  so  vast  a  momentum,  something  would  certainly 
give  way ;  but  a  rope  of  very  moderate  size,  kept  tight  between  the  shore 
and  the  ship,  and  from  time  to  time  allowed  to  slip  a  little  round  a  wooden 
block,  when  the  tightness  threatened  its  breaking,  would  accomplish  the  end 
very  soon  and  easily. 

The  following  are  farther  proofs  that  forces  are  to  be  measured  as  much  by 
the  time  or  space  through  which  they  act,  as  by  their  difference  of  inten- 
sity or  momentary  power. 

A  door  standing  open,  and  which  would  yield  readily  on  its  hinges  to  the 
gentle  push  of  a  finger,  is  not  moved  by  a  cannon-ball  piercing  through  it. 
Now  the  ball  really  overcomes  the  whole  force  of  cohesion  among  the  atoms 


70  MOTIONS    AND    FORCES. 

of  tough  wood  :  but  that  force  is  allowed  to  act  or  resist  for  so  short  a  time, 
owing  to  the  rapid  passage  of  the  ball,  that  it  is  not  sufficient  to  affect  the 
inertia  of  the  door,  in  a  degree  to  produce  a  sensible  motion.  The  cohesion 
of  the  circle  in  the  door,  cut  out  by  the  ball,  would  have  borne  a  weight  of 
more  than  a  hundred  pounds  laid  quietly  upon  it,  but  supposing  the  bullet 
to  fly  twelve  hundred  feet  in  a  second,  and  the  door  to  be  one  inch  thick, 
the  cohesion  being  allowed  to  act  for  only  the  14,400th  part  of  a  second,  its 
influence  is  not  perceived.  The  following  are  other  examples  of  the  same 
kind. 

A  leaden  bullet  pressed  slowly  against  a  pane  of  glass,  breaks  it  irregu- 
larly, where  the  strength  happens  to  be  least ;  but  the  same  bullet  shot  at  it 
from  a  pistol,  makes  only  a  small  round  hole.  It  has  been  amusingly  said 
of  such  a  case,  that  the  particles  struck  and  carried  away  have  not  time  to 
warn  their  neighbours  of  what  is  happening. 

A  cannon-ball,  having  very  great  velocity,  passes  through  a  ship's  side, 
and  leaves  but  a  little  mark ;  while  one  with  less  speed  splinters  and  breaks 
the  wood  to  a  considerable  distance  around.  A  near  shot  thus  often  injures 
a  ship  less  than  one  from  a  greater  distance. 

A  sheet  of  paper  standing  edgeways  on  a  table,  is  not  driven  down  by  a 
pistol-ball  fired  through  it. 

The  truth  at  present  under  consideration  explains,  with  respect  to  gun-shot 
wounds,  why  the  man  often  remains  ignorant  for  a  time  of  his  misfortune, 
and  why  a  rapid  bullet  only  kills  the  parts  which  it  touches,  while  a  spent 
ball  may  bruise  and  injure  all  around.  In  many  cases  of  injury  popularly 
attributed  to  the  wind  of  a  ball,  the  ball  itself  has  really  reached  the  part. 

A  man  lying  down  and  receiving  the  blow  of  a  great  hammer  on  his  chest 
would  be  killed  by  it ;  but  if  a  heavy  anvil  be  first  laid  upon  the  chest,  and 
the  blow  then  received  upon  the  anvil,  the  man  bears  it  with  impunity. 
Here  the  quantity  of  motion  in  the  hammer  being  diffused  through  the  great 
mass  of  the  anvil,  produces  but  a  trifling  velocity,  which  the  elasticity  of  the 
chest,  in  its  slow  yielding,  easily  overcomes. 

A  circular  plate  of  soft  iron,  made  to  turn  with  extreme  rapidity,  will  cut 
through  the  hardest  steel  file,  almost  as  a  knife  cuts  through  a  carrot.  In 
cases  where  a  soft  powder  suffices  to  polish  a  hard  body,  it  acts  partly  like 
this  plate,  by  the  motion  or  velocity  given  to  the  wearing  particles. 

"  There  is  no  motion  or  action  in  the  universe,  without  a  concomitant  and 
opposite  action  of  equal  amount."    (See  the  Analysis.) 

This  truth  has  otherwise  been  expressed — "  action  and  reaction  are  equal 
and  contrary."  It  is  evident,  that  if  no  action  or  movement  takes  place  on 
earth  but  in  consequence  of  either  Attraction  or  Repulsion — and  this  has 
now  been  shown — there  must  always  be  two  objects  or  masses  concerned, 
and  each  must  be  attracted  or  repelled  just  as  much  as  the  other,  although 
one  will  have  less  velocity  than  the  other,  as  it  may  be  itself  greater,  or 
fixed  to  another  mass. 

If  a  man  in  one  boat  pull  at  a  rope  attached  to  another,  the  two  boats  will 
approach.  If  they  be  of  equal  size  and  load,  they  will  both  move  at  the 
same  rate,  in  whichever  of  the  boats  the  man  may  be ;  and  if  there  be  a 
difference  in  the  sizes,  and  resistances,  there  will  be  a  corresponding  differ- 
ence in  the  velocities,  the  smaller  boat  moving  the  fastest. 

A  magnet  and  a  piece  of  iron  attract  each  other  equally,  whatever  dispro- 
portion there  is  between  the  masses.  If  either  be  balanced  in  a  scale,  and 


ACTION    AND    REACTION    EQUAL.  71 

the  other  be  then  brought  within  a  certain  distance  beneath  it,  the  very  same 
counterpoise  will  be  required  to  prevent  their  approach,  whichever  be  in  the 
scale.  If  the  two  were  hanging  near  each  other  as  pendulums,  they  would 
approach  and  meet;  but  the  little  one  would  perform  more  of  the  journey 
in  proportion  to  its  littleness. 

A  man  in  a  boat  pulling  a  rope  attached  to  a  large  ship,  seems  only  to 
move  the  boat :  but  he  really  moves  the  ship  a  little,  for  supposing  the  re- 
sistance of  the  ship  to  be  just  a  thousand  times  greater  than  that  of  the 
boat,  a  thousand  men  in  a  thousand  boats,  pulling  simultaneously  in  the 
same  manner  would  make  the  ship  meet  them  half  way. 

A  pound  of  lead  and  the  earth  attract  each  other  with  equal  force,  but 
that  force  makes  the  lead  approach  sixteen  feet  in  a  second  towards  the  earth, 
while  the  contrary  motion  of  the  earth  is  of  course  as  much  less  than  this  as 
the  earth  is  weightier  than  one  pound, — and  is  therefore  unnoticed.  Speak- 
ing strictly,  it  is  true,  that  even  a  feather  falling  lifts  the  earth  towards  it, 
and  that  a  man  jumping  kicks  the  earth  away. 

A  spring  unbending  between  two  equal  bodies,  throws  them  off  with  equal 
velocity ;  if  between  bodies  of  different  magnitudes,  the  velocity  of  the  smaller 
body  is  greater  in  proportion  to  its  smallness. 

On  firing  a  cannon,  the  gun  recoils  with  even  more  motion  or  momentum 
in  it  than  the  ball  has,  for  it  suffers  the  reaction  of  the  expelled  gunpowder 
as  well  as  of  the  ball ;  but  the  momentum  in  the  gun  being  diffused  through 
a  greater  mass,  the  velocity  is  small,  and  easily  checked. 

The  recoil  of  a  light  fowling-piece  will  hurt  the  shoulder,  if  the  piece  be 
not  held  close  to  it. 

A  ship  in  chase,  by  firing  her  bow  guns,  retards  her  motion ;  by  firing 
from  her  stern  she  quickens  it. 

A  ship  firing  a  broadside,  heels  or  inclines  to  the  opposite  side. 

A  vessel,  of  water  suspended  by  a  cord  hangs  perpendicularly  ;  but  if  a 
hole  be  opened  on  one  side,  so  as  to  allow  the  water  to  jet  out  there,  the 
vessel  will  be  pushed  to  the  other  side  by  the  reaction  of  the  jet,  and  will  so 
remain  while  it  flows.  If  the  hole  be  oblique,  the  vessel  will  constantly  turn 
round. 

A  vessel  of  water  placed  upon  a  floating  piece  of  plank,  and  allowed  to 
throw  out  a  jet,  as  in  the  last  case,  moves  the  plank  in  the  opposite  direction. 

A  steamboat  may  be  driven  by  making  the  engine  pump  or  squirt  water 
from  the  stern,  instead  of  making  it,  as  usual,  move  paddle-wheels.  There 
is  a  loss  of  power,  however,  in  this  mode  of  applying  it,  as  will  be  explained 
under  the  head  of  "  Hydraulics." 

A  man  floating  in  a  small  boat,  and  blowing  strongly  with  a  bellows  to- 
wards the  stern,  pushes  himself  onwards  with  the  same  force  with  which  the 
&ir  issues  from  the  bellows-pipe. 

A  sky-rocket  ascends,  because,  after  it  is  lighted,  the  lower  part  is  always 
producing  a  larger  quantity  of  aeriform  fluid,  which,  in  expanding,  presses 
not  only  on  the  air  below,  but  also  on  the  rocket  above,  and  thus  lifts  it. 
The  ascent  is  aided  also  by  the  recoil  of  the  rocket  from  the  part  of  its  sub- 
stance, which  is  constantly  bursting  downwards. 

He  was  a  foolish  man  who  thought  he  had  found  the  means  of  command- 
ing always  a  fair  wind  for  his  pleasure-boat,  by  erecting  an  immense  bellows 
in  the  stern.  The  bellows  and  sails  acted  against  each  other,  and  there  was 
no  motion  :  indeed,  in  a  perfect  calm,  there  would  be  a  little  backward  mo- 
tion, because  the  sail  would  not  catch  all  the  wind  from  the  bellows. 


72  MOTIONS    AND    FORCES. 

A  man  supported  on  a  floating  plank,  by  walking  towards  one  end  of  it 
gives  it  a  motion  in  the  direction  opposite. 

A  man  using  an  oar,  or  a  steam-engine  turning  paddle-wheels,  advances 
exactly  with  the  force  that  drives  the  water  astern. 

A  swimmer  pressing  the  water  downwards  and  backwads  with  his  hands, 
is  sent  forwards  and  upwards  with  the  same  force,  by  the  reaction  of  the 
water. 

And  a  bird  flying,  is  upheld  with  exactly  the  force  with  which  it  strikes 
the  air  in  the  opposite  direction. 

A  man  pushing  against  the  ground  with  a  stick}  may  be  considered  as 
compressing  a  spring  between  the  earth  and  the  end  of  his  stick,  which 
spring  is  therefore  pushing  him  up  as  much  as  he  pushes  down ;  and  if,  at 
the  time,  he  were  balanced  in  the  scale  of  a  weighing  beam,  he  would  find 
that  he  weighed  just  as  much  less  as  he  was  pressing  with  his  stick. 

Thus  an  invalid,  on  a  spring  plank  or  chair,  who,  by  a  trifling  downward 
pressure  of  his  hand  on  a  staff  or  on  a  table,  causes  his  body  to  rise  and  fall 
through  a  great  range,  and  thus  obtains  the  advantange  of  almost  passive 
exercise,  is  really  lifting  himself  while  he  presses  downward. 

When  a  boy  cries  on  knocking  his  head  against  a  table  or  pane  of  glass, 
he  is  commonly  told,  and  truly,  that  he  has  given  as  hard  a  blow  as  he  has 
received  ;  although  his  philosophy  probably,  looking  chiefly  to  results, 
blames  the  table  for  his  head  hurt,  and  his  head  for  the  glass  broken. 

The  difference  of  momentum  acquired  in  a  fall  of  one  foot  or  of  several, 
is  well  known  :  the  corresponding  intensities  of  reaction  are  unpleasantly 
experienced  by  a  man  who  sits  down  in  an  easy  chair,  or  who,  in  sitting 
down  where  he  supposed  a  chair  to  be,  unexpectedly  reaches  the  floor. 

What  motion  the  wind  has  given  to  a  ship  it  has  itself  lost,  that  is  to  say, 
the  ship  has  reacted  on  the  moving  air  :  as  is  seen  when  one  vessel  is  be- 
calmed under  the  lee  of  another. 

When  one  billiard-ball  strikes  directly  another  ball  of  equal  size,  it  stops, 
and  the  second  ball  proceeds  with  the  whole  velocity  which  the  first  had — 
the  action  which  imparts  the  new  motion  being  equal  to  the  reaction  which 
destroys  the  old.  Although  the  transference  of  motion,  in  such  a  case,  seems 
to  be  instantaneous,  the  change  is  really  progressive,  and  as  follows.  The 
approaching  ball,  at  a  certain  point  of  time,  has  just  given  half  of  its  motion 
to  the  other  equal  ball,  and  if  both  were  of  soft  clay,  they  would  then  proceed 
together  with  half  the  original  velocity  ;  but,  as  they  are  elastic,  the  touching 
parts  at  the  moment  supposed  are  compressed  like  a  spring  between  the  balls, 
and  by  then  expanding,  and  exerting  force  equally  both  ways,  they  double 
the  velocity  of  the  foremost  ball,  and  destroy  altogether  the  motion  of  that 
behind. 

If  a  billiard  ball  be  propelled  against  the  nearest  one  of  the  row  of  balls 
equal  to  itself,  it  comes  to  rest  as  in  the  last  case  described,  while  the  farthest 
ball  of  the  row  darts  off  with  its  velocity, — the  intermediate  balls  having 
each  received  and  transmitted  the  motion  in  a  twinkling,  without  appearing 
themselves  to  move 

As  farther  illustrative  of  the  truths,  that  action  and  reaction  are  equal  and 
contrary,  and  that  in  every  case  of  hard  bodies  striking  each  other,  they  may 
be  regarded  as  compressing  a  very  small  strong  spring  between  them,  we 
may  mention,  that  when  any  elastic  body,  as  a  billiard-ball,  strikes  another 
body  larger  than  itself,  and  rebounds,  it  gives  to  that  other,  not  only  all  the 
motion  which  it  originally  possessed,  this  being  done  at  the  moment  when 
it  comes  to  rest,  but  an  additional  quantity,  equal  to  that  with  which  it  recoils 


ACTION    AND    REACTION    EQUAL.  73 

— owing  to  the  equal  action  in  both  directions  of  the  repulsion  or  spring 
which  causes  the  recoil.  When  the  difference  of  size  between  the  bodies  is 
very  great,  the  returning  velocity  of  the  smaller  is  nearly  as  great  as  its 
advancing  motion  was,  and  thus  it  gives  a  momentum  to  the  body  struck 
nearly  double  of  what  it  originally  itself  possessed.  This  phenomena  con- 
stitutes the  paradoxical  case  of  an  effect  being  greater  than  its  cause,  and  has 
led  persons,  imperfectly  acquainted  with  the  subject,  to  seek  from  the  prin- 
ciple, a  perpetuum  mobile.  A  hamifler  on  rebounding  from  an  anvil  has 
given  a  blow  nearly  double  the  force  which  it  had  itself,  for  the  anvil  felt  its 
full  original  force  while  stopping  it,  and  then,  equally  with  itself,  was  affected 
by  the  repulsion  which  caused  its  return. 

Many  other  interesting  facts  might  be  adduced  as  examples  of  equal  action 
and  reaction,  but  these  will  suffice. 

This  second  section  of  the  work  has  now  explained  the  nature  of  INERTIA 
in  matter,  and  has  shown  that  the  infinitely  varied  phenomena  of  motion, 
which  the  universe  exhibits,  are  only  attraction  and  repulsion,  acting  on 
inertia  of  atoms  separate  or  conjoined,  under  diversified  circumstances. 
And  such  is  the  sublime  simplicity  of  the  whole  scheme  of  nature. 


74  APPENDIX. 

APPENDIX 

TO  PAKT  I.  — SECTION  II. 


"V 

BY    THE   AMERICAN    EDITOR. 


THE  attentive  perusal  of  the  preceding  section  will  prepare  the  reader  to 
understand  the  following  propositions. 

Definitions. 

Prop.  1. — When  a  body  is  successively  changing  its  place,  it  is  said  to  be 
in  motion,  p.  42. 

The  idea  of  motion  involves  those  of  space,  time,  velocity,  direction,  the 
quantity  of  matter  and  momentum. 

Prop.  2. — The  space  described  is  the  distance  passed  over  by  a  body 
during  its  motion ;  and  is  measured  by  the  number  of  units  of  length,  as  a 
foot,  a  yard,  a  mile,  &c.,  contained  in  this  distance. 

Prop.  3. — The  time  consists  of  a  certain  number  of  units  of  time  adopted 
as  its  measure,  as  a  second,  a  minute,  &c.;  which  have  elapsed  during  the 
motion  of  a  body. 

Prop.  4. — The  velocity  of  a  body  is  the  rate  at  which  it  moves,  or  the 
number  of  these  assumed  units  of  space  that  it  passes  over  during  the  as- 
sumed unit  of  time. 

All  the  above  measures  may  be  represented  graphically  by  lines  that  are 
proportioned  to  them,  p.  65. 

Prop.  5. — The  direction  of  a  body  may  be  straight  or  curved ;  when 
straight  or  rectilinear,  it  is  the  angle  which  its  path  makes  with  any  straight 
line  in  the  same  plane,  adopted  as  an  axis ;  when  the  path  of  a  body  is  a 
curve,  its  direction  at  any  point  is  the  angle  which  the  tangent  to  the  curve 
at  the  point  makes  with  the  fixed  axis. 

Prop.  6. — The  momentum  of  a  body  is  its  quantity  of  motion,  both  the 
mass  and  velocity  being  taken  into  consideration,  and  its  proper  measure  is 
the  product  of  the  mass  into  the  velocity,  pp.  58,  54. 

Prop.  7. — A  body  is  said  to  have  a  uniform  motion  when  its  velocity 
remains  constant,  that  is,  when  it  describes  equal  spaces  in  equal  successive 
intervals  of  time,  p.  47. 

Prop.  8. — Every  motion  that  is  not  uniform  is  said  to  be  varied,  and  is 
called  accelerated  or  retarded  as  the  velocity  increases  or  decreases. 

Prop.  9. — When  the  velocity  constantly  increases  or  decreases  in  the 
direct  ratio  of  the  time  that  the  body  has  been  moved,  the  motion  is  said  to 
be  uniformly  accelerated  or  retarded,  pp.  43,  58,  59,  60. 

.  Prop.  10* — Whatever  is  capable  of  producing  or  destroying  the  motion 
of  a  body  is  called  force. 


APPENDIX.  75 

• 

Prop.  11. — A  force  that  produces  its  effect  instantaneously,  and  then 
ceases  to  act,  is  called  an  impulsive  force. 

Prop.  12. — A  force  that  acts  continually  and  equally  is  termed  a  con- 
stant force. 

Prop.  13. — When  the  constant  force  acts  in  lines  directed  towards  a 
single  point  or  centre,  it  is  called  centripetal,  and  the  path  of  the  body  its 
orbit,  p.  59. 

Prop.  14. — That  part  of  the  impulsive  force  which  tends  to  make  a  body 
move  directly  from  the  centre,  is  termed  the  centrifugal  force,  p.  49. 

Prop.  15. — A  force  that  is  capable  of  destroying  motion  without  being 
able,  under  any  circumstances,  to  produce  motion,  is  termed  a  passive  force. 

Prop.  16. — The  state  of  rest  produced  by  the  action  of  opposite  forces  is 
termed  equilibrium. 

Prop.  17. — When  a  body  is  struck,  its  particles  yield  to  the  impulse,  and 
the  form  of  the  body  is  changed.  When  the  body  possesses  the  inherent 
power  when  thus  changed,  of  restoring  its  form,  it  is  said  to  be  elastic; 
when  it  has  not  this  power,  it  is  called  non-elastic,  p.  37. 

Prop.  18. — A  body  oscillating  below  a  point  to  which  it  has  in  any  way 
attached,  is  termed  a  pendulum,  p.  60. 

Laws  of  Motion. 

Prop.  19. — 1st.  If  a  body  be  at  rest  it  will  continue  at  rest,  and  if  in 
motion,  it  will  continue  to  advance  uniformly  in  a  right  line  unless  com- 
pelled to  change  its  state  by  some  external  force,  pp.  47,  49. 

Prop.  20. — 2d.  The  motion  of  a  body  is  in  the  direction  of  the  force  that 
produces  it,  and  is  proportional  to  that  force,  pp.  53,  55. 

Prop.  21. — 3d.  Action  and  reaction  are  always  equal  and  opposed  to  each 
other;  or  when  a  body  communicates  motion  to  another,  it  loses  of  its  own 
momentum  as  much  as  it  gives  to  the  other  body,  pp.  70,  72. 

Of  Impulsive  force  and  Rectilinear  motion. 

Prop.  22. — The  effect  of  an  impulsive  force  is  to  produce  uniform  rec- 
tilinear motion,  p.  49. 

For  during  the  moment  of  its  action  on  any  body,  it  must  set  it  in  motion 
with  a  certain  velocity  j  and  by  the  first  law  of  motion,  the  body  must  con- 
tinue to  advance  in  a  straight  line  with  that  velocity. 

Prop.  23. — In  rectilinear  motion  the  space  is  as  the  velocity  multiplied 
into  the  time. 

For  if  a  body  move  with  the  velocity  of  three  feet  per  second,  it  is  evi- 
dent, that  it  will  move  over  6  feet  in  two  seconds,  i.  e.  X  32 ;  and  9  feet  in 
seconds,  i.  e.  X  33,  and  12  feet  in  four  seconds,  &c.  &c. 

Prop.  24. — The  time  is  as  the  space  divided  by  the  velocity. 

For  if.a  body  passes  over  12  feet  for  instance,  when  its  velocity  is  3  feet 
per  second,  it  is  evident,  that  in  order  to  find  the  number  of  seconds,  which 
the  body  has  employed  in  passing  over  12  feet  of  space,  we  need  only 
divide  12  by  3,  (i.  e.,  the  space  by  the  velocity)  and  the  quotient  4  is  the 
time  sought. 

Prop.  25. — The  velocity  is  as  the  space  divided  by  the  time. 

For  if  a  body  move  over  12  feet  in  4  seconds,  its  velocity  is  evidently 
3  feet  per  second  or  12-^-4. 

The  velocities  of  two  bodies  may  be  compared,  in  the  same  manner  :  the 
velocities  of  two  bodies  A  and  B,  for  instance,  of  which  A  moves  over  54 


76  APPENDIX. 

• 

feet  in  9  seconds,  and  B,  96  feet  in  6  seconds ;  their  velocities  will  be  as 
6  (54-9)  to  16  (96-f6.*) 

Of  a  constant  force  and  uniformly  accelerated  motion. 

Prop.  26. — The  effect  of  a  constant  force  acting  upon  a  body,  is  to  pro- 
duce in  it  a  uniformly  accelerated  motion,  p.  58. 

For  since  the  effect  of  force  is  to  produce  velocity,  a  constant  force  must, 
in  successive  instants  of  time,  afford  continual  and  equal  additions  to  the 
velocity  of  the  body  it  has  set  in  motion ;  that  is,  the  velocity  will  increase 
in  the  direct  ratio  that  the  body  has  been  moving,  which  is  the  definition  of 
uniformly  accelerated  ti^otion. 

Prop.  27. — In  uniformly  accelerated  motion  the  space  described  is  as  the 
square  of  the  time,  pp.  58,  59. 

Thus  it  is  found  by  experiment,  that  if  a  body  move  with  a  gradually  and 
constantly  increasing  velocity  that  would  carry  it  through  a  mile  in  one 
minute,  that  at  the  end  of  this  time  it  has  acquired  such  a  velocity  as  would 
carry  it  through  two  miles  the  next  minute,  if  the  force  that  communicated 
its  motion  ceased  to  act  at  the  end  of  the  first  minute ;  but  if  the  force  con- 
tinues to  act,  it  acquires  a  velocity  that  would  carry  it  over  an  additional 
mile,  so  that  it  will  pass  over  three  miles  the  second  minute,  or  four  miles 
in  two  minutes.  At  the  end  of  the  second  minute  it  has  acquired  a  velocity 
that  will  carry  it  over  double  the  space  in  the  third  minute,  that  it  moved 
over  in  the  first  two  minutes,  or  a  velocity  of  8  miles  in  2  minutes,  or  4 
miles  a  minute.  But  the  force  still  continuing  to  act,  it  will  move  a  mile 
farther  or  five  miles  in  the  third  minute.  Hence,  if  a  body  acted  upon  by  a 
continued  force  move  a  mile  the  first  minute,  it  will  move  3  miles  the  second, 
5  the  3d,  7  the  4th,  9  the  5th,  &c. 

Thus  the  spaces  described  in  successive  equal  parts  of  time,  by  uniformly 
accelerated  motion,  are  always  as  the  odd  numbers  1,  3,  5,  7,  9,  &c.,  and 
consequently  the  whole  spaces  are  as  the  squares  of  the  times  or  of  the  last- 
acquired  velocities.  For  the  continued  addition  of  the  odd  numbers  yields 
the  squares  of  all  numbers  from  unity  upwards.  Thus  1  is  the  first  odd 
number  and  the  square  of  1  is  1 ;  3  is  the  second  odd  number,  and  this 
added  to  one  makes  4,  the  square  of  2 ;  5  is  the  third  odd  number  and  this 
added  to  4  makes  9,  the  square  of  3  ;  and  so  on  for  ever.  Since,  therefore, 
the  times  and  velocities  proceed  evenly  and  constantly  as  1,  2,  3,  4,  &c.,  but 
the  spaces  described  in  equal  times  are  as  1,  3,  5,  7,  &c.,  it  is  evident  that 
the  space  described, 

In  1  minute  will  be  l=squareofl 

In  2      "          "  1+3=4=       "       2 

In  3      "          "  -  -      •     1+3+5=9=       "       3 

In  4      «  -  1+3+5+7=16=       "       4 

*  For  the  benefit  of  those  who  are  acquainted  with  algebra,  we  subjoin  >he  follow- 
ing equation,  which  expresses  all  the  circumstances  of  uniform  motion. 
Let  t  —  the  time  of  motion, 

*  =  the  space  described  in  the  time  j?, 
v  =  the  velocity; 

Then,  s  =  v  t  from  which  we  obtain 

* 

v  =  — 

t 

s 

and  t  =  — 


APPENDIX.  77 

Of  Gravity. 

Prop.  28. — The  force  which  causes  bodies  to  fall  to  the  earth  is  of  the 
kind  named  constant,  and  is  called  gravity,  p.  58. 

Prop.  29. — The  direction  of  gravity  is  in  lines  perpendicular  to  the  earth's 
surface. 

Prop.  30. — The  force  of  gravity  is  directly  proportional  to  the  mass  of 
the  body. 

For  however  small  the  parts  into  which  we  divide  a  body,  we  find  them  all 
affected  by  gravity,  since  this  force  must  act  upon  all  the  particles  of  a  body. 

Hence,  in  an  unresisting  medium,  all  bodies  setting  out  from  a  state  of 
rest,  fall  through  the  same  space  in  the  same  time,  because  the  force  of 
gravity  acting  upon  them  increases  in  proportion  to  the  mass  to  be  moved. 

Prop.  31. — The  force  of  gravity  decreases,  as  the  square  of  the  distance 
from  the  attracting  body  increases. 

This  is  proved  by  astronomical  observations. 

Motion  produced  by  joint  forces. 

Prop.  32. — When  a  body  is  acted  upon  at  the  same  moment  by  a  plurality 
of  forces,  each  of  these  forces  produces  its  full  effect :  and  the  place  of  the 
body  at  the  end  of  any  given  time  is  the  same  as  it  would  have  been  if  the 
forces  had  acted  in  succession  each  during  that  time,  pp.  55,  56,  57. 

Thus  let  A  B  represent  the  direction  of  a 
force  that  would  move  a  body,  A  the  distance 
from  A  to  B  in  a  certain  interval  of  time,  (a 
second  for  example,)  and  A  0,  the  direction  of 
a  force  that  would  propel  the  same  body  from 
A  to  C  in  the  same  interval  of  time.  Suppose 
the  first  force  acted  alone,  it  would  move  the 
body  from  A  to  B  in  one  second ;  if  the  force  A  C  then  acted  at  B,  by  drawing 
B  R  equal  and  parallel  to  A  C,  B  R  will  represent  the  direction  and  velocity 
of  the  force  A  C,  and  R  the  position  in  which  the  body  would  be  in  at  the 
end  of  the  second  interval  of  time.  Unite  A  and  R  and  the  line  A  R  will 
represent  the  course  of  the  body  A  if  acted  upon  at  the  same  moment  by 
the  two  forces  A'  B  and  A  C,  and  R  the  position  of  the  body  at  the  end  of 
the  first  interval  of  time. 

In  the  same  manner  the  action  of  anynum-  Fig.  19. 

ber  of  forces  may  be  represented.  Thus  let 
A  B,  A  C,  A  D,  A  E,  represent  the  separate 
effects  of  four  different  forces  acting  in  the 
same  plane,  capable  of  moving  a  body  the 
distances  A  B,  A  C,  A  D,  A  E,  in  a  given 
interval  of  time.  Draw  B  c,  c  d,  d  R,  equal 
and  parallell  to  A  C,  A  D,  A  E,  respectively, 
and  join  A  R,  A  B,  c  d  R,  will  represent  the 
path  of  the  body  if  these  forces  had  acted 
successively  each  during  one  interval  of  time, 
and  A  R  the  path  of  the  body  if  they  all  act 
together,  and  R  the  position  of  the  body  at  the  end  of  the  first  interval  of  time. 

Prop.  33. — The  line  A  R  in  figures  given  to  illustrate  the  preceding 
propositions  represents  the  direction  and  measure  of  a  single  force  equivalent 
to  all  the  others  in  each  figure ;  and  hence  the  process  by  which  it  is  deter- 
mined called  the  composition  of  forces,  pp.  55,  56,  57. 


78  APPEXDIX. 

Prop.  34. — Any  force  may  be  decomposed  into  any  number  of  other  forces, 
that  shall  be  equivalent  to  it,  by  the  reverse  of  the  foregoing  operation. 
This  process  is  called  the  Resolution  of  forces,  p.  57. 

Thus  the  force  A  R,  fig.  18,  may  be  separated  into  two  forces  A  B,  A  C, 
and  the  force  A  R,  fig  19,  into  four  forces,  A  B,  A  C,  A  D,  and  A  E. 

Prop.  35. — When  the  forces  act  in  the  same  right  line,  we  have  only,  in 
order  to  ascertain  the  spaces  described  by  their  combined  action,  to  add  or 
substract  the  spaces  which  would  be  described  by  their  separate  action,  ac- 
cording as  these  forces  act,  in  the  same  or  opposite  direction. 

Equilibrium. 

Prop.  36. — A  body  acted  upon  by  a  plurality  of  forces,  in  opposite  direc- 
tions, will  remain  at  rest,  or  in  equilibria;  when  these  forces  were  supposed 
to  act  in  succession  each  during  the  same  interval  of  time,  the  body  would 
arrive  at  its  point  of  departure. 

The  simplest  and  most  evident  case  of  equilibrium  is  that  in  which  a 
body  is  acted  upon  by  two  equal  and  opposite  forces. 

On  the  joint  action  of  an  impulsive  and  a  constant  force. 
A.      When  these  forces  act  in  the  same  manner. 

Prop.  37. — When  the  forces  act  in  the  sam'e  direction,  the  place  of  the 
body  at  the  end  of  any  given  time,  may  be  determined,  as  in  the  problem 
of  the  composition  of  forces,  by  supposing,  first,  that  the  impulsive  force  acts 
during  that  time,  and  then  that  the  action  of  the  constant  force  commences 
and  acts  alone  during  the  same  time :  the  spaces  added  altogether  will  give 
the  space  passed  over  by  the  joint  action  of  these  forces  during  the  assumed 
time. 

Prop.  38. — When  the  forces  act  in  opposite  directions,  the  place  of  the 
body  may  be  ascertained  by  a  similar  process ;  in  this  case,  however,  the 
spaces  are  to  be  substracted  one  from  the  other,  pp.  58,  59. 

When  a  constant  force  is  acting  in  a  direction  contrary  to  that  of  a  moving 
body  set  in  motion  by  an  impulsive  force,  the  retardation  that  the  former 
produces  may  be  determined  by  comparing  the  motion  wifh  that  of  a  body 
moved  by  the  same  force. 

The  degrees  by  which  an  ascending  body  loses  its  motion,  are  the  same 
as  those  by  which  it  is  again  accelerated  at  the  same  points,  when  it  has 
acquired  its  greatest  height  and  again  descends,  for  the  velocity  at  the  corres- 
ponding parts  of  the  ascent  and  descent  are  equal.  Thus  we  may  calculate 
to  what  height  a  body  will  rise  when  projected  upwards  by  an  impulsive 
force,  gunpowder,  for  instance,  and  retarded  by  the  force  of  gravity.  Since 
the  force  of  gravitation  produces  or  destroys  a  velocity  of  32  feet  in  every 
second,  a  velocity  of  320  feet  will  be  destroyed  in  10  seconds;  and  accord- 
ing to  what  has  been  premised,  a  body  will  fall  in  10  seconds  through  a  hun- 
dred times  16  feet  or  1600  feet,  which  is  therefore  the  height  to  which  a 
velocity  of  320  feet  in  a  second  will  carry  a  ball  projected,  without  resistance 
from  other  cause  than  gravity, 'in  a  vertical  direction,  p.  60. 

B.      When  these  forces  act  in  different  directions. 
*  When  the  successive  directions  of  the  constant  forces  are  parallel. 
Prop.  39. — If  the  constant  force  be  that  of  gravity,  the  successive  direc- 


APPENDIX. 


79 


Fig.  20. 


tions  of  which  are  assumed  to  be  parallel,  the  investigation  of  the  effects 
produced  constitutes  the  doctrine  of  projectiles;  a  projectile  being  a  body 
thrown  in  any  direction  by  an  impulsive  force  and  at  the  same  time  acted 
upon  by  the  force  of  gravity,  pp.  59,  60. 

Prop.  40. — The  place  of  a  projectile  at'  the  end  of  any  given  time  may 
be  determined  as  in  the  problem  of  the  composition  of  forces,  by  supposing 
first  that  the  impulsive  force  alone  has  acted  during  that  time,  and  then  that 
the  action  of  gravity  commences,  and  acts  alone  during  the  same  time. 

Thus  let  A  H  represent  a  hori- 
zontal plane,  and  A  B  the  initial 
direction  and  velocity  of  a  body 
projected  from  the  point  A  in 
the  same  plane.  If  the  impul- 
sive force  alone  acted  on  the 
body  it  would  describe  the  path 
ABB' B"  B'" &c.  with  uniform 
velocity.  But  as  the  force  of 
gravity  acts  from  the  moment  of 
projection,  the  body  will  be 
drawn  downwards  from  the  line 
A  B'"  so  as  to  be  found  after  the 
successive  intervals  of  time,  at 
the  points  g  g'  g",  &c.,  and  as 
the  force  of  gravity  produces  a 
velocity  which  increases  as  the 

squares  of  the  distances^  if  the  distances  A  B,  B  B',  B'  B",  B"  B'"  be 
equal,  B  g,  B'  g',  B"  g",  B"'  g"',  &c.,  will  be  as  the  squares  of  these  dis- 
tances7  and  the  path  of  the  projectile  through  the  points  g  g'  g"  g'"  will 
be  a  curve,  and  this  curve  mathematicians  have  called  a»parabola. 

**  When  the  successive  directions  of  the  constant  force  tend  to  a  common 

centre. 

Prop.  41. — This  case  constitutes  the  doctrine  of  central  forces,  see  prop. 
13,  p.  75. 

Prop.  42. — The  place  of  the  body  at  the  end  of  any  given  time  may  be 
determined  here  also  by  the  problem  of  the  resolution  of  forces. 

Thus,  suppose  A  represent  a  body  impelled 
towards  H  with  such  a  force,  as,  by  itself,  would 
enable  it  to  run  over  the  equal  spaces  A  B,  B  F, 
F  G-,  &c.,  in  equal  portions  of  time  :  suppose  like- 
wise that  it  is  acted  upon  the  same  time  by  con- 
stant force,  which  would  enable  it  to  pass  over  the 
unequal  spaces  A  I,  I  K,  K  L,  &c.,  in  the  same 
equal  portions  of  time.  It  is  evident,  that  the 
joint  action  of  both  these  forces  would  compel  the 
body  A  to  pass  over  the  curvilinear  path  A  N  0 
P,  &o.  Through  B  draw  the  line  B  C,  (viz.,  in 
the  centre  of  attraction);  through  I  draw  I N  par- 
allel to  A  B ;  and  at  the  end  of  the  first  portion  of 
time  the  body  will  be  found  at  N,  whence  it  would 
proceed  in  the  straight  direction  N  R  (by  the  first 
law  of  motion),  if  the  constant  force  then  ceased 


Fig.  21. 


80  APPENDIX. 

to  act.  But  as  this  force  continues  to  act,  the  body  at  the  end  of  the  second 
portion  of  time  will  be  found  in  0 ;  for  the  like  reason,  at  the  end  of  the 
third  portion  of  time,  it  will  be  found  in  P,  and  so  on.  The  course  then  A 
N  0  P  is  not  straight,  but  consists  of  the  lines  A  N,  N  0, 0  P,  forming  certain 
angles  with  each  other.  Now  it  will  not  be  difficult  to  conceive  that,  because 
the  attractive  force  acts  not  by  intervals  but  constantly  and  unremittedly, 
the  real  path  of  the  body  must  be  a  polygonal  course,  consisting  of  an  infi- 
nite number  of  sides;  or  more  justly  speaking,  a  continuate  curved  line,  which 
passes  through  the  points  A,  N,  0,  P,  &c.,  as  is  shown  by  the  dotted  line. 

Prop.  43. — Should  the  action  of  the  centripetal  force  cease  at  any  instant, 
the  body  would  proceed  straight  forward,  p.  49. 

The  portion  of  the  impulsive  force  by  which  this  is  affected  is  called  the 
centrifugal,  prop.  14. 

Prop.  44. — Whilst  the  distance  from  the  centre  remains  unchanged,  as 
when  the  body  moves  in  a  circular  orbit,  the  centripetal  and  centrifugal 
forces  are  equal. 

Laws  of  Central  forces. 

Prop.  45. — When  bodies  revolve  in  equal  circles,  their  centrifugal  forces 
are  proportional  to  the  squares  of  their  velocities. 

Prop.  46. — When  two  bodies  revolve  with  equal  velocities  at  different 
distances,  the  centrifugal  forces  are  inversely  as  the  distances. 

Consequently  (prop  45,46)  the  centrifugal  forces  are  in  all  cases  directly 
as  the  squares  of  the  velocities,  and  inversely  as  the  distances. 

Prop.  47. — When  two  bodies  revolve  in  equal  times  at  different  distances, 
their  centripetal  forces  are  simply  as  their  distances. 

In  general,  the  centripetal  forces  are  as  the  distances  directly  and  as  the 
squares  or  the  times  of  revolution  inversely. 

Prop.  48. — WheR  the  forces  vary  inversely  as  the  squares  of  the  distances, 
as  in  the  case  of  gravitation,  the  squares  of  the  times  of  revolution  are  pro- 
portional to  the  cubes  of  the  distances. 

Thus,  if  the  distance  of  one  body  be  four  times  as  great  as  that  of  another, 
the  cube  of  4  being  64,  which  is  the  square  of  8,  the  times  of  its  revolution 
will  be  8  times  as  great  as  that  of  the  first  body. 

Prop.  49. — Where  the  orbit  deviates  more  or  less  from  a  circular  form,  a 
right  line  joining  the  revolving  body  and  its  centre  of  attraction,  always  de- 
scribes equal  areas  in  equal  times,  and  the  velocity  of  the  body  is  therefore 
always  inversely  as  the  perpendicular  drawn  from  the  centre  to  the  tangent ; 
and  the  velocity  at  any  point  less  than  three-eighths,  greater  than  that  neces- 
sary to  make  the  body  describe  a  circle. 

Prop.  50. — To  propel  a  body  in  an  elliptical  orbit,  the  force  directed  to 
its  focus  must  be  inversely  as  the  square  of  the  distance. 

This  is  proved  by  astronomical  observations,  but  we  have  no  other  proof  of  it. 

The  motion  of  the  planets  round  the  sun  in  the  solar  system  is  governed 
by  the  laws  of  central  forces,  the  centripetal  force  in  this  case  being  that  of 
gravity. 

On  the  joint  effect  of  active  and  inactive  forces. 
A.    When  they  have  opposite  directions. 

Prop.  51. — The  effect  of  passive  forces  is  to  restrain  and  modify  the  action 
of  other  forces  so  as  to  confine  the  motion  of  a  body  to  a  particular  course  or 
path,  and  the  direction  of  the  passive  force  affecting  a  body  at  any  moment 


APPENDIX 


81 


s  the  line  perpendicular  to  that  part  of  this  path  at  which  the  body  is  found 
at  this  moment.  If  the  direction  of  the  active  force  be  also  perpendicular 
to  this  path,  the  body  must  evidently  remain  at  rest,  since  no  part  of  this 
force  can  be  resolved  into  the  direction  of  the  path  in  which  alone  the  body 
can  move. 

B.    When  they  have  different  directions. 

General  rule. 

Prop.  52. — Resolve  the  active  force  into  two,  one  perpendicular,  and  the 
other  a  tangent  to  the  path  of  the  body,  the  effect  of  the  former  force  will  be 
entirely  destroyed  (prop.  51,)  and  the  body  will  advance  by  the  latter  alone, 

,       *  On  the  motion  of  a  body  impelled  obliquely  against  a  plane. 

Prop.  53.— Let  M  N  represent  the  plane,  Fig.  22 

and  A  B  the  direction  and  velocity  from  the 
impulsive  force,  resolve  A  B  into  the  forces 
A  C  perpendicular  to  the  plane,  and  C  B  in 
its  direction,  then  by  the  general  rule  (prop. 
52)  the  body  will  move  along  the  plane  witlra 
velocity  of  which  C  B  is  the  measure. 

K   On  the  motion  of  a  body  impelled  obliquely  against  a  curved  surface. 

Prop.  54. — Let  M  N  represent  the  curve  and  Fig.  23. 

A  B  the  direction  and  velocity  from  the  impul- 
sive force.  Resolve  A  B  into  two  forces,  C  B 
perpendicular  to  the  curve  at  B,  and  B  D  (equal 
to  A  C)  a  tangent  to  the  curve  at  the  same  point. 
Then  B  D  will  represent  the  velocity  at  the  point 
B. 

Prop.  55. — If  the  curve  be  interrupted  at  any 
point,  or  change  the  direction  of  its  concavity,  the  body  will  advance  with 
its  last  velocity  in  a  tangent  to  the  curve  at  that  point. 

***  On  the  descent  of  a  body  along  an  inclined  plane. 

Prop.  56. — Let  M  N  represent  an  in- 
clined plane  and  A  B  (perpendicular  to  the 
horizontal  base  H  NJ  the  force  of  gravity 
as  measured  by  the  distance  which  would 
cause  a  body  to  descend  in  the  first  second 
of  time.  Resolve  A  B  into  two,  A  C,  per- 
pendicular to  the  plane,  and  C  B  in  its 
direction,  then  the  body  will  be  urged 
down  the  plane  by  the  constant  force 
measured  by  C  B. 

Laws  of  the  descent  of  bodies  down  inclined  planes. 

Prop.  57. — 1st.  The  motion  of  a  body  drawn  down  an  inclined  plane  is 
uniformly  accelerated. 

Prop.  58. — 2d.  The  velocity  acquired  is  proportional  to  the  perpendicular 

6 


82  APPENDIX.. 

descent,  so  that  a  body  falling  from  M  to  H  has  the  same  velocity  at  H  as 
one  descending  the  whole  length  of  the  plane  at  N. 

Prop.  59. — 3d.  The  times  of  descent  down  planes  of  the  same  heights 
are  as  their  lengths. 

Prdp.  60. — 4th.  The  times  of  descent  down  all  planes  which  are  cords 
drawn  to  the  lowest  point  of  the  same  circle,  are  equal. 

Thus,  if  the  balls  A,  B,  C,  be  placed  at  different 
Fig-  25«  points  of  the  circle  and  suffered  to  descend  at  the 

same  instant  along  as  many  planes  which  meet  at 
the  lowest  point  of  the  circle,  they  will  arrive  there 
at  the  same  time. 

Or  it  may  be  enunciated  in  the  following  terms  : 
the  times  of  descent  down  all  the  cords  drawn  from 
the  same  point  or  circumference  of  a  circle  will  be 
the  same. 

This  will  be  made  evident  by  supposing  the  above 
figure  inverted,  D  being  made  the  upper  point  and  the 
balls  allowed  to  fall  from  that  point  to  A,  B,  and  C.* 

****  On  the  descent  of  a  body  down  a  vertical  curved  line. 

Prop.  61. — The  times  of  descent  down  the  cords  of  different  circles  are 
to  each  other  as  the  square  roots  of  their  diameters. 

Prop.  62. — If  a  body  fall  from  a  state  of  rest  down  a  curve,  the  velocity 
acquired  is  equal  to  that  which  it  would  have  by  falling  through  the  same 
perpendicular  height. 

For  if  the  curve  be  considered  as  made  up  of  an  infinite  number  of  con- 
tiguous planes,  it  is  evident  that  the  angle  of  inclination  of  any  two  of  these 
adjacent  planes  is  infinitely  small,  or  nothing,  and  consequently  there  is  no 
velocity  lost  by  a  change  of  direction  in  passing  from  one  to  the  other. 
Therefore,  as  the  effect  of  gravity  is  not  impeded,  the  truth  of  the  proposi- 
tion becomes  evident. 

Prop.  63. — If  a  body  be  projected  up  a  curve,  the  perpendicular  height  to 
which  it  will  rise  is  equal  to  that  through  which  it  roust  fall  to  acquire  the 
velocity  of  projection.  For  the  body  in  its  ascent  will  be  retarded  in  the 
same  degree  that  it  was  accelerated  in  its  descent* 

Thus,  let  B  A  B'  be  a  curve  in  which  the  lowest 
Fig.  26.  point  is  A,  and  the  parts  A  B,  A  B'  are  similar ; 

a  body  in  falling  down  B  A  will  acquire  a  velocity 
that  will  carry  it  to  B',  and  since  the  velocities  in 
all  equal  altitudes  in  the  ascent  and  descent  are 
equal,  the  times  of  ascent  and  descent  are  equal. 

The  foregoing  proposition  is  equally  true  whether 
the  body  actually  move  over  a  solid  surface  or  be 
retained  in  its  path  by  a  string  which  is  in  every  part  perpendicular  to  it. 

Of  the  simple  Pendulum. 

Prop.  64. — The  simple  pendulum  is  conceived  to  be  a  mere  material  point 
suspended  by  an  imponderable  and  inextensible  thread,  p.  60. 

Prop.  65. — If  the  simple  pendulum  vibrates  through  very  small  arcs, 
these  may,  without  sensible  error,  be  conceived  to  coincide  with  their  chords, 
and  we  may  derive  from  this  consideration  the  following  theorems : 

1st.  As  the  times  of  descent  of  the  body  down  different  chords  of  the  same 


APPENDIX.  83 

verticle  circle  are  equal  (prop.  60.)  the  vibrations  of  the  same  pendulum, 
although  performed  through  unequal  arcs,  will  be  very  nearly  equal, 
p.  61. 

2d.  The  times  of  vibrations  of  different  pendulums  will  be  to  each  other 
as  the  square  roots  of  the  lengths  of  these  pendulums,  or,  which  is  the  same 
thing,  their  lengths  are  proportioned  to  the  squares  of  the  times  of  vibration, 
p.  61. 

The  times  of  descent  down  the  chords  of  different  circles  are  the  same  as 
would  be  occupied  in  descending  vertically  through  their  diameters,  and  are 
consequently  proportional  to  the  square  roots  of  these  diameters. 

Of  the  impact  of  bodies. 

Prop.  66. — When  a  body  in  motion  strikes  directly  another  body,  it  always 
communicates  motion  to  the  second  body,  and  loses  part  of  its  own,  and  from 
the  third  law  of  motion  it  is  evident  that  the  momentum  gained  by  the 
second  body  is  exactly  equal  to  that  lost  by  the  first. 

Prop.  67. — When  one  non-elastic  body  strikes  against  another,  the  two 
bodies  will  move  on  together  since  there  is  no  force  to  separate  them ;  and  as 
one  of  the  bodies  gains  all  the  momentum  which  the  other  loses,  the  momen- 
tum after  impact  will  be  equal  to  the  sum  of  the  momentum  before  impact. 

Prop.  68. — When  an  elastic  body  strikes  against  another,  the  second  is 
impelled  forward  with  double  the  momentum  which  it  would  have  received 
under  the  same  circumstances  if  non-elastic. 

For  at  the  moment  of  impact  the  form  of  the  body  struck  is  changed  by  a 
force  equivalent  to  the  momentum  which  it  receives  from  the  striking  body, 
and  if  this  body  be  perfectly  elastic,  its  form  will  be  restored  to  it  by  a  force 
exactly  equal  to  that  by  which  it  was  changed,  and  this  force  (which  we 
have  just  seen  to  be  equal  to  the  original  impulse,)  will  be  exerted  in  driving 
the  body  forward.  The  body  thus  receives,  besides  its  original  impulse,  the 
equal  force  of  the  re-bound. 

Prop.  69. — The  striking  body,  when  elastic,  is  also  acted  upon  by  the 
rebound,  and  loses  twice  as  much  momentum  as  it  would  have  lost  if 
non-elastic. 

In  this  case,  as  in  the  former,  the  sum  of  the  momenta  is  the  same  after 
impact  as  before  it ;  but  the  bodies  after  impact  do  not  move  on  together. 

Prop.  70. — If  an  elastic  body  strike  against  a  firm  plane,  the  angle  of 
reflection  will  be  equal  to  the  angle  of  incidence,  p.  66. 


84  MECHANICS. 

PART    II. 

PHENOMENA   OF  SOLIDS. 

THE  FOUR  FUNDAMENTAL  TRUTHS  USED  TO  EXPLAIN  THE  PECULIARI- 
TIES OF  STATE  AND  MOTION  WHICH  DEPEND  ON  THE  SOLID  FORM  OF 
BODIES  j  A  DEPARTMENT  COMMONLY  CALLED  MECHANICS. 

ANALYSIS   OF    THE   CHAPTER.* 

A  force,  which  moves  part  of  a  solid  body,  must  effect  the  whole  or  break 
off  the  part. 

If  the  force  be  directed  towards  a  certain  central  point  in  the  mass,  it  will 
effect  the  whole  equally,  whether  simply  to  support  the  mass,  or  to  move  it 
or  to  stop  it  when  in  motion.  The  point,  according  to  circumstance,  is 
called  THE  CENTRE  OF  GRAVITY  OF  INERTIA,  OT  OF  ACTION. 

In  solid  bodies  moving  about  an  axis,  as  exemplified  in  a  wheel  or  weigh- 
ing beam  the  various  parts  describe  circles  or  move  through  spaces  which 
are  greater  in  proportion  to  their  respective  distances  from  the  centre  of 
motion.  Hence  forces  differing  as  to  speed,  may  still,  through  a  solid 
medium,  be  brought  exactly  to  co-operate  or  to  oppose  one  anotJier  j  a  slow 
force  counter -balancing  or  being  equivalent  to  a  quicker  one,  provided 
tliat  it  be  more  intense  in  proportion  as  it  is  slower.  The  SIMPLE 
MACHINES,  or  MECHANICAL,  MEDIA  Called  LEVER,  WHEEL  AND  AXLE, 
PULLEY,  INCLINED  PLANE,  WEDGE,  SCREW,  &c.,  are  so  many  arrange- 
mentsof  solid  parts,  by  which  forces  of  different  velocities  and  intensities 
maybe  thus  connected  or  opposed,  or  may  be  conveniently  substituted  one 
for  another . 

By  solid  connecting  parts,  also,  the  direction  of  any  existing  motion  or  force 
may  be  changed,  as  when  the  straight  motion  of  running  water  is  con- 
verted into  the  rotary  motion  of  a  water-wheel,  &c.  Hence  arises  an  end- 
less variety  of  COMPLEX  MACHINES. 

In  all  machines,  an  important  circumstance  to  be  considered  is  the  resist- 
ance among  moving  parts  which  arises  from  FRICTION  : — and  in  solid 
structures  generally,  the  forms  and  positions  of  parts  have  to  be  adjusted 
to  the  STRENGTH  OF  THE  MATERIALS,  and  to  the  strains  which  the  parts 
have  to  bear. 

"  Solid"  is  the  term  applied  to  a  mass  in  which  the  mutual  attraction  of 
the  atoms  is  so  strong,  that  the  mass  may  be  moved  about  as  one  body,  with- 
out the  relative  positions  of  the  component  parts  being  thereby  disturbed. 

"  Force  moving  part  of  a  solid  must  effect  the  whole  or  break  off  the  part." 

This  is  a  necessary  consequence  of  the  description  or  definition  of  a  solid 
just  given.  And  it  follows  that  in  all  cases  of  breaking,  the  cohesion  of  the 

*  The  reader  .should  here  re-peruse  the  general  table  or  synopsis  at  page  19. 


CENTRE    OF    GRAVITY.  85 

atoms  at  the  fractured  part  must  have  been  less  strong  than  the  weight  of 
the  remaining  mass,  or  its  inertia  resisting  the  degree  of  change  attempted,  or 
the  force  fixing  it  to  its  place,  or  than  some  combination  of  these  particulars. 

The  sharp  blow  of  a  hammer  given  to  'an  ivory  ball,  causes  it  to  dart  off 
swiftly,  but  does  not  injure  it,  because  the -cohesion  among  the  atoms  struck 
is  stronger  than  the  opposing  inertia  of  the  mass,  even  under  a  rapid  change ; 
but  the  blow  of  a  hammer  on  a  large  elephant's  tusk  indents  or  breaks  the 
part  because  the  opposing  inertia  of  the  larger  mass  is  stronger  than  the 
cohesion  of  the  atoms  which  receive  the  blow. 

A  vessel  of  pottery- ware  may  be  safely  suspended  by  its  handle ;  proving 
that  the  cohesion  which  fixes  the  handle  to  it  is  stronger  than  the  weight  of 
the  vessel  ;  but  if  the  attempt  be  made  to  lift  the  vessel  quickly,  the  handle 
may  rise  and  leave  the  vessel  behind ;  because  then  the  weight  and  inertia 
are  acting  together  to  destroy  the  cohesion.  Thus  servants  attempting  to 
lift  too  quickly  the  loaded  stone-ware  dishes  at  a  dinner-table,  often  break 
off  the  part  by  which  they  take  hold. 

Centre  of  Gravity  or  Inertia. 

If  any  uniform  beam  or  rod  be  supported  by  its  middle,  like  a  weighing 
beam,  the  two  ends  will  just  balance  each  other.  This  is  in  accordance  with 
the  general  truth  or  law  of  attraction  already  explained;  for  as  there  is  just 
as  much  similarly  situated  matter  on  one  side  of  the  support  as  on  the  other, 
there  will  also  be  just  as  much  attraction,  and  therefore  no  reason  why  the 
matter  on  one  side  should  overpower  that  on  the  other.  If  equal  weights  be 
afterwards  attached  in  corresponding  situations  on  the  two  arms  of  the  beam 
the  balance  will  not  be  thereby  disturbed;  and  the  operation  of  adding 
weights  that  counterpoise,  above  and  below,  and  near  and  far  from  the  centre 
may  be  continued,  until  a  bulky  mass  is  built  up  upon  the  beam — and  in- 
stead of  a  beam  a  wheel  may  be  used — yet  the  whole  will  remain  perfectly 
supported  and  in  equilibrium  about  the  original  centre.  In  the  pages  now 
to  follow,  it  will  be  shown  that,  in  every  body  or  mass,  or  system  of  con- 
nected masses,  in  the  universe,  there  is  a  point  of  this  kind  about  which  all 
the  parts  balance  or  have  equilibrium,  and  it  is  this  point  which  is  called 
the  centre  of  gravity  or  of  inertia.  Although  in  any  mass,  therefore,  every 
atom  has  its  separate  gravity  and  inertia,  and  the  weight  and  inertia  of  the 
whole  are  really  diffused  through  the  whole,  still  by  supporting  this  one 
point,  either  from  above  or  from  below,  the  whole  mass  is  equally  supported ; 
by  lifting  it,  the  whole  is  lifted ;  by  stopping  it,  the  whole  is  brought  to 
rest ;  and  when  it  rises  or  falls,  the  general  mass  is  really  rising  or  falling. 
Thus  for  many  purposes,  a  body,  however  large,  may  be  considered  as  com- 
pressed into  or  existing  only  in  the  single  point  called  its  centre  of  gravity 
or  of  inertia. 

This  centre  in  a  mass  of  regular  shape  and  of  uniform  substance,  as  a  ball 
or  cube  of  metal,  is  easily  found,  because  it  is  the  evident  centre  of  the 
form ;  but  in  bodies  that  are  irregular,  either  as  to  density  or  form,  it  must 
be  found  by  rules  of  calculation  hereafter  explained. 

To  say  that  the  centre  of  gravity  will  always  take  the  lowest  situation 
which  the  support  of  the  body  will  allow,  is  only  to  repeat,  that  bodies  tend 
by  their  gravity  towards  the  centre  of  the  earth.  In  a  suspended  body,  there- 
fore, as  the  lowest  situation  which  the  centre  of  gravity  can  find  is,  when  it 
is  immediately  under  the  point  of  suspension,  all  bodies  hanging  freely  must 
have  their  centre  of  gravity  directly  under  that  point.  A  plummet  is  an 
interesting  example  of  this;  and  the  truth  furnishes,  in  many  cases  of  irre- 
gular masses,  a  very  simple  practical  mode  of  finding  the  centre. 


86 


MECHANICS. 


Thus  if  an  irregular  piece  of  plank  or  of  pasteboard,  represented  here  by 
the  figure  a  e  b  d,  be  suspended  from  any  point,  as  a,  and 
the  cord  of  a  plummet  a  g  be  attached  at  the  same  point,  the 
centre  of  gravity  of  the  board  must  be  somewhere  in  the 
direction  of  the  plummet,  and  a  chalk  line  left  on  the  board 
where  the  cord  touched  it,  must  pass  over  the  centre  of 
gravity.  If  the  board  be  then  suspended  by  another  point, 
as  d,  and  another  chalk  line  d  e  be  made  in  the  same  man- 
ner, the  place  c,  where  the  two  lines  cross  or  cut  each  other, 
will  indicate  the  centre  of  gravity ;  and  the  board  when  sup- 
ported by  a  cord  attached  there,  will  hang  evenly  balanced. 
The  following  cases  further  illustrate  the  truth,  that  the 
centre  of  gravity  always  seeks  the  lowest  place.  They  seem 
at  first  to  be  exceptions  to  the  law ;  but  when  more  fully 
considered,  are  interesting  proofs  of  it. 

A  wooden  cylinder  or  roller  e  d  c,  placed  on  a  slope 
or  inclined  plane  a  b,  will  naturally  descend,  because 
its  centre  of  gravity  is  thereby  approaching  the  earth ; 
but  if  there  be  a  heavy  mass  of  lead  c  introduced  at  one 
side,  which  must  rise  before  the  roller  can  descend,  the 
rise  of  the  mass  being  contrary  to  gravity,  the  motion 
will  be  arrested.  Indeed  if  the  roller  were  placed  on 
the  plane  with  the  lead  in  the  position  d,  the  lead  would 
fall  down  to  the  position  c,  and  so  would  move  the  roller  towards  &,  exhibit- 
ing the  singular  phenomenon  of  a  body  rolling  up  hill  by  the  action  of  its 
weight. 

If  a  billiard-ball  be  placed  upon  the  small  ends  of  two  billiard  sticks  or  cues 

a  b  and  c  d,  laid  on  a  table  with 
their  points  c  and  a  in  contact,  but 
with  the  larger  ends  b  and  d  so  far 
apart  that  there  may  be  just  room 
for  the  ball  to  touch  the  table  be- 
'tween  them,  the  ball  will  roll  along 
between  the  cues,  sinking  gradu- 
ally from  its  high  situation  near 
their  points,  to  its  lower  situation  near  b.  To  a  careless  observer,  it 
would  then  have  the  appearance  of  rolling  upwards,  because  the  cues  on 
which  it  rests  are  thicker  towards  the  end  d  and  b ;  but  it  would  really  be 
descending  in  obedience  to  gravity.  If  a  double  cone,  as  represented  at/, 
were  substituted  for  the  ball,  it  would  similarly  roll  from  c  to  e,  and  with  still 
niore  of  the  fallacious  appearance  of  rolling  upwards,  because  its  ends  would 
always  be  resting  on  the  upper  and  rising  surfaces  of  the  cues. 

The  board  or  stick  c  d  resting  on  the  edge  of 
the  table  a  b  would  naturally  fall  if  left  to  itself, ' 
because  more  than  half  of  it  is  beyond  the  edge 
of  the  table ;  but  strange  to  say,  an  additional 
weight  e  attached  to  its  projecting  part  as  at  b 
by  the  cord  b  e,  instead  of  pulling  it  down  fast- 
er, shall  fix  or  steady  it  on  the  table,  provided 
the  weight  be  pushed  inwards  a  little  by  a  rod 
d  e  resting  against  it  and  against  a  niche  in  the 
stick  at  d.  It  is  evident  that  the  stick  c  d,  in 
falling,  must  turn  round  the  edge  of  the  table  at 


Fig.  30. 


CENTRE    OF    GRAVITY. 


87 


b;  but  in  so  doing,  after  the  arrangement  now  supposed,  it  must  lift  the 
weight  e  along  the  path  e  f- — which  rise,  as  the  weight  is  heavier  than  the 
stick  (that  is  to  say,  as  the  common  centre  of  gravity  of  the  connected  objects 
is  near  e, )  gravity  forbids,  and  therefore  the  stick  and  weight  will  both  remain 
supported  by  the  table.  •  An  umbrella  or  walking  cane,  hanging  on  the  edge 
of  a  table  by  a  crooked  handle,  is  another  instance  of  the  same  kind.  And 
the  common  toy  of  a  little  man  standing  on  tiptoe  upon  the  top  of  a  pillar, 
and  supporting  two  leaden  bullets  by  wires  descending  from  his  hands,  is 
another  combination  of  parts  which  places  the  centre  of  gravity  of  the  whole 
the  support,  making  the  combination  a  kind  of  pendulum. 

By  attending  to  the  centre  of  gravity  of  the  bodies  around  us  on  earth,  we 
are  enabled  to  explain  why,  from  the  influence  of  gravity,  some  of  them  are 
stable 'or  firmly  fixed,  others  tottering,  others  falling. 

If  we  find  that  a  body,  from  its  form  or  position,  cannot  be  overturned 
without  its  centre  of  gravity  being  lifted, — knowing  now  that  the  general 
mass  is  then  lifted  in  the  same  degree,  we  see  why  a  weak  cause  cannot  effect 
the  change.  The  rise  of  the  centre  of  gravity,  or  body,  in  any  case  of  falling 
over  when  the  centre  of  gravity  is  over  the  middle  of  the  sustaining  base, 
will  be  proportioned  to  the  breadth  of  the  base  of  the  body,  compared  with 
the  height  of  the  centre  of  gravity  above  the  base.  This  is  shown  in  the 
annexed  figures  of  which  the  two  particulars  of  base  and  height &rv  combined 


in  a  series  of  proportions.  In  the  figures,  the  dot  c  marks  the  place  of  the  cen- 
tre of  gravity,  and  the  curved  line  beginning  from  the  dot  marks  the  path 
of  the  centre  of  gravity,  when  the  body  is  overturned.  This  curved  line  is  a 
portion  of  a  circle  which  has^the  edge  or  extremity  of  the  base  (Z>,  in  fig.  A.) 
as  a  centre,  because  the  body  in  turning  must  rest  upon  such  extremity  or 
corner  as  the  centre  of  its  motion.  The  farther  inwards,  therefore,  from  this 
extremity  that  the  centre  of  gravity  is,  as  marked  by  where  a  plumb-line  as 
p,  hanging  from  it,  crosses  the  base,  the  farther,  of  course,  is  the  centre  of 
gravity  from  the  top  of  the  circle  which  it  has  to  describe  in  moving,  and  the 
steeper,  consequently,  will  be  its  commencing  path  j  and  as  in  the  case  of 
bodies  made  to  roll  up  slopes,  the  steeper  the  ascent,  the  greater  will  be  the 
force  necessary  to  give  motion. — The  line  of  a  plummet  hanging  from  the 
centre  of  gravity  is^called  the  line  of  direction  of  the  centre,  or  that  in 
which  it  tends  naturally  to  descend  to  the  earth. 

In  fig.  A,  which  has  a  broad  base  and  little  height  of  the  centre  of  gravity, 
we  see  that  the  centre  must  rise  almost  perpendicularly  before  it  can  fall 
over,  and  the  resistance  to  overturning  is  therefore  nearly  equal  to  the 
whole  weight  of  the  body.  Hence  the  firmness  of  a  pyramid. 

In  figures  B,  C,  and  D,  progressively,  the  commencing  path  of  the  centre 
is  less  steep,  because  the  base  is  narrower,  and  hence  the  bodies  are  so  much 
the  less  stable.  B,  may  represent  an  ordinary  house,  C,  a  tall  narrow  house, 
and  D.  a  lofty  chimney. 


88 


MECHANICS. 


Fig.  E,  shows  a  tottering  position,  for  the  centre  of  gravity  being  directly 
over  a  base  which  is  a  mere  point,  the  least  inclination  places  it  on  a 
descending  slope,  and  the  body-  must  fall. 

Fig.  32. 


K. 


In  F,  the  position  is  tottering  on  one  side,  and  stable  on  the  other.  This 
explains  how  the  least  inclination  of  a  standing  body  virtually  narrows,  in 
one  direction,  its  sustaining  base. 

In  Gr,  which  represents  a  ball  upon  a  level  plane,  the  whole  mass  is  sup- 
ported on  a  single  point,  as  in  E,  yet  the  body  has  no  tendency  to  move, 
because,  in  any  other  possible  position,  the  centre  would  still  be  as  far  from 
the  sustaining  plane.  In  moving,  the  centre  describes  the  straight  level- 
line  a  b. 

In  H,  the  ball  is  on  an  inclined  plane,  and  rolls  down,  the  centre  of 
gravity  describing  the  oblique  line  b  a. 

In  I,  which  is  an  oval  body  resting  on  a  level  plane,  when  the  body  is 
moved  to  either  side,  the  centre  of  gravity  must  rise,  as  in  the  case  of  a  pen- 
dulum. Hence  an  oval  body  on  a  level  will  rock  or  vibrate  like  a  pendulum. 

K,  is  a  true  pendulum,  whose  centre  of  gravity  describes  the  curve  here 
shown,  as  explained  in  Section  II.,  at  page  60. 

The  importance  of  the  subject  of  the  centre  of  gravity  will  be  farther  judged 
of  by  the  facts  which  are  now  to  be  reviewed. 

A  cart  loaded  with  metal  or  stone  may  go  safely  along  a  road  of  which  one 
side  is  higher  than  the  other,  as  here  shown,  but  were  the  same  cart  loaded 
with  wool  or  hay  it  would  be  overturned ;  because,  although  the  sustaining 
base  be  the  same  in  the  two  cases,  the  line  of  direction  falls  much  within  it 
from  the  low  centre  of  gravity  of  the  metal  at  (^but  falls  very  near  the  wheel 
at  P,  or  altogether  on  the  outside,  from  the  high  centre 
of  the  wool  at  a,  and  in  the  latter  case  the  centre  has 
offered  to  it  a  descending  path. 

This  explains  why  lofty  stage  coaches  or  vans  are  so 
dangerous,  and  particularly  when  heavy  luggage  is 
placed  on  the  top,  and  why  lofty  gigs  and  curricles 
have  led  to  so  many  fatal  accidents.  As  regards  any 
of  these,  a  defect  of  smoothness  or  of  level  in  the  road, 
or  even,  in  a  case  of  quick  driving,  a  slight  lateral 
bend  often  suffices  to  produce  the  catastrophe.  The 
safety-coaches  of  late  times  are  made  with  the  wheels 
far  apart  to  give  a  broad  base,  and  with  the  luggage 
receptacles  and  seats  for  outside  passengers  placed  low  down  before  and 
behind  the  body  of  the  carriage,  instead  of  on  the  top,  as  formerly. 

The  feet  of  tripods  are  generally  expanded  below  to  give  a  broad  base. 
The  same  is  true  of  our  common  chairs  ;  but  a  thoughtless  child  often  leans 
so  far  over  the  back  of  the  chair,  that  he  causes  the  line  of  the  general  centre 
of  gravity  to  fall  beyond  the  base,  and  the  chair  with  its  load  is  overturned. 


Fig.  33. 


CENTRE    OF    GRAVITY.  89 

The  small  lofty  chairs  made  to  raise  children  to  the  parents'  elbow  at  the 
dinner-table,  are  very  dangerous  if  the  feet  are  not  made  to  spread  much. 
Pillar-and-claw  tables,  candle-sticks,  table-lamps,  and  many  other  articles  of 
household  furniture,  have  stability  given  in  the  same  manner. 

The  least  inclination  of  a  standing  body  virtually  narrows  the  supporting  base. 

This  truth  is  explained  by  jig.  F.  It  shows  the  necessity  of  building  the 
thin  walls  and  tall  chimneys  of  modern  houses  perfectly  upright.  And  hence 
the  extreme  importance  and  utility  of  that  simple  instrument,  the  plummet 
or  plumb-line,  which,  when  applied  to  a  body,  is  a  visible  indication  of  the 
line  of  its  centre  of  gravity.  The  mason  and  many  other  workmen  cannot 
proceed  a  step  without  their  guiding  plummet. 

The  brick  walls  of  ordinary  houses  are  so  thin,  that,  to  have  standing 
strength,  they  require  to  rest  against  one  another ;  and  hence  they  occasion- 
ally exhibit  the  kind  of  stability  which  belongs  to  a  child's  house  built  of 
cards.  As  contrasted  with  the  masses  of  masonry  which  remain  to  us  from 
antiquity,  resting  on  firm-spreading  basements,  they  are  examples  of  what 
is  truly  ephemeral,  in  comparison  with  that  which  has  partaken  of  the  per- 
manency of  nature's  own  works,  covering  regions  with  mighty  ruins.  What 
magnificent  illustrations  of  strength  and  durability,  dependent  on  propor- 
tions, are  those  ancient  pyramids  and  temples,  which  still  give  such  interest 
to  the  banks  of  the  Nile,  and  to  the  valleys  and  plains  of  Asia ! 

There  are  many  remarkable  structures  on  earth  which  lean  or  incline  a 
little }  yet  so  long  as  the  line  of  their  centre  of  gravity  remains  within  the 
base,  and  the  parts  of  the  mass  have  tenacity  among  themselves  sufficient  to 
hold  together  the  structure  will  stand.  The  famous  tower  of  Pisa  was  built 
intentionally  inclining,  to  frighten  and  surprise  :  with  a  height  of  one  hun- 
dred and  thirty  feet,  it  overhangs  its  base  sixteen  feet,  and  assumes  nearly 
the  air  of  fig.  F.  in  page  88. 

The  tall  monument  near  London  Bridge  inclines  so  much,  that  in  high 
winds,  from  a  particular  quarter,  timid  minds  have  doubted  of  its  stability. 
And  many  of  the  most  lofty  and  beautiful  of  our  cathedral  spires  or  tow- 
ers, as  that  of  Salisbury,  have  lost  something  of  their  perpendicularity. 

An  oval  body  on  a  flat  level  surface,  as  already  explained  by  fig.  I,  page 
88,  oscillates  somewhat  like  a  pendulum,  because,  when  disturbed  from  its 
middle  position,  its  centre  of  gravity  has  risen  and  seeks  to  return.  The 
same  is  true  of  any  regular  slice  or  portion  of  a  solid  globe,  which  will  con- 
sequently always  come  to  rest  with  its  plane  .face  turned  directly  upwards. 

The  rocking-horse  of  children  and  the  common  cradle  are  exemplifications 
of  the  same  class. 

But  perhaps  the  most  curious  instances  are  those  rocks  called  Loggan  or 
Laggan  stones,  of  which  there  are  several  among  the  picturesque  barriers 
of  the  British  coast.  An  immense  mass,  loosened  in  some  convulsion  of 
nature,  is  found  with  a  slightly  rounded  base  resting  on  a  flatter  surface  of 
rock  below;  and  is  so  nearly  balanced,  that  the,  force  of  a  man  suffices  to 
move  it.  Some  of  these  have  been  objects  of  much  superstitious  veneration 
to  their  neighbourhood. 

There  is  an  amusing  Chinese  toy,  made  in  obedience  to  the  same  princi- 
ple. It  has  the  appearance  of  a  little  fat,  laughing  man,  sitting  on  the 
ground  with  his  feet  concealed  under  him;  but  where  the  feet  should  be, 
there  is  only  a  rounded  smooth  surface,  with  heavy  lead  ballast  placed  in  it, 
so  low,  as  always,  when  allowed,  to  raise  the  body  to  the  erect  or  sitting 


90  MECHANICS. 

attitude.  A  child  pushes  the  little  fellow  down  again  and  again,  and  would 
persuade  him  to  be  still,  but  is  surprised  to  see  him  always  up  the  moment 
after,  shaking  about  and  as  lively  as  ever. 

The  vibratory  motion  of  a  pendulum,  as  dependent  upon  the  circumstance 
of  the  centre  of  gravity  having  been  moved  from  its  lowest  place,  which  it 
again  constantly  seeks,  was  so  fully  considered  in  the  last  chapter,  that  it 
need  not  be  again  dwelt  upon  here ;  but  we  have  to  enumerate  the  follow- 
ing phenomena  as  being  of  the  same  class  : 
— The  vibrations  of  a  common  swing. 
— The  rocking  of  a  balloon  when  it  first  ascends. 

— The  spontaneous  shutting  of  those  gates  or  doors  of  which  the  upper 
hinge  overhangs  or  projects  beyond  the  lower,  causing  the  gate,  when  in  the 
shut  position,  to  have  its  lock  lower  than  when  in  any  other.  Such  a  gate 
always  returns  of  itself,  from  either  side,  to  the  shut  position,  just  as  a  pen- 
pulum  returns  to  the  lowest  part  of  its  arc : — the  gate,  in  fact,  is  but  a 
sloping  pendulum. 

Of  the  same  nature  also  is  the  rocking  or  rolling  of  a  ship,  in  particular 
states  of  wind  and  sea.  When  the  centre  of  gravity  of  a  ship  is  too  low, 
owing  to  all  the  heavy  load  being  placed  near  the  keel,  this  pendulum- 
motion,  in  rough  weather,  becomes  excessive  and  dangerous. 

The  actions  and  postures  of  animals,  and  particularly  of  man,  illustrate  beau- 
tifully the  observations  made  above  with  respect  to  the  centre  of  gravity. 

A  body,  we  have  seen,  is  tottering  in  proportion  as  it  has  great  altitude 
and  narrow  base — but  it  is  the  noble  prerogative  of  man  to  be  able  to  sup- 
port his  towering  figure  with  great  firmness,  on  a  very  narrrow  base,  and 
under  constant  change  of  attitude.  This  faculty  is  acquired  slowly  because 
of  the  difficulty.  A  child  does  well  who  walks  at  the  end  of  ten  or  twelve 
months  j  while  the  young  of  quadrupeds,  which  have  a  broad  supporting  base, 
are  able  to  stand  and  even  to  move  about  almost  immediately  after  birth. 

The  supporting  base  of  a  man  is  the  space  occupied  by  and  included  be- 
tween the  feet.  The  advantage  of  turning  out  the  toes  is,  that  without  taking 
much  from  the  length  of  the  base,  it  adds  considerably  to  the  breadth. 

If  there  be  much  art  in  walking  on  two  perfect  feet,  there  is  still  more  in 
walking  on  two  slender  wooden  legs,  with  rounded  extremities : — which, 
however,  we  often  see  done,  by  mutilated  soldiers  and  sailors. 

All  the  ladies  of  the  empire  of  China  have  to  acquire  nearly  the  same 
talent  as  these  victims  of  war,;  for  barbarous  custom  has  crippled  them,  by 
confining  their  feet  for  life  in  such  shoes  as  fitted  them  in  infancy. 

But  surpassing  in  difficulty  any  of  these  instances  is  the  practice,  which  is 
general  among  the  inhabitants  of  the  sandy  plains,  called  the  Landes,  in  the 
south-west  of  France,  of  walking  on  stilts.  The  Landes  afford  tolerable 
pasture  for  sheep ;  but  during  one  portion  of  the  year  are  half  covered  with 
water,  and  during  the  remainder  are  still  very  unfit  walking  ground,  by 
reason  of  their  deep  loose  sand  and  thick  furze.  The  natives  meet  the  incon- 
veniences of  all  seasons  by  doubling  the  length  of  their  natural  legs,  through 
the  addition  to  them  of  the  stilts  mentioned,  which  they  call  des  echasses. 
Mounted  on  these,  which  are  wooden  poles,  put  on  and  off  as  regularly  as 
the  other  parts  of  dress,  they  appear  to  strangers  a  new  and  extraordinary 
race  of  long-legged  beings,  marching  over  the  loose  sand,  or  through  the  water, 
with  steps  of  eight  or  ten  feet  in  length,  and  with  the  speed  of  a  trotting 
horse ;  their  moderate  journeys  being  of  thirty  or  forty  miles  in  a  day.  While 


CENT  RE    OF    GRAVITY.  91 

•watching  their  flocks,  they  fix  themselves  in  convenient  stations  by  means  of  a 
third  staff  which  supports  them  behind,  and  then  with  their  rough  sheep-skin 
cloaks  and  caps,  like  thatched  roofs  over  them,  they  appear  like  little  watch- 
towers,  or  singular  lofty  tripods,  scattered  over  the  face  of  the  country. 

Still  beyond  the  art  of  walking  on  stilts  is  that  which  some  persons 
attain  of  walking  and  dancing  on  a  single  rope  or  wire ;  or  even  of  keeping 
the  centre  of  gravity  above  the  base,  while  standing  on  the  moveable  support 
of  a  galloping  horse. — A  rope-dancer  usually  carries  a  long  pole  in  his  hand, 
to  balance  him  ]  it  is  loaded  at  each  end,  and  when  he  inclines,  he  throws 
it  a  little  towards  the  side  required,  that  the  reaction  may  restore  his 
perpendicularity. 

Much  art  of  the  same  sort  is  shown,  in  the  attitudes  and  evolutions  of  the 
skater ;  in  the  amusements  of  supporting  a  stick  upright  on  the  end  of  the 
finger ;  and  many  other  feats  of  a  like  kind. 

Attitudes  generally  depend  on  the  necessity  of  keeping  the  centre  of 
gravity  of  the  body  over  the  base  under  variety  of  circumstances,  as  in  the 
straight  or  upright  part  of  a  man  who  carries  a  load  on  his  head ; — the 
leaning  forward  of  one  who  carries  it  on  his  back  ;  the  hanging  backwards 
of  one  who  bears  it  between  his  arms; — the  leaning  to  one  side  of  him  who 
is  carrying  a  weight  on  the  other  side ; — the  habitual  carriage  of  very  fat 
people,  whose  head  and  shoulders  are  thrown  back,  giving  a  certain  air  of 
self-satisfaction, — an  air  which  belongs  also  to  the  expectant  mother,  and 
even  to  the  dropsical  patient,  although  producing  in  the  latter  so  sad  an 
incongruity. 

When  a  man  walks  or  runs,  he  inclines  forward,  that  the  centre  of  gravity 
may  overhang  the  base  :  and  he  must  then  be  constantly  advancing  his  foot 
to  prevent  his  falling.  He  makes  his  body  incline  just  enough  to  produce 
the  velocity  which  he  desires. 

A  man,  in  pulling  horizontally  at  a  load,  is  merely  causing  his  body  to 
overhang  its  base,  so  that  its  tendency  to  fall  may  become  a  force  or  power 
applicable  to  the  work. 

When  a  man  rises  from  a  chair,  he  is  seen  first  to  bend  the  body  forward, 
or  to  draw  the  feet  backward,  so  as  to  bring  the  feet  or  base  under  the  centre 
of  gravity,  and  then  he  lifts  the  body  up.  If  he  lifts  too  soon,  that  is, 
before  the  body  be  sufficiently  advanced,  he  falls  back  again. 

A  man  standing  with  his  heels  close  to  a  perpendicular  wall,  cannot  with- 
out falling,  bend  forward  sufficiently  to  pick  up  any  object  that  lies  before 
him  on  the  ground }  because  the  wall  prevents  him  from  throwing  part  of 
his  body  backward,  to  counterbalance  the  head  and  arms  which  must  pro- 
ject forward.  A  person  little  versed  in  such  matters,  might  agree  to  give 
ten  guineas  for  permission  to  possess  himself,  if  he  could,  of  a  purse  of  twenty, 
laid  on  the  ground  before  him  :  he  of  course  would  lose  his  stake. 

When  a  man  walks  at  a  moderate  rate,  his  centre  of  gravity  comes  alter- 
nately over  the  right  and  over  the  left  foot.  This  is  the  reason  why  the 
body  advances  in  a  waving  line,  and  why  persons  walking  arm  in  arm  shake 
each  other,  unless  they  make  the  movements  of  their  feet  to  correspond,  as 
soldiers  do  in  marching. 

Sea  Sickness  is  a  subject  closely  related  to  the  present.  Man  requiring, 
as  now  explained,  so  strictly  to  maintain  his  perpendicularity,  that  is,  to  keep 
the  centre  of  gravity  always  over  the  supporting  part  of  his  body,  ascertains 
the  required  position  in  various  ways,  but  chiefly  by  comparing  the  perpen- 
dicularity, or  other  known  position  of  things  about  him,  with  his  own  posi- 


92  MECHANICS. 

tion.  Vertigo  and  sickness  are  the  consequences  of  depriving  him  of  his 
standards  of  comparison,  or  of  disturbing  them. 

Hence  on  shipboard,  where  the  lines  of  the  masts,  windows,  furniture,  &c. 
are  constantly  changing,  sickness,  vertigo  and  other  affections  of  the  same 
class  are  common  to  persons  unaccustomed  to  ships.  Many  persons  experi- 
ence similar  effects  in  carriages,  and  in  swings  j  or  on  looking  from  a  lofty 
precipice,  where  known  objects  being  distant,  and  viewed  under  a  new  as- 
pect, are  not  so  readily  recognized ;  also  in  walking  on  a  wall  or  roof;  in 
looking  directly  up  to  a  roof,  or  to  the  stars  in  the  zenith,  because  then  all 
standards  disappear ;  on  entering  a  round  room,  where  there  are  no  perpen- 
dicular lines  of  light  and  shade,  as  when  the  walls  and  roof  are  covered  with 
a  paper  which  has  no  regular  arrangement  of  spot ;  on  turning  round,  as  in 
waltzing,  or  if  placed  on  a  wheel ;  because  the  eye  is  not  then  allowed  to 
rest  long  enough  on  any  standard,  &c. 

People  when  in  the  dark,  and  therefore  blind  people,  always  use  standards 
belonging  to  the  sense  of  touch ;  and  it  is  because,  on  board  of  a  ship, 
the  standards  both  of  sight  and  touch  are  lost,  that  the  effect  is  so  very 
remarkable. 

But  sea  sickness  also  partly  depends  on  the  irregular  pressure  of  the 
bowels  among  themselves  and  against  the  containing  parts,  when  the  influ- 
ence of  their  inertia  and  weight  varies  with  the  rising  and  falling  of  the  ship. 

From  the  nature  of  sea  sickness,  as  discovered  in  these  facts,  it  is  seen 
•why  persons  unaccustomed  to  the  motion  of  a  ship,  often  find  relief  by  keep- 
ing their  eyes  directed  to  the  fixed  shore,  where  visible  ;  or  by  lying  down 
on  their  backs  and  shutting  their  eyes  ;  or  by  taking  such  a  dose  of  exhilar- 
ating drink  as  shall  diminish  their  sensibility  to  all  objects  of  external  sense. 

As  no  condition  or  form  of  matter  escapes  from  the  great  laws  of  nature, 
we  find  the  attitudes  and  general  condition  of  vegetable  as  well  as  of  animal 
bodies,  characterized  by  the  necessity  of  having  the  centre  of  gravity  sup- 
ported over  tho  base.  With  what  admiration  may  we  contemplate  the  pine 
and  other  trees  in  the  forests  or  nature,  springing  up  to  heaven  as  perpen- 
dicularly as  if  the  plummet  had  been  at  work  to  direct  them  j  and  no  less 
on  the  brows  of  precipitous  hills  than  in  the  level  plains.  On  a  smaller  scale, 
we  see  the  grasses  and  corn-stalks  of  our  fields  illustrating  the  same  truth. 
And  whenever,  in  tree  or  shrub,  accident  or  peculiar  nature  causes  a  devia- 
tion from  perpendicularity,  additional  strength  and  support  are  provided. 

Beauty  of  form  or  position  is  often  felt  to  exist  in  bodies,  merely  because 
they  possess  the  shape  and  support  required,  that  the  centre  of  gravity  may 
be  stable. 

In  architecture,  how  displeasing  is  a  wall  or  pillar  that  is  not  quite  upright ; 
or  a  column  with  too  small  a  base ;  or  a  very  tall  narrow  house  ;  or  a  long 
slender  chimney.  On  the  other  hand,  how  beautiful  in  a  lofty  edifice  is  the 
suitable  succession  of  columns,  from  the  massive  Doric  of  the  basement, 
supporting  the  whole  superstructure,  to  the  light  Corinthian  or  kindred  forms 
seen  above.  The  Chinese  pagoda  is  a  fine  example  of  the  union  of  certain 
requisites  for  stability,  viz.,  perpendicularity  and  expanding  base,  with  the 
other  qualities  of  perfect  symmetry,  graceful  proportion,  and  fanciful  orna- 
ment. When  seen  crowning  a  rising  ground  in  a  wooded  island,  or  spring- 
ing up  from  the  centre  of  a  rich  garden,  it  forms,  perhaps,  one  of  the  most 
beautiful  objects  which  fancy  has  ever  designed. 

Beauty  of  attitude  and  grace  of  carriage  in  the  human  individual  are  in 
great  part  referable  to  the  same  principle. 


CENTRE     OP    GRAVITY.  93 

The  postures  of  opera  dancers  might  pass  as  intentional  illustrations  of  the 
number  of  ways  in  which  the  centre  of  gravity  may  be  kept  above  a  narrow 
base,  by  counteracting  one  disturbing  motion  or  extension  of  a  limb  by  some 
opposite  and  corresponding  motion.  The  common  statute  of  the  god  Mercury 
on  tip-toe  is  a  permanent  familiar  illustration  of  such  a  beautifully  balanced 
attitude. 

Grace  of  carriage  includes  not  only  a  perfect  freedom  of  motion,  but  also 
a  firmness  of  step,  or  steady  bearing  of  the  centre  of  gravity  over  the  base. 
It  is  usually  possessed  by  those  who  live  in  the  country,  taking  much  and 
varied  exercise,  or  who  make  gymnastics  a  part  of  their  discipline.  What 
a  contrast  is  there  between  the  gait  of  the  active  mountaineer,  enjoying  the 
consciousness  of  perfect  nature,  and  that  of  the  mechanic  or  shop-keeper, 
whose  confinement  to  the  cell  of  his  trade  soon  produces  in  his  body  a  shape 
and  air  corresponding  to  it — and  in  the  softer  sex  what  a  difference  is  there, 
between  that  active  and  graceful  fair  one  who  recalls  to  us  the  fabled  Diana 
of  old,  and  that  other  sedentary  being  who,  having  scarcely  trodden  but  on 
smooth  pavements  and  carpets,  under  any  new  circumstances,  carries  her 
person  as  if  it  were  a  load  quite  new  and  foreign  to  her. 

The  centre  of  gravity  is  also  the  centre  of  inertia.  When  a  person  lifts 
a  uniform  rod  by  its  middle,  the  inertia  of  both  ends  being  equal,  he  over- 
comes it  equally,  and  raises  them  evenly  together.  When  he  lifts  by  a  part 
nearer  to  one  end,  the  shorter  and  lighter  portion  having  less  inertia  will  rise 
the  first,  and  there  will  be  a  turning  motion  of  the  rod  round  the  finger  as  a 
centre,  proportioned  to  the  excess  of  inertia  in  the  greater  side. 

The  centre  of  gravity,  or  inertia ,  however,  is  not  necessarily  in  the  centre 
of  the  mass;  for  if  a  weight  of  three  pounds,  a, 
be  affixed  to  one  end  of  a  rod,  and  a  weight  of  Fig.  34. 

only  one  pound,  b,  be  affixed  to  the  other,  the         J>  «^ — -v 

two  will  still  be  balanced,  if  supported  or  lifted       CD  &  (  "*  J 

by  a  point  of  the  rod,  c,  three  times  nearer  to  the  ^— S 

centre  of  the  large  weight  than  to  the  centre  of 

the  small  one.  This  fact  is  explained  under  the  head  of  lever,  a  few  pages 
hence.  For  the  sake  of  simplicity,  in  describing  such  experiments,  the 
weight  of  the  connecting  rod  itself  is  neglected. 

The  centre  of  gravity  or  inertia  is  also  the  centre  of  centrifugal  force : — 
for  if  the  balls  a  and  b  of  the  last  figure  were  made  to  spin  round  a  common 
centre,  as  by  making  the  connecting  rod  rest  and  turn  upon  a  point  or  pivot 
at  c;  unless  the  point  c  were  the  centre  of  inertia  of  the  two,  the  pivot 
would  always  be  drawn  in  the  direction  of  that  end  of  the  rod  at  which  there 
was  the  greatest  centrifugal  force.  It  is  on  this  account  that  in  the  case  of 
a  mill-stone,  or  great  fly-wheel,  or  of  the  balance  wheel  of  a  watch,  the  axis 
must  pass  through  the  centre  of  inertia,  to  prevent  its  being  more  worn  on 
one  side  than  on  the  other. 

When  we  say  in  astronomy,  that  the  earth  revolves  round  the  sun,  or  that 
the  moon  revolves  round  the  earth,  we  do  not  speak  with  absolute  correct- 
ness, for  in  all  such  cases,  both  bodies  are  revolving  round  the  common  cen- 
tre of  inertia  of  the  two.  In  the  case  of  the  sun  and  earth,  as  the  former  is 
about  a  million  times  larger  than  the  latter,  the  common  centre  of  inertia  of 
the  two  is  a  million  times  nearer  to  its  centre  than  to  the  centre  of  the  earth, 
and  is  therefore  within  its  body  or  circumference. 

The  centre  of  inertia  in  a  body  moving  evenly  is  also  its  centre  of  action 
OY  percussion  ;  because,  if  such  centre  come  against  an  obstacle,  the  whole 


94  MECHANICS. 

momentum  of  the  body  acts  there  and  is  destroyed ;  while  if  any  other  part 
than  the  centre  hit,  the  body  loses  only  part  of  its  momentum,  and  revolves 
round  the  obstacle  as  a  pivot  or  centre  of  motion,  to  pass  it  on  the  side  to- 
wards which  the  greater  inertia  happened  to  'be. 

In  a  hammer,  or  a  bar  of  iron  used  as  a  hammer,  or  in  a  pendulum,  the 
motion  is  not  said  to  be  even,  because  the  velocity  of  the  different  parts  is 
different,  being  greatest  far  from  the  hand  or  centre  of  motion,  and  the  centre 
of  all  the  motal  inertia  is  nearer  to  the  fast  moving  end  than  to  the  other. 
Its  exact  place,  in  many  cases,  is  easily  ascertained  by  calculation.  In  a  uni- 
form rod  moving  as  a  pendulum,  for  instance,  it  is  at  a  distance  of  one-third 
from  the  lower  end.  In  the  pendulum  it  is  called  the  centre  of  oscillation. 
If  a  man  use  a  bar  or  rod  of  iron  as  a  hammer,  he  must  take  care  to  make 
it  strike  the  object  by  its  centre  of  action,  or  his  own  hand  will  receive  a  part 
of  the  shock.  A  very  heavy  mass  thus  carelessly  used  will  seriously  strain 
the  wrist.  In  a  common  hammer,  as  the  chief  part  of  the  matter  is  at  the 
end,  the  centre  of  percussion  is  there  too,  and  no  precaution  of  the  kind  men- 
tioned is  required. 

If  a  rod  or  small  log  of  wood  be  suspended  horizontally  by  a  string  tied  to 
its  middle,  or  be  floating  in  water,  and  if  a  forward  blow  be  given  directly 
across  it  near  to  one  end,  the  other  end  will  be  found,  in  the  first  instant  to 
have  moved  a  little  backward,  or  in  a  direction  contrary  to  the  blow,  as  if  the 
rod  had  been  fixed  upon  an  axis.  The  inertia  of  the  general  mass,  by  re- 
sisting the  motion  becomes  in  effect  a 

Fig.  35.  fixed  axis.     This  fact  is  amusingly 

a  n  i  illustrated  by  laying  the  ends  of  a 

long  stick  on  two  wine-glasses,  and 
then  breaking  the  stick  by  a  smart 
downward  blow  of  a  poker  on  its  cen- 
tre. Instead  of  breaking  the  glasses 
also,  as  by  such  a  blow  might  be  expected,  the  ends  of  the  stick  rise  at  the 
instant  of  the  stroke,  to  turn  round  certain  centres  of  resistance  in  the  frag- 
ments, as  at  a  and  b  ,  and  then  fall  harmless  on  the  table. 

In  this  section  we  have  seen  what  admirable  simplicity  is  given  to  many 
of  our  reasonings  and  operations,  by  considering  bodies  in  reference  only  to 
their  centre  of  inertia,  under  one  or  other  of  its  names. 

u In  a  solid  body  moving  about  an  axis,  like  a  wheel  or  weighing-beam, 
the  different  parts  have  different  velocities,  according  to  their  respective 
distances  from  the  axis  or  centre"  (  Read  the  Analysis.  ) 

The  truth  of  this  proposition  is  perceived  at  pnce  on  comparing  the  motion 
in  the  rim  of  a  wheel,  or  near  the  ends  of  a  weighing-beam,  with  that  in 
parts  nearer  the  centres.     Suppose  a  d  to  be  a 
Fig.  36.  line  drawn  across  a  wheel,  or,  along  a  weighing- 

beam,  the  centre  of  motion  in  either  case  being 
at  c;  then  the  outer  circular  line  or  path,  «c, 
which  a  point  in  a  describes  when  moving,  is 
longer  than  the  "corresponding  inner  line,  b  f, 
which  a  point  at  b  describes  in  the  same  time,  as 
a  is  farther  from  the  centre  than  b.  This  admits 
of  easy  mathematical  'demonstration,  and  is  in- 
deed merely  an  instance  of  the  truth,  that  the 
proportions  existing  between  any  parts  or  lines  in 
one  circle,  hold  with  respect  to  the  corresponding 
parts  and  lines  in  all  circles. 


T 


f 

SIMPLE     MACHINES.  95 

"  Hence  forces  with  different  speed  may  still  be  placed  in  continued  connec- 
tion or  opposition  ;  and  they  will  balance  or  be  equivalent,  if  the  one  be  as 
much  more  intense  than  the  other  as  it  is  slower."  (Read  the  Analysis.) 

This  is  the  important  truth  unon  which  the  whole  of  mechanics  may  be 
said  to  hinge.  It  gives  to  man  the  simple  machines  or  mechanical  powers, 
as  they  have  been  called, — the  Lever,  Wedge,  Pulley,  &c  ,  which  enable  him 
to  adapt  any  species  and  speed  of  power  which  he  can  command  to  almost 
any  work  which  he  has  to  accomplish  :  and  the  discovery  of  it  and  of  means 
to  apply  it  may  be  said  to  have  subjected  external  nature  to  his  control.  His 
works  are  of  a  thousand  kinds,  from  the  displacing  of  a  rock  to  the  spinning 
of  a  delicate  thread ;  while  the  natural  powers  or  forces  at  his  command  are 
chiefly  wind,  waterfalls,  fire  and  animal  effort — and  of  which  in  any  particu- 
lar case,  he  may  have  only  one  kind  at  his  service ; — still,  being  able  to  con- 
nect together  his  power  and  resistance  by  solid  media,  of  which  different 
parts  move  with  any  desired  difference  of  velocities,  he  can  employ  any  force 
for  a  purpose  of  almost  any  kind. 

There  is,  however,  a  false  and  most  pernicious  prejudice  very  generally 
existing  with  respect  to  the  simple  machines,  which  we  must  begin  by 
removing,  viz.,  that  they  increase  the  quantity  of  power  or  force  applied  to 
them.  For  instance,  when  one  pound,  as  a  at  the  end  of  a  beam  or  lever, 
is  seen  balancing  two  pounds,  as  b,  at  half  the  distance  on  the  other  side  of 
the  axis,  or  four  pounds  as  c,  at  a  quarter  of  the  distance,  many  persons  be- 
lieve that  the  lever  itself  gives  or  begets 

a  force  equal  to  the  difference  of  the  Fig.  37. 

weights  so  balanced.   But  we  shall  now  <s 

show,  that  levers  and  all  the  other  me- 
chanical powers  (as  from  the  erroneous 

idea  above  mentioned  they  have  been       #    ,      .      ,  fl'\     ^     & 
called,)  merely  enable  us  to  make  such       ,JJ 

substitutions,  so  that  of  a  small  weight       U  j   p£>, 

descending  far,  in  place  of  a  greater        *  J... !— 2-:< 

weight  descending  a  little  way,  or  of  an  £.".^ 

inferior  force  working  long,  instead  of  a  |        \ 

superior  force   working  for  a  shorter  '*"£" 

time, — and  thus  often  to   accomplish 
ends  to  which  the  force  possessed  would 

be  quite  unsuited  if  applied  directly.  In  other  words  the  simple  machines 
enable  us  to  concentrate  or  divide  any  kind  or  quantity  of  force  which  we 
possess,  so  as  to  suit  it  to  our  various  purposes,  just  as  mill-ponds  and 
branching  channels  enable  us  to  accumulate  or  divide  the  force  of  a  stream 
of  water;  but  they  no  more  increase  the  quantity  of  power  than  a  mill-pond 
increases  the  quantity  of  water.  When  any  slender  force  is  caused  through 
a  machine,  to  produce  some  effect  which  seems  proportioned  to  an  intense 
force,  it  has  always  to  act  longer,  or  through  more  space  than  the  other, 
just  in  proportion  as  it  is  more  slender;  as  a  small  stream  of  water  acting 
for  ten  minutes,  may  produce  the  same  effect  as  a  greater  gush  in  one 
minute.  Twenty  feet  of  the  action  of  a  small  horse  near  the  circumference 
of  a  great  wheel,  may  be  rendered,  by  intervening  machinery,  equivalent  to 
ten  feet  of  the  action  of  a  heavy  ox  or  elephant  nearer  the  centre.  And  one 
horse  in  drawing  through  six  hundred  feet,  or  a  hundred  horses  in  drawing 
through  six  feet,  or  the  piston  of  a  great  steam-engine,  in  raising  one  from  the 
bottom  to  the  top  of  its  cylinder,  &c.,  may  all  be  made  to  do  the  same  work. 


96  MECHANICS. 

To  illustrate  this  subject  farther;  we  shall  suppose  a  weighing-beam  x 
with  a  weight  of  one  pound  hanging  at  the  end  x  ;  then  if  a  spring  issuing 
from  the  fixed  box  at  E,  with  uniform  force  of  one  pound,  be  made  to  push 
at  the  other  end  of  the  beam  y,  it  will  just  balance  the  weight;  and  if  it  be 

in  the  slightest  degree  stronger  than 

FiS-  38-  the  weight,  it  will  push  the  end  of 

fl.  the  beam  y  down  to  B,  and  will  raise 

/  3>  33  tne  weight  to  F.  If,  instead  of  the 
single  spring  of  one  pound  at  the  end 
of  the  beam,  two  such  springs  be 


A 

•p  T 

T  ............  I  / 

""  '""•' 


.^ 
......  -U  ........    +          applied  at  half  way  from  the  centre 

"Jj  to  the  end  so  as  to  press  at  A,  where 

there  is  just  half  the  extent  of  mo- 
tion, or  room  to  act,  as  at  B,  exactly 

the  same  effect  will  follow.  Now  because  one  spring  at  the  end  of  the  beam 
is  seen  here  doing  the  same  work  as  two  similar  springs,  or  a  single  spring 
of  double  strength  at  the  middle,  it  might  at  first  appear  that  there  was  a 
saving  of  power  by  using  the  single  spring  and  longer  lever  ;  but  let  it  be 
observed,  that  the  two  middle  springs  have  each  issued  from  their  box  only 
one  inch,  while  the  single  spring  at  the  end  has  issued  two  inches  :  in  both 
cases,  therefore,  exactly  two  inches  of  one-pound  spring  have  been  used. 

In  the  last  experiment,  pound  weights  or  little  buckets  of  water  might  be 
used  instead  of  the  springs,  and  with  exactly  the  same  result  —  one  pound  or 
pint  at  the  end  of  the  arm  producing  the  same  effect  as  two  pounds  or  pints 
at  the  middle  of  it  ;  but  it  would  be  observed  that  the  single  quantity  fell  two 
inches,  while  the  double  quantity  at  half  distance  fell  only  one  inch  ;  and  to 
•  replace  them  after  they  had  done  their  work,  there  would  evidently  be  the 
same  labour,  whether  a  person  had  to  lift  the  single  quantity  first  one  inch, 
and  then  another,  or  had  to  lift,  first,  one  half  of  the  double  equipoise  an  inch, 
and  then  the  other  half  as  much.  —  Each  atom  of  matter  may  be  considered 
as  held  to  the  earth  by  its  thread  of  attraction,  a.nd  if  one  atom  rise  or  fall 
ten  inches,  just  as  much  of  the  supposed  thread  of  attraction  will  be  drawn 
out  or  returned  as  if  ten  atoms  rise  or  fall  one  inch.  And  so  where  a  weight 
of  one  pound  is  made  to  do  any  work,  instead  of  a  weight  of  two  pounds, 
there  is  no  more  saving  than  in  giving  away  two  yards  of  single  rope  instead 
of  one  yard  of  double  rope  ;  and  in  like  manner  for  all  other  differences  of 
intensity. 

If  a  man  were  to  exert  a  force  of  one  hundred  pounds  at  A,  in  the  above 
figure,  to  lift  the  weight,  a  boy  at  B,  with  force  of  fifty  pounds,  might  do 
the  same  work  ;  but  the  man  would  only  have  worked  or  pressed  down 
through  one  foot,  while  the  boy  would  have  worked  through  two;  and  there- 
fore, although  the  boy,  with  the  assistance  of  the  lever,  seemed  to  become  as 
strong  as  the  man,  the  case  would  merely  be,  again,  that  of  the  one-pound 
spring  unbending  two  inches,  to  produce  an  effect  equal  to  that  of  the  two- 
pound  spring  unbending  one  inch.  The  boy  would  be  using  two  feet  of  his 
smaller  force,  where  the  man  used  one  foot  of  his  greater  force  ;  and  if  the 
work  had  to  be  long  continued,  the  boy  would  have  completely  exhausted 
himself,  when  the  man  remained  yet  fresh. 

A  case  of  the  lever,  exhibited  in  this  diagram,  serves  well  to  explain  the 
nature  of  mechanical  powers  in  general.  Suppose  A  to  be  a  weight  of  four 
pounds  at  the  end  of  a  rod  or  lever  A  B,  (p.  97)  made  to  turn  on  c  as  an 
axis  or  fulcrum,  and  having  the  arm  c  B  four  times  as  long  as  the  arm  c  A, 
(but  the  two  arms  of  the  lever  being  equipoised  so  as  not  to  conceal  the  action 


SIMPLE     MACHINES.  97 

* 

of  weights  subsequently  attached  to  them  ;)  then  one  pound  at'  the  end  B, 
would  balance  the  four  pounds  at  the  end  of  A,  and  with  the  slightest  addi- 
tional weight  would  preponderate.  Now  let  us  suppose  the  arc  B  b  to  have 
been  fixed  to  the  long  arm  of  the  lever  with  the  four  projections  or  shelves 
here  shown  on  which  balls  of  one 

pound  might  rest;  then  if  one  of  the  Fig.  39. 

four  balls  from  the  plane  d  were  to  roll 
upon  the  first  shelf,  it  would  just 
balance  A,  and,  with  one  grain  more, 
would  descend  to  the  plane  e,  one  inch 
below ;  then  a  second  ball  of  one  pound 
would  occupy  the  second  shelf,  and 
would  descend  in  the  aume  way,  to  be 

followed  by  a  third,  and  afterwards  by  / "" 

a  fourth ;  and  when  the  whole  four  had 

fallen  from  d  to  e,  they  would  Just  have  lifted  the  four  pound  mass,  at  the 
other  end  of  the  lever,  one  inch.  So  that,  although  one  pound  was  seen 
here  lifting  four  pounds,  it  would  only  have  lifted  them  one-fourth  part  as 
far  as  it  fell  itself,  and  the  sum  of  the  phenomena  would  be,  that  four  pounds 
by  falling  one  inch  at  the  long  end  of  the  lever,  had  raised  four  pounds 
through  the  same  distance  of  one  inch  at  the  short  end.  No  mechanical 
power  or  machine  generates  force  more  than  the  lever  does  in  this  case. 

It  appears,  then,  from  all  this,  that  as  the  quantity  of  motion  in  a  body  is 
measured  by  its  velocity  and  the  number  of  atoms  in  it  conjointly,  so  the 
quantity  of  force  exerted  in  any  case,  is  measured  by  the  intensity  of  the 
force  conjointly  with  the  space  through  which  it  moves.  A  clear  mode, 
therefore,  of  comparing  forces,  is  to  state  the  lengths  and  the  intensities — 
for  instance,  to  speak  of  ten  feet  of  one-pound  force,  as  equal  to  one  foot  of 
ten-pound  force,  &c. 

A  horse  pulling  with  the  force  of  fifty  pounds  goes  generally  at  the  rate  of 
six  miles  an  hour ;  the  steam-engine  piston  is  generally  made  to  move  at  the 
rate  of  two  hundred  feet  per  minute,  bearing  a  pressure  of  steam 'of  about 
twenty  pounds  to  each  square  inch  of  its  surface ;  a  particular  mill-stream 
may  have  a  force  of  one  hundred  pounds,  with  a  velocity  of  a  hundred  and 
fifty  feet  per  minute : — now  it  is  easy,  by  simple  arithmetic,  and  the  rule  of 
length  and  intensity  above  explained,  to  compare  all  these  and  other  forces 
as  applicable  to  any  given  work.  We  must  warn  the  reader,  however,  that 
there  are  many  important  considerations  connected  with  the  practical  employ- 
ment of  forces,  according  to  their  respective  nature  and  that  of  the  resistance 
to  be  overcome,  which  cannot  be  entered  upon  in  this  elementary  work.  In 
very  many  cases  there  is  a  great  waste  or  unavoidable  loss  of  force,  because 
the  resistance,  in  yielding,  runs  away  or  escapes  from  the  force ;  as  when  a 
ship  runs  away  from  the  wind  which  is  driving  her,  or  the  floats  of  a  quick 
moving  water-wheel,  from  the  stream  which  turned  it.  Horses  drawing  boats 
or  carriages  at  the  rate  of  five  miles  an  hour,  might  exert  great  force,  but  to 
have  a  speed  exceeding  twelve  miles  they  might  require  tl^ir  whole  effort 
to  move  their  own  bodies.  As  a  general  rule,  although  equul  quantities  of 
force  balance  each  other  when  applied  to  parts  of  a  lever  or  wheel  altogether 
or  nearly  at  rest,  still  when  a  force  is  made  to  act  near  its  axis  or  fulcrum, 
to  produce  considerable  velocity  in  a  more  distant  part  of  the  machinery, 
much  of  it  is  wasted  in  pressure  against  the  fixed  fulcrum. 

What  an  infinity  of  vain  schemes — yet  some  of  them  displaying  great 
ingenuity — for  perpetual  motion,  and  new  mechanical  engines  of  power,  &c.; 

7 


98  MECHANICS. 

% 

would  have*  been  checked  at  once,  had  the  great  truth  been  generally  under- 
stood, that  no  form  or  combination  of  machinery  ever  did  or  ever  can  increase, 
in  the  slightest  degree,  the  quantity  of  power  applied.  Ignorance  of  this  is 
the  hinge  on  which  most  of  the  dreams  of  mechanical  projectors  have  turned. 
No  year  passes,  even  now,  in  which  many  patents  are  not  taken  out  for  such 
supposed  discoveries;  and  the  deluded  individuals,  after  selling  perhaps  even 
their  household  necessaries  to  obtain  the  means  of  securing  the  expected 
advantages,  often  sink  into  despair,  when  their  attempts,  instead  of  bringing 
riches  and  happiness  to  their  families,  end  in  disappointment  and  ruin.  The 
frequency,  and  eagerness,  and  obstinacy,  with  which  even  talented  individuals, 
owing  to  their  imperfect  knowledge  of  this  part  of  natural  philosophy,  have 
engaged  in  such  undertakings,  is  a  remarkable  phenomenon  in  human  nature. 
Examples  of  such  schemes  will  be  noticed  in  different  parts  of  this  work, 
where  they  may  serve  to  illustrate  points  under  consideration. 

" Lever )  wheel  and  axley  &c"     (Read  the  Analysis,  at  page  84.) 

These  are  the  simplest  of  the  contrivances  which  the  circumstances  of 
solidity  in  masses  has  enabled  man  to  adopt  for  the  purpose  of  connecting 
or  opposing  forces  and  resistances  of  different  intensities.  We  proceed  to 
describe  them,  and  to  explain  some  of  their,  useful  applications. 

"  Lever." 

A  beam  or  rod  of  any  kind,  resting  at  one  end  of  a  prop  or  axis,  which 
becomes  its  centre  of  motion,  is  a  lever ;  and  it  has  been  so  called,  probably, 
because  such  a  contrivance  was  first  employed  for  lifting  weights. 

This  figure  represents  a  lever  employed  to  move  a  block  of  stone ;  a,  is 
the  end  to  which  the  power  or  force  is  applied,/,  is  the  prop  or  fulcrum,  and 
the  mass  6,  is  the  weight  or  resistance.  According  to  the  rule  already  given 

and  explained  at  page  96,  the  power  may  be 

•    Fig.  40;  as  much  less  intense  than  the  resistance  as 

it  is  fajrther  from  the  fulcrum,  or  moving 
through  a  greater  space.  A  man  at  a,  there- 
fore, twice  as  far  from  the  prop  as  the  centre 
of  gravity  of  the  stone  5,  will  be  able  to  lift 
a  stone  twice  as  heavy  as  himself;  but  he 
will  lift  it  only  one  inch  for  every  two  inches 

that  he  descends  :   and  two  men  would  be  required,  acting  at  half  the  dis- 
tance, to  do  the  same  work. 

There  is  no  limit  to  the  difference,  as  to  intensity,  of  forces  which  may  be 
made  to  balance  each  other  by  the  lever,  except  the  length  and  strength  of 
the  material  of  which  levers  have  to  be  formed.  Archimedes  said,  "  Give  me 
a  lever  long  enough,  and  a  prop  strong  enough,  and  with  my  own  weight  I 
will  lift  the  world. "  But  he  would  have  required  to  move  with  the  velocity 
of  a  cannon-ball  for  millions  of  years,  to  alter  the  position  of  the  earth  by 
a  small  part  of  an  inch.  As  stated  in  a  former  part  of  the  volume,  this  feat 
of  Archimedes  is,  in  mathematical  truth,  performed  by  every  man  who  leaps 
from  the  ground ;  he  kicks  the  world  away  from  him  when  he  rises,  and 
attracts  it  again  when  he  falls  back. 

To  calculate  the  effect  of  a  lever,  in  practice,  we  must  always  take  into 
account  the  weight  of  the  lever  itself,  and  the  fact  of  its  bending  more  or 
less ;  but  in  expounding  the  theory  of  the  lever,  it  is  usual  to  consider,  first, 


SIMPLE     MACHINES.  99 

what  would  be  the  result  if  the  lever  were  a  rod  without  weight  and  without 
flexibility. 

The  rule  for  the  lever,  that  the  opposing  forces,  to  balance  each  other,  must 
be  more  or  less  intense,  exactly  as  they  act  nearer  to  or  farther  from  the 
centre,  holds  in  all  cases,  whether  the  forces  be  on  different  sides  of  the  prop 
or  both  on  the  same  side,  and  whether  the  force  nearest  to  the  prop  have  the 
office  of  power  or  of  resistance ;  it  holds,  also,  whether  the  lever  be  straight 
or  crooked. 

The  following  are  examples  of  levers  with  the  prop  between  the  forces. 

The  handspiJce,  represented  in  page  98,  is  a  lever  moving  a  block  of  stone* 
The  same  form,  when  made  of  iron,  with  the  extremity  formed  into  claws,  is 
called  croio-bar.  Both  kinds  are  used  by  gunners,  in  working  cannon  during 
battle  :  they  are  also  used  generally  for  lifting  and  moving  heavy  masses 
through  small  spaces,  as  the  materials  of  the  mason,  the  ship-builder,  the 
warehouse-man,  &c.  A  short  crow-bar  is  the  instrument  of  house-breakers, 
for  wrenching  open  locks  or  bolts,  tearing  off  hinges,  &c. 

The  common  daw-hammer,  for  drawing  nails,  is  another  example.  A 
boy  who  cannot  exert  a  direct  force  of  fifty  pounds,  may  yet,  by  means  of 
this  kind  of  hammer,  extract  a  nail  to  which  half  a  ton  might  be  quietly  sus- 
pended,— because  his  hand  moves  through,  perhaps,  eight  inches,  to  make 
the  nail  rise  one-quarter  of  an  inch.  The  claw-hammer  also  proves,  that 
it  is  of  no  consequence  whether  the  lever  be  straight  or-  crooked,  provided 
"it  produces  the  required  difference  of  velocity  between  power  and  resistance. 
The  part  of  the  hammer  resting  on  the  plank  is  the  fulcrum. 

A  pincers  or  forceps  consists  of  two  levers,  of  which  the  hinge  is  the 
common  prop  or  fulcrum.  In  drawing  a  nail  with  steel  forceps  or  nippers, 
we  have  a  good  example  of  the  advantages  of  using  a  tool  :  1,  the  nail  is 
seized  by  the  teeth  of  steel  instead  of  by  the  soft  fingers :  2,  instead  of  the 
griping  force  of  the  extreme  fingers  only,  there  is  the  force  of  the  whole 
hand  conveyed  through  the  handles  of  the  nippers  :  3,  the  force  is  rendered, 
perhaps,  six  times  more  effective  by  the  lever-length  of  the  handles  :  and  4, 
by  making  the  nippers,  in  drawing  the  nail,  rest  on  one  shoulder  as  a 
fulcrum  it  acquires  all  the  advantages  of  a  lever  or  claw-hammer  for  the 
same  purpose. 

Common  scissors  are  also  double  levers,  and  those  stranger  shears  with 
which,  under  the  power  of  a  steam-engine,  bars  and  plates  of  iron  are  now 
cut  as  readily  as  paper  is  cut  by  the  force  of  the  hand. 

The  common  fire-poker  is  a  lever.  It  rests  on  the  bar  of  the  grate  as  its 
prop,  and  displaces  or  breaks  the  caked  coal  behind  as  the  resistance. 

The  mast  of  a  ship,  with  sails  set  upon  it,  may  be  regarded  as  a  long 
lever,  having  the  sails  as  the  power,  turning  upon  the  centre  of  buoyancy  of 
the  vessel  as  the  fulcrum,  and  lifting  the  ballast  or  centre  of  gravity  as  the 
resistance.  For  this  reason  lofty  sails  make  a  ship  heel  or  lean  over  greatly, 
and  if  used  in  open  boats,  are  dangerous.  In  some  of  the  islands  in  the 
Eastern  and  Pacific  Oceans,  for  the  sake  of  sailing  swiftly,  boats  are  used 
so  extremely  narrow  and  sharp,  that  to  counteract  the  overturning  tendency 
of  their  large  sails,  they  have  an  outrigger  or  projecting  plank  to  wind- 
ward, on  the  extremity  of  which  one  or  more  of  the  crew  may  sit  as  a 
balance. 

Perhaps  no  instance  of  the  lever,  with  the  prop  between  the  forces,  is 
more  interesting  than  the  weighing -beam  ;  whether  with  equal  arms,  form- 
ing the  common  scale-beam  ;  or  with  unequal  arms,  forming  the  steel-yard. 


100  MECHANICS. 

We  have  seen  why  quantities  of  matter  attached  at  equal  distances  from 
the  prop,  must  be  equal  to  each  other  in  order  to  balance.  A  lever,  there- 
fore, which  enables  us  to  place  quantities  thus  exactly  in  opposition  to  each 
other,  and  which  turns  easily  on  its  axis,  becomes  a  weighing-beam.  Of  this 

the  annexed  figure  shows  a  common  form. 
41-  The  axis  or  pivot  at  c  is  sharpened  below, 

wedge-like,  that  the  beam  may  turn  easily, 
and  that  its  centre  of  motion  may  be  nicely 
determined ;  in  a  delicate  balance  for  philo- 
sophical purposes,  the  axis  is  almost  as  sharp 
as  a  knife  edge,  and  rests  on  some  hard  smooth 
surface  of  support,  so  as  to  turn  with  the 
weight  of  a  small  part  of  a  grain.  The  scales 

also  of  a  weighing-beam  are  suspended  on  sharp  edges  to  facilitate  motion, 
and  to  determine  nicely  the  points  of  suspension.  If  the  two  arms  of  a  beam 
be  not  of  perfectly  equal  length,  a  smaller  weight  at  the  end  of  the  longer  will 
balance  a  greater  weight  at  the  end  of  the  shorter.  An  excess  of  half  an 
inch  in  the  length  of  a  beam-arm,  to  which  merchandise  is  attached,  where 
the  arm  should  be  eight  inches  long,  would  cheat  the  buyer  of  exactly  one 
ounce  in  every  pound.  This  case  might  be  detected  instantly,  by  changing 
the  places  of  the  two  things  balanced ;  for  so,  the  lightest  would  be  at  the 
short  arm,  and  would  then  appear  doubly  too  light.  A  beam  intended  for 
delicate  purposes,  and  required,  therefore,  to  turn  easily,  must  have  its  centre 
of  gravity  very  near  the  axis  on  which  the  beam  turns;  for  if  otherwise,  th<? 
beam  will  be  in  the  predicament  of  a  ship  with  the  ballast  too  high  or  too  low  : 
in  the  former  case,  when  once  inclined,  it  would  fall  over,  and  not  to  recover 
itself:  in  the  latter,  it  would  tend  to  remain  horizontal,  and  therefore  would 
be  less  free  to  move.  The  proper  situation  of  the  centre  of  gravity  is  a  little 
below  the  axis  or  line  of  support,  that  the  beam  may  return  with  sufficient 
readiness  from  any  state  of  inclination,  to  its  horizontal  position  of  rest. 

There  is  a  mode  of  arriving  at  very  accurate  results,  even  with  a  weigh- 
ing-beam which  is  not  itself  accurately  made,  provided  it  has  very  free 
motion,  viz.,  first,  very  nicely  to  balance  in  one  scale  the  substance  to  be 
weighed,  and  then  to  remove  it,  and  to  put  weights  into  the  same  scale,  until 
a  perfect  balance  is  produced.  Such  weights  must  be  the  exact  equivalent 
or  weight  of  the  substance,  however  unlike  to  each  other  the  arms  of  the 
balance  may  be.  A  projecting  rod,  or  plank,  or  branch  of  a  tree,  may  thus 
be  made  to  answer  the  purpose  of  a  weighing-beam,  by  attaching  any  sub- 
stance to  its  extremity  and  observing  minutely  how  far  such  substance 
bends  it,  and  then  trying  what  weights  would  bend  it  as  much. 

The  steel-yard  is  a  lever  with  unequal  arms,  and  any  weight  as  6,  on  the 

long   arm,  will  balance  as  much    more 

Fig.  42.  weight  as  a  on  the  short  arm,  as  the  for- 

mer is  supported  farther  from  the  fulcrum 

j      2      3      .4    .5    G     than  the  latter.     Thus,  if  the  hook  at  the 
1       '       '  — T — — *     short  end  be  one  inch  from  the  centre  of 
(5  £  support,  c,  a  pound  weight  5,  on  the  long 

arm  at   four   inches,  will    balance   four 

\a,  pounds  a,  at  the  short  arm.     This  sup- 

poses, however,  that  the  steel-yard  when 
bare,  hangs  horizontally,  from  having  a 
greater  mass  of  matter  in  the  short  arm  to  counterbalance  the  long  slender 


SIMPLE     MACHINES.  101 

arm  from  which  the  shifting  weight  hangs.  When  this  is  not  the  case,  a 
corresponding  allowance  has  to  be  made. 

The  Chinese,  who  are  so  remarkable  for  the  simplicity  to  which  they  have 
reduced  all  their  common  implements,  weigh  any  small  objects  by  a  delicate 
pocket  steel-yard.  It  is  a  rod  of  wood  or  ivory,  about  six  inches  long,  with 
a  silk  cord  passing  through  it  at  a  particular  part,  to  serve  as  a  fulcrum,  and 
with  a  sliding  weight  on  the  long  arm,  and  a  small  scale  attached  to  the 
short  one. 

The  following  are  examples  of  levers  with  both  forces  on  the  same  side  of  the 
prop,  and  where  the  more  distant  force  acts  as  the  power. 

A  common  wheel-barrow  is  a  lever,  in  using  which  a  man  bears  as  much 
less  than  the  whole  weight  of  the  load  as  the  centre  of  gravity  of  the  load  is 
nearer  to  the  axle  of  the  wheel  than  to  his  hands. 

When  two  porters  carry  a  load  placed  midway  between  them,  on  a  pole, 
they  share  it  equally,  that  is  to  say,  each  bears  a  half,  for  the  pole  becomes  a 
lever,  of  which  each  porter  is  a  fulcrum,  as  regards  the  other;  but  if  the  load 
be  nearer  to  one  end  than  to  the  other,  he  to  whom 
it  is  nearest  bears  proportionably  more  of  its  weight.  F  g  43. 

A  load  at  c  is  equally  borne  by  a  porter  at  a  and    a      &       G  ~b 

by  one  at  b  ;  but  a  load  at  d  gives  thre.e-quarters  i        i 

of  its  weight  to  the  man  at  a  and  only  one-quarter          <Ja    f±* 
to  him  at  b.  LJ    LJ 

Two  horses  drawing  a  plough,  act  from  the  ends 

of  a  cross  bar,  of  which  the  middle  is  usually  hooked  to  the  plough.  The 
horses  must  thus  pull  equally,  to  keep  the  bar  directly  across.  When  on 
heavy  land,  three  horses  are  yoked,  and  two  of  them  are  made  to  draw  from 
one  end  of  the  bar,  it  must  be  attached  to  the  plough  by  a  hook,  not  at  its 
middle,  but  half  as  far  from  one  end  of  it  as  from  the  other. 

The  oar  of  a  boat  is  a  lever  of  this  kind,  where  singularly  the  purpose  of 
fulcrum  is  served  by  the  unstable  water. 

The  common  nut-crackers  furnish  another  instance,  by  the  lever-power  of 
which  a  person  can  break  a  shell  many  times  stronger  than  he  could  break 
with  the  bare  fingers. 

The  consideration  of  this  kind  of  lever  explains  why  a  finger  caught  near 
the  hinge  of  a  shutting  door  is  so  much  injured.  The  momentum  of  the 
door  acts  by  a  comparatively  long  lever,  upon  a  resistance  placed  very  near 
the  fulcrum.  Children  pinching  their  fingers  near  the  hinge  of  a  door, — or  of 
the  fire-tongs,  which  furnishes  a  similar  case — wonder  why  the  bite  is  so  keen. 

The  phenomenon  of  the  branch  of  a  tree  giving  way,  when  in  autumn 
overloaded  with  fruit,  or  in  winter  with  snow,  also  exhibit  the  action  of  this 
kind  of  lever.  The  resistance  is  the  cohesion  of  the  upper  side  of  the  branch 
to  the  tree,  and  the  fulcrum  is  the  part  below  which  is  last  broken. 

The  following  are  examples  of  the  lever,  where  the  two  forces  are  on  the 
same  side  of  the  pivot,  but  where  that  nearest  to  the  pivot  acts  as  the  power. 
In  this  kind,  the  power  is  more  intense  than  the  resistance. 

The  hand  of  a  man  who  pushes  open  a  gate  while  standing  near  the  hinges, 
moves  through  much  less  space  than  the  end  of  the  gate,  and  hence  must  act 
with  great  force. 

When  a  man  uses  the  common  fire-tongs,  the  ends  move  much  further 


102  MECHANICS. 

than  his  fingers,  and  therefore  with  less  strength.     No  one  fears  a  pinch 
with  the  ends  of  the  fire- tongs. 

The  most  beautiful  and  remarkable  instances  of  this  modification  of  lever 
are  in  the  limbs  of  animals.  The  object  in  these  was  to  give  to  the  extremi- 
ties great  range  and  freedom  of  motion,1  without  clumsiness  of  form ;  and  it 
has  been  attained  most  perfectly  by  the  tendons  or  ropes  which  move  them, 
being  attached  near  to  the  joints,  which  are  the  pivots  or  fulcra  of  the  bone 
levers. 

In  the  human  arm,  the  deltoid  muscle,  which  forms  the  cushion  of  the 
shoulder,  by  contracting  its  fibres  less  than  an  inch,  raises  the  elbow  twenty 
inches,  and  of  course,  if  it  overcome  a  force  of  fifty  pounds  at  the  elbow,  it 
must  itself  be  acting  with  a  force  at  least  twenty  times  as  intense,  or  of  one 
thousand  pounds. — What  extraordinary  strength  of  muscle,  then,  is  displayed 
by  a  man  who  lifts  another  at  the  end  of  his  extended  arm ;  yet  this  feat  is 
frequently  accomplished,  and  even  on  both  sides  of  the  person  at  once. 

How  powerful  again  must  be  the  wing-muscles  of  birds,  which,  by  this 
kind  of  action,  sustain  themselves  in  the  sky  for  many  hours  together.  The 
great  albatros,  with  wings  extended  fourteen  feet  or  more,  is  seen  in  the 
stormy  solitudes  of  the  Southern  Ocean,  accompanying  ships  for  whole  days, 
without  ever  resting  on  the  waves. 

A  little  contraction  of  the  glutaei  muscles  of  the  hips  gives  to  the  human 
step  a  length  of  four  feet. 

While  the  erroneous  opinion  prevailed,  that  machines  increased  power, 
instead  of,  as  they  do,  merely  accommodating  forces  to  purposes,  this  last 
kind  of  lever,  where  a  great  force  acting  through  a  short  distance  is  made  to 
gain  great  extent  of  motion  and  other  benefits,  was  regretted  by  many  as  a 
most  unprofitable,  contrivance  and  was  called  the  losing  lever. 

It  is  almost  unnecessary  to  say,  that  the  same  rule  of  comparative  veloci- 
ties ascertains  the  relations  required  between  power  and  resistance,  where  a 
combination  of  levers  is  used,  as  where  there  is  only  one.  If  a  lever  which 
makes  one  balance  four,  be  applied  to  work  a  second  lever  which  does  the 
same,  one  pound  at  the  long  arm  of  the  first  will  balance  sixteen  pounds  at 
the  short  arm  of  the  second,  and  would  balance  sixty-four  at  the  short  arm 
of  a  third  such,  &c. 

The  general  rule  for  the  lever,  that  a  force  may  be  less  intense  the  farther 
it  is  from  the  pivot,  supposes  always  that  the  force  acts  at  right  angles,  or 
directly  across  the  lever;  for  if  there  be  any  obliquity,  there  is  a  correspond- 
ing diminution  of  effect,  as  explained  under  the  head  of  resolution  of  forces, 
at  page  57.     For  instance,  one  pound  at  b  on  the  end  of  the  long  arm  of  the 
bent  lever  b  d  a,  because  its  weight  does  not  act 
Fig-  44.  directly  across  b  d,  has  influence  only  as  if  it  were 

acting  directly  at  the  end  of  a  shorter  horizontal 
arm  df;  and  the  two-pound  weight  at  a  acts  only 
as  if  it  were  on  a  horizontal  arm  at  e;  now  e  being 
only  half  as  far  from  the  centre  as/,  two  pounds 
at  a,  in  the  position  of  the  lever  here  shown, 
would  just  balance  the  one  pound  at  b.  In  every 
case,  the  exact  influence  of  weights  is  known  by 
referring  them  to  places  directly  above  or  below 
them,  on  a  supposed  horizontal  lever  ef.  What 
is  called  a  bent-lever  balance,  is  made  on  the  principle  here  explained.  It  has 
on  one  side  a  heavy  weight  <fs  at  a,  and  on  the  other  side  a  scale  attached  at 
b  ;  and  the  weight  of  any  thing  put  into  the  scale  is  indicated  by  the  position 


SIMPLE     MACHINES 


103 


then  assumed  by  the  lever,  marked  by  the  point  at  which  it  cuts  an  arc  of 
divisions  placed  behind  it.  In  any  common  weigh-beam-,  the  point  of  sus- 
pension of  the  scales  being  a  little  below  the  axis  of  motion  of  the  beam, 
there  is  a  degree  of  the  property  of  the  bent-lever  balance,  and  enough  to 
require  notice  in  very  nice  experiments. 

«  The  Wheel  and  Axle" 


Fig.  45. 


a, 


is  the  next  to  be  mentioned  of  the  simple  machines.  The  letter  d  here  marks 
a  wheel,  and  e  an  angle  affixed  to  it;  and  we  see  that  in  turning  together,  the 
wheel  would  take  up  or  throw  off  as  much 
more  rope  than  the  axle,  as  its  circumference 
or  diameter  were  greater  than  that  of  the  axle. 
If  the  proportions  were  as  four  to  one,  one 
pound  at  b  hanging  from  the  circumference  of 
the  wheel,  would  balance  four  pounds  at  a, 
hanging  from  the  opposite  side  of  the  axle. 
The  proportions  are  equally  indicated,  and  are 
usually  expressed  by  comparison  of  the  diame- 
ters of  the  wheel  and  the  axle. 

This  figure  represents  the  same  object  as  the 
last,  viewed  endways.  It  explains  why  the 
wheel  with  its  axle  has  been  called  a  perpetual 

lever ;  for  the  two  weights  hanging  in  opposition,  on  the  wheel  at  a,  and  on 
the  axle  at  5,  are  always  as  if  they  were  connected  by  a 
horizontal  lever  at  a  c  6,  of  which  the  arms  are  respectively 
the  diameters  of  the  wheel  and  the  axle  turning  on  the 
centre  c  as  the  prop;  and  while  a  simple  lever  could  only 
lift  through  a  small  space,  it  is  evident  that  this  construc- 
tion will  lift  as  ^mg  as  there  is  rope  to  be  wound  up. 

A  common  crane  for  raising  weights  consists  of  an  axle 
to  wind  up  or  receive  the  rope  which  carries  the  weight, 
and  of  a  large  wheel  at  the  circumference  of  which  the 
power  is  applied.  The  power  may  be  animal  effort  exerted 
on  the  rim  or  outside  of  the  wheel,  or  the  weight  of  a  man 
or  beast,  walking  within  it,  and  moving  it  as  a  squirrel 
moves  the  cylinder  of  his  cage. 

The  capstan  used  on  board  of  ships,  is  merely  a  large  upright  axle  or 


Fig.  46. 


Fig.  47. 


spindle  b,  which  by  turning,  pulls  the 
cable  or  rope  a  b  c  ;  and  it  is  moved  by 
the  men  pushing  at  the  capstan-bars  d,  e, 
/,  &c.,  which  for  the  time  are  stuck  into 
holes  made  for  them  in  the  broader  part 
or  drum,  usually  'appearing  above  the 
deck  at  the  top  of  the  spindle.  These 
bars  may  be  considered  as  the  spokes  of 
a  large  wheel,  and  the  effect  produced  by 
a  man  working  at  one  of  them  is  in  pro- 
portion to  his  distance  from  the  centre.  The  capstan  is  chiefly  used  on  board 
ships  for  lifting  the  anchor,  and  for  doing  any  other  very  heavy  work ;  but  it 
is  also  applied  for  certain  purposes  on  shore. 

The  common  winch  (represented  as  attached  to  the  wheel  and  axle  at  the 
letter  c,)  with  which  a  grindstone  is  turned,  or  a  crane  worked,  or  a  watch 


104 


MECHANICS. 


wound  up,  is  really  in  principle  a  wheel :  for  the  hand  of  the  worker 
describes  a  circle,  and  there  is  no  difference  in  the  result  whether  an  entire 
wheel  be  turning  with  the  hand  or  only  a  single  spoke  of  a  wheel. 

That  part  of  a  common  watch  called  the  fusee  is  as  beautiful  an  illustra- 
tion of  the  principle  of  the  wheel  and  axle  now  under  consideration,  as  it  is 

a  useful  and  ingenious  contri- 

Fig.  48.  vance.  The  spring  of  a  watch, 

r)  <i  immediately  after  winding  up, 

»  being  more  strained,  is  acting 
more  powerfully  than  after- 
wards when  slacker,  and  if 
there  were  no  means  of  equal- 

izing  its  action,  it  would  de- 

[j  !]  stroy  the  wished-for  uniform- 

ity in  the  motion  of  the  time- 
piece. The  fusee  is  this  means.  It  may  be  considered  as  a  barrel  or  spindle, 
gradually  diminishing  from  its  large  end  6,  to  its  small  end  a,  with  the 
surface  cut  into  a  spiral  grove  to  receive  the  chain,  by  pulling  at  which  the 
spring  in  the  box  c  moves  the  watch.  Now  when  the  watch  has  been  wound 
up,  by  a  key  applied  on  the  axis  of  the  fusee,  the  fusee  is  covered  with  the 
chain  up  to  the  small  end  a,  and  the  newly  bent  and  strong  spring  begins 
to  pull  by  this  small  end  or  short  lever;  and  afterwards,  exactly  as  the 
spring  becomes  relaxed  and  weaker,  it  is  pulling  at  a  larger  and  larger  part 
of  the  fusee-barrel,  and  so  keeps  up  an  equal  effect  on  the  general  move- 
ment. 

A  large  fusee  in  place  of  a  common  cylindrical  axle,  is  often  used  with  a 
winch,  for  drawing  water  by  bucket  or  rope  from  very  deep  wells.  When 
the  bucket  is  near  the  bottom  of  the  well,  and  the  Iab6rer  has  to  overcome 
the  weight  of  the  long  rope,  in  addition  to  that  of  the  bucket  and  water,  he 
does  so  more  easily  by  beginning  to  wind  the  rope  on  a  sm|ill  axle,  that  is 
to  say,  on  the  small  end  of  the  fusee ;  and  in  proportion  as  the  length  of  rope 
diminishes,  he  lifts  by  a  larger  axle. 

By  the  double  axle  a  b,  very  unequal  .intensities  of  force  maybe  balanced. 

We  see  that  in  turning  it,  a  rope  unwinding 
from  the  small  end  a  is  taken  up  by  the  large 
end  b,  turn  for  turn,  and  that  the  rope  below 
must  be  shortened  at  each  turn  by  the  differ- 
ence between  the  circumference  of  the  ends 
a  and  b.  If  the  weight  rise  half  an  inch 
only,  while  the  handle  of  the  winch  describes 
a  circle  of  fifty  inches,  one  pound  force  at  the 
winch  would  balance  one  hundred  pounds 
at  d. 

By  means  of  a  wheel,  which  is  very  large 
in  proportion  to  its  axle,  forces  of  very  differ- 
ent intensities  may  be  balanced,  but  the  machine  becomes  of  inconvenient 
proportions.  It  is  found  preferable,  therefore,  when  such  an  end  is  desired, 
to  use  a  combination  of  wheels  of  moderate  size.  In  the  adjoining  figure, 
three  wheels  are  seen  thus  connected.  Teeth  on  the  axle  rf,  of  the  first 
wheel  c,  acting  on  six  times  the  number  of  teeth  in  the  circumference  of 
the  second  wheel  g,  turn  it  only  once  for  every  six  times  that  c  turns;  and 
in  the  same  manner  the  second  wheel,  by  turning  six  times,  turns  the 


Fig.  49. 


SIMPLE    MACHINES. 


105 


third  wheel  h  once  ;  the  first  wheel,  there- 
fore, turns  thirty -six  times  for  one  turn  of 

the  last ;  and  as  the  diameter  of  the  wheel 

c,  to  which  the  power  is  applied,  is  three 

times  as  great  as  that  of  the  axle/,  which 

has  the  resistance,  three  times  thirty-six, 

or  one  hundred  and  eight,  is  the  difference 

of  velocity,  and  therefore  of  intensity,  be- 
tween weights  or  forces  that  will  balance 

here. — An  axle  with  teeth  upon  it,  as  d 

or  e,  is  called  a  pinion. 

On  the  principle  of  combined  wheels, 

cranes  are  made,,  by  which  one  man  can 

lift  many  tons.     It  is  even   possible  to 

make  an  engine,  by  means  of  which  a  little  windmill,  of  a  few  inches  in 
(-  diameter,  should  tear  up  the  strongest  oak  by  the  roots ;  but  of  course  it 
would  require  a  very  long  time  for  its  work. 

The  most  familiar  instances  of  wheel-work  are  in  our  clocks  and  watches. 
One  turn  of  the  axle. on  which  the  watch-key  is  fixed,  is  rendered  equiva- 
lent, by  the  train  of  wheels,  to  about  four  hundred  turns  or  beats  of  the 
balance-wheel ;  and  thus  the  exertion  during  a  few  seconds,  of  the  hand 
which  winds  up,  gives  motion  for  twenty-four  or  thirty  hours.  By  increas- 
ing the  number  of  wheels,  time-pieces  are  made  which  go  for  a  year  ]  if  the 
material  would  last,  they  might  easily  be  made  to  go  for  a  hundred  or  a 
thousand  years. 

Wheels  may  be  connected  by  bands  as  Fig.  61. 

well  as  by  teeth.  This  is  seen  in  the  com- 
mon spinning-wheel,  turning-lathes,  grind- 
stones, &c.  &c.  A  spinning-wheel,  as  a  c, 
of  thirty  inches  in  circumference,  turns  by 
its  band  a  pirn  or  spindle  of  half  an  inch,  6, 
sixty  times  for  every  turn  of  itself. 

"  The  Inclined  Plane" 

is  the  third  means  which  we  shall  describe,  of  balancing,  by  solid  media, 
forces  of  different  intensities.    A  force  push- 
ing a   weight  from  c  to  d}  only  raises  it  Fig.  52. 
through  the  perpendicular  height  e  d,  by 
acting  along  the  whole  length  of  the  plane 
c  d;  and  if  the  plane  be  twice  as  long  as  it 
is  high,  one   pound  at   b  acting  over  the 
pulley  d  would  balance  two  pounds  at  a,  or 

anywhere  on  the  plane :  and  so  of  all  other      

quantities  and  proportions,  as  already  ex- 
plained under  the  head  of  "  Eesolution  offerees,"  at  page  .86. 

A  horse  drawing  on  a  road  where  there  is  a  rise  of  one  foot  in  twenty, 
is  really  lifting  one-twentieth  of  the  load,  as  well  as  overcoming  the  friction 
and  other  resistances  of  the  carriage.  Hence  the  importance  of  making  roads 
as  level  as  possible ;  and  hence  our  forefathers  often  erred  in  carrying  their 
roads  directly  over  hills,  for  the  sake  of  straightness  considered  vertically, 
where  by  going  round  the  bases  of  the  hills  they  would  scarcely  have  had 
greater  distance,  and  would  have  avoided  all  rising  and  falling.  Hence,  also, 


106  MECHANICS. 

•« 

a  road  up  a  very  steep  hill  must  be  made  to  wind  or  zig-zag  all  the  way ;  for 
to  reach  a  given  height,  the  ease  of  the  pull  to  the  horses  is  greater  exactly 
as  the  road  is  made  longer.  This  rule  of  road-making  is  exhibited  remark- 
ably in  various  parts  of  the  world,  where  hills  with  almost  perpendicular 
faces  have  very  safe  and  commodious  roads  upon  them,  leading  to  forts  or 
residences  near  their  summits.  An  intelligent  driver,  in  ascending  a  steep 
hill  on  which  there  is  a  broad  road,  winds  from  side  to  side  of  the  road  all 
the  way  to  save  his  horses  a  little. 

The  railways  of  modern  times  offer  a  beautiful  illustration  of  this  subject. 
They  are  made  generally  quite  level,  so  that  the  drawing  horse  or  steam- 
engine  has  only  to  overcome  the  friction  of  the  carriage ;  or  where  heavy 
loads  are  passing  only  in  one  direction',  as  from  mines,  they  are  made  to  slope 
a  very  little,  leaving  to  the  horse  or  other  power  only  the  office  of  regulating 
the  movement. 

A  hogshead  of  merchandize,  which  twenty  men  could  not  lift  directly,  is 
often  seen  moved  into  or  out  of  a  wagon,  by  one  or  two  men,  who  have  the 
assistance  of  an  inclined  plane.  In  some  canals,  or  rather  particular  situa- 
tions on  canals,  the  loaded  boats  are  drawn  up  by  machinery  or  inclined 
planes,  instead  of  being  raised  by  water  in  locks,  as  is  the  usual  mode. 

It  is  supposed  that  the  ancients  (the  Egyptians  particularly)  must  have 
used  the  inclined  plane,  to  assist  in  elevating  and  placing  those  immense 
masses  of  stone,  which  still  remain  from  their  times,  specimens  of  their 
gigantic  architecture. 

Our  common  stairs  are  inclined  planes  in  principle  ;  but  being  so  steep, 
are  cut  into  horizontal  and  perpendicular  surfaces,  called  steps,  that  they 
may  afford  a  firm  footing. 

We  may  here  recall,  that  a  body  falling  freely,  in  obedience  to  gravity, 
descends  about  sixteen  feet  in  the  first  second,  and  that  if  made  to  descend 
on  an  inclined  plane,  it  moves  just  as  much  less  quickly  (besides-  the  loss 
from  the  friction  and  the  turning  produced)  as  the  length  of  the  plane  is 
greater  than  the  height.  On  a  plane  sloping  one  foot  in  sixteen  of  its  length, 
a  body  would  descend  only  one  foot  in  the  first  second. 

The  descent  of  a  pendulum  in  its  arc  is  investigated  mathematically  by 
the  laws  of  the  inclined  plane.  And  the  laws  of  the  inclined  plane  itself 
are  mathematically  explained  by  the  principle  of  the  resolution  of  forces, 
explained  at  p.  57. 

"  The  Wedge" 

is  merely  an  inclined  plane  forced  in  between  resistances  to  separate  or  over- 
come them,  instead  of,  as  in  the  last  case,  being  stationary  while  the  resistance 
is  moved  along  its  surface.  The  same  rule  as  to  mechanical 
Fig.  53.  advantage  has  been  applied  to  the  wedge  as  to  other  simple 
machines ;  the  force  acting  on  a  wedge  being  considered  as 
moving  through  its  length  c  d,  while  the  resistance  yields  to 
the  extent  of  its  breadth  a  b.  But  this  rule  is  far  from  ex- 
plaining the  extraordinary  power  of  a  wedge.  During  the 
tremor  produced  by  the  blow  of  the  driving-hammer,  the 
wedge  insinuates  itself,  and  advances  much  more  quickly 
than  the  above  rule  anticipates. 

The  wedge  is  used  for  many  purposes;  as  for  splitting 
blocks  of  stone  and  wood :  for  squeezing  strongly,  as  in  the 
oil-press ;  for  lifting  gfeat  weights,  as  when  a  ship  of  war,  in 
dock  is  raised  by  wedges  driven  under  the  keel,  £c. 


SIMPLE     MACHINES..  107 

An  engineer  in  London,  who  had  built  a  very  lofty  and  heavy  chimney, 
common  to  all  his  steam-engines  and  furnaces,  found  after  a  time  that, 
owing  to  a  defect  in  the  foundation,  it  was  beginning  .to  incline.  However, 
by  driving  wedges  under  one  side  of  it,  he  succeeded  in  restoring  it  to 
perfect  perpendicularity. 

Nails,  awls,  needles,  &c.,  are  examples  of  the  wedge;  as  are  also  all  our 
cutting  instruments,  knives,  razors,  the  axe,  &c.  These  latter  are  often 
used  somewhat  in  the  manner  of  a  saw — which  is  a  series  of  small  wedges — • 
by  pulling  them  lengthwise  at  the  same  time  that  they  are  pressed  directly 
forward  against  the  object.  They  themselves,  indeed,  when  viewed  through 
a  microscope,  are  seen  to  be  but  finer  saws.  It  appears  that  the  vibration  of 
the  particles  produced  by  the  drawing  of  a  saw,  enables  its  edge  to  insinuate 
itself  more  easily.  The  sharpest  razor  may  be  pressed  directly  against  the 
hand  with  considerable  force,  and  will  not  enter,  but  if  then  drawn  along 
ever  so  little,  it  starts  into  the  flesh. 

"  The   Screw" 

is  another  of  the  simple  machines.     It  may  be  called  a  winding  wedge,  for 
it  has  the   same  relation  to  a  straight  wedge  that 
a  road  winding  up  a  hill  or  tower  has  to  a  straight 
road  of  the  same  length  and  acclivity. 

A  screw  may  be  described  as  a  spindle  a  d,  with 
a  thread  wound  spirally  round  it, — turning  or  work- 
ing in  a  nut  c,  which  has  a  corresponding  spiral  fur- 
row fitted  to  receive  the  thread.  The  nut  is  some- 
times called  the  female  screw.  Every  turn  of  the 
screw  carries  it  forward  in  a  fixed  nut,  or  draws  a 
moveable  nut  along  upon  it,  by  exactly  the  distance 
between  two  turns  of  its  thread  :  this  distance,  there- 
fore, is  the  space  passed  through  by  the  resistance, 
while  the  force  moves  in  the  circumference  of  the 
circle  described  by  the  handle  of  the  screw,  as  at  /,  in  the  figure.  The  dis- 
parity between  these  lengths  or  spaces  is  often  as  a  hundred  or  more  to  one ; 
hence  the  prodigious  effects  which  a  screw  enables  a  small  force  to  produce. 

Screws  are  much  used  in  presses  of  all  kinds :  as  in  those  for  squeezing 
oil  and  juices  from  such  vegetable  bodies  as  linseed,  rapeseed,  almonds, 
apples,  grapes,  sugar-cane,  &c. :  they  are  used  also — in  the  cotton  press, 
which  reduces  a  great  spongy  bale,  of  which  a  few,  comparatively,  would 
fill  a  ship,  to  a  compact  package,  heavy  enough  to  sink  in  water ; — also,  in 
the  common  printing-press,  which  has  to  force  the  paper  strongly  against  the 
types  : — a  screw  is  the  great  agent  in  our  coining  machinery, — and  in  letter- 
copying  machines  : — it  is  a  screw  which  draws  together  the  iron  jaws  of  a 
smith's  vice,  &c.  The  screw,  although  producing  so  -much  friction  as  to  con- 
sume a  considerable  part  of  the  force -used  in  working  it,  is  an  exceedingly 
useful  contrivance. 

As  a  screw  can  easily  be  made  with  a  hundred  turns  of  its  thread  in  the 
space  of  an  inch,  at  perfectly  equal  distances  from  each  other,  it  enables  the 
mathematical  instrument  maker  to  mark  divisions  on  his  work,  with  a  minute- 
ness and  accuracy  quite  extraordinary.  If  we  suppose  such  a  screw  to  be 
pulling  forward  a  plate  of  metal,  or  pulling  round  the  edge  of  a  circle,  over 
which  a  sharp-pointed  steel  marker  can  be  let  down  perpendicularly,  always 
in  the  same  place,  the  marker,  if  let  down  once  for  every  turn  of  the  screw, 
will  make  just  as  many  lines  on  the  plate  as  the  screw  makes  turns;  but  if 


103 


MECHANICS. 


made  to  mark  at  every  hundredth  or  a  thousandth  of  a  turn  of  the  screw, 
which  it  will  do  with  equal  accuracy,  it  may  draw  a  hundred  thousand  dis- 
tinct lines  in  one  inch. 

The  instruments  called  micrometers,  by  which  the  sizes  of  the  heavenly 
bodies,  and  of  microscopic  objects,  are  ascertained,  are  worked  by  fine 
screws. 

A  perpetual  screw  is  the  name  given  where  a  screw  acts  on  the  teeth  of  a 
wheel,  so  as  to  produce  a  continued  rotation  of  the  wheel. 

A  commoii  cork-screw  is  the  thread  of  a  screw  without  the  spindle,  and 
is  used,  not  to  connect  opposing  forces,  but  merely  to  enter  and  fix  itself  in 
•the  cork  Complicated  cork-screws  are  now  made,  which  draw  the  cork  by 
the  action  of  a  second  screw,  or  of  a  toothed  rod  or  rack  and  pinion. 

«  The  Pulley" 

is  another  simple  machine,  by  which  forces  of  different  intensities  may  be 
balanced.     A  simple  pulley  consists  of  a  wheel  as 
Fig.  55.  •       a  b,  which  rests  with  its  grooved   circumference 

.  — -ft  on  the  bend  of  a  rope,  c  a  b  d,  and  to  the  axis  of 
which  the  weight  or  resistance  is  attached,  as 
at  e.  I 

In  such  a  construction,  it  is  evident  that  the 
weight  (let  it  be  supposed  ten  pounds)  is  equally 
supported  by  each  end  of  the  rope,  and  that  a  man 
holding  up  one  end,  only  bears  half  of  the  weight, 
or  five  pounds ;  but  to  raise  the  weight  one  foot, 
he  must  draw  up  two  feet  of  rope ;  therefore, 
with  the  pulley,  he  is  as  if  lifting  five  pounds  two 
feet,  where,  without  the  pulley,  he  would  have  to 
lift  ten  pounds  one  foot. 

Many  wheels  may  be  combined  together,  and 
in  many  ways  to  form  compound  pulleys.  Where- 

ever  there  is  but  one  rope  running  through  the  whole,  as  shown  here,  the 
relation  of  power  and  resistance  is  known  by  the  number  of  folds  of  the 
rope  which  support  the  weight.     Here  there  are  four  supporting  folds,  and 
a  power  of  one  hundred  pounds  would  balance  a  resistance  of  four  hundred. 
As  persons  using  pulleys  generally  find  it  more  convenient 
Fig.  56.  to   stand  upon  the  ground  than  to  go  up  and  apply  their 

force  directly  to  one  of  the  supporting  ropes,  the  last  of 
these  is  commonly  made  to  pass  over  a  wheel  above,  and 
to  come  down  apart  from  the  others,  as  shown  here.  This 
portion  not  being  directly  connected  with  the  weight,  adds 
convenience  to  the  pulley,  but  is  not  to  be  counted  with 
the  others,  in  estimating  the  relation  of  the  power  and 
resistance. 

In  fixed  pulleys,  like  those  shown  at  a  and  c,  p.  109, 
there  is  no  mechanical  advantage,  for  the  weight  just  moves 
as  fast  as  the  power ;  yet  such  pulleys  are  of  great  use  in 
changing  the  direction  of  forces.  A  sailor  without  mov- 
ing from  the  deck  of  his  ship,  by  means  of  such  a  pulley, 
may  hoist  the  sail  or  the  signal-flag  to  the  top  of  the 
loftiest  mast.  And  in  the  building  of  lofty  edifices,  where 
heavy  loads  of  material  are  to  be  sent  up  every  few 


SIMPLE    MACHINES.  109 

minutes,  a  horse,  trotting  away  with  the  end  of  the  rope  from  d,  in  a  level 
courtyard,  causes  the  charged  basket  b  to  ascend  to  the  summit  of  the 
building  as  effectually  as  if  he  had  the  power  of  climbing,  at  the  same  rate, 
the  perpendicular  wall.  jijg  5^ 

There  is  a  case,  however  in  which  a 'fixed 
pulley  may  seem  a  balancer  of  different  intensi- 
ties of  force;  viz.,  where  one  end  of  a  rope  is 
attached  to  a  man's  body,  and  the  other  is  carried 
over  a  pulley  above,  and  brought  down  again  to 
his  hands; — for  safety  this  end  also  should  be 
attached  to  his  body.  By  using  the  hands  then 
to  pull  with  force  equal  to  half  his  weight,  he 
supports  himself,  and  may  easily  raise  himself  to 
the  pulley.  A  man,  by  a  pulley  thus  employed,  { 

may  let  himself  down  into  a  deep  well,  or  from, 
the  brow  of  a  cliff,  with  assurance  of  being  able 
easily  to  return,  although  no  one  be  near  to  help 
him ;  and  cases  have  often  occurred  where,  by 
such  means,  a  fellow-creature's  life  might  have  been  saved,  or  other  im- 
portant objects  attained.  How  easily,  for  instance,  might  persons  either 
reach  or  escape  from  the  elevated  windows  of  a  house  on  fire,  by  such  a 
pulley,  which  might  readily  be  found  and  used  where  ladders  could  not  be 
obtained !  This  kind  of  pulley  furnishes  a  convenient  means  of  taking  a 
bath  from  a  ship's  stern  windows,  &c. 

The  chief  use  of  the  pulley  is  on  ship-board.  It  is  there  called  a  block, 
although,  strictly  speaking,  the  block  is  only  the  wooden  mass  which  sur- 
rounds the  wheel  or  wheels  of  the  pulley.  It  aids  so  powerfully  in  over- 
•coming  the  heavy  strains  of  placing  the  anchor,  hoisting  the  masts  and  sails, 
&c.,  that,  by  means  of  it,  a  smaller  number  of  sailors  are  rendered  equal  to 
the  duties  of  the  ship.  Pulleys  are  also  used  on  shore,  instead  of  cranes 
and  capstans,  for  lifting  weights,  and  overcoming  other  resistances. 

Surgeons,  in  former  days,  when  they  trusted  rather  to  force  than  to  the 
address  which  better  information  gives,  used  pulleys  much  to  help  in  the 
reductions  of  luxations, — but  often  hurtfully,  from  not  understanding  the 
force  of  the  pulley.  A  good  surgeon  now  rarely  needs  a  pulley,  and  he  who 
should  ignorantly  stretch  his  patient  on  the  rack,  would  be  .well  requited  by 
similar  treatment. 

The  cranks  of  bell-wires,  seen  in  the  corners  of  our  rooms,  are  bent  levers 
nearly  equivalent  to  fixed  pulleys.  * 

There  is  no  reason,  but  old  usage,  why  the  appellation  of  mechanical  power 
should  be  confined  to  the  six  contrivances  now  explained,  for  those  of 
which  the  account  is  yet  to  follow  equally  deserve  it ;  and,  as  will  be  seen 
under  hydrostatics  and  pneumatics,  the  most  powerful  mechanical  engines 
do  not  belong  to  solids  at  all. 

Engine  of  oblique  action,  is  a  title  which  may  include  a  considerable 
variety  of  contrivances  for  connecting  different  velocities. 

Suppose  c  a  and  c  b  to  represent  two  strong  rods  connected  together,  like 
a  carpenter's  folding  rule,  by  a  hinge  or  joint  at  c.  If  the  distant  ends  be 
made  to  bear  against  notches  in  two  obstacles,  at  a  and  6,  and  by  force  then 
applied  to  c,  either  to  push  or  to  pull,  the  joint'  c  be  straightened  or  carried 
towards  d,  the  joint  c  will  move  through  a  much  greater  space  than  the  sirnul- 


110 


MECHANICS. 


Fig.  58. 


Fig.  59. 


taneous  increase  of  distance  produced  between  a  and  b  ;  and,  in  proportion  to 
the  disparity,  the  power  applied  at  c  will  overcome 
a  more  intense  resistance  at  the  extremities.  The 
mechanical  power  of  this  contrivance  increases 
rapidly,  the  nearer  the  jointed  rods  approach  to 
straightness. 

If  we  suppose  the  end  a  to  be  steadied  by  a  hinge 
on  frame-work,  and  .the  end  b  to  bear  upon  that 
part  of  a  printing-press  which  carries  the  paper 
against  the  types,  we  have  imagined  the  simple 
press  called,  from  its  contriver,  the  Russell-press.  A 
man's  force  at  d,  at  the  moment  when  the  rods  are 
drawn  nearly  to  a  straight  line,  becomes  equivalent 
to  a  pressure  of  many  tons. 
For  the  same  reason,  that  by  urging  c  toward  d,  in  the  last  figure,  the 
extremities  a  and  b  are  separated  with  great  force,  so  by  urging  c  in  the  con- 
trary direction,  the  extremities  would  be  drawn  together  with  corresponding 
force :  and  if  we  suppose  a  c  b  to  be  part  of  a  rope  coming  through  pulleys 
at  a  and  6,  to  one  end  of  which  rope,  beyond  a,  great  resistance  is  attached, 
one  man,  by  pulling  at  c,  may  move  a  weight  or  resistance  many  times 
greater  than  he  could  move  by  his  direct  power. 

The  following  is  another  mode  of  connecting  an  oblique  and  a  direct  force 
so  as  to  balance  them,  although  of  different  inten- 
sities. If  to  turn  a  wheel  (represented  here  by 
the  circle)  a  weight  be  suspended  from  d,  it  is  act- 
ing directly,  for  it  descends  just  as  fast  as  the  cir- 
cumference of  the  wheel  moves,  and  would,  there- 
fore, be  impelling  with  its  whole  strength :  but  if 
it  were  suspended  from  the  point  e,  it  would  then 
be  acting  obliquely  to  the  motion  of  that  part  of  the 
wheel,  and  from  not  descending  so  fast  as  if  at  c?, 
it  would  have  as  much  less  effect  on  the  wheel 
than  if  there,  as  the  line  e  b  is  shorter  than  the  line 
d  c.  The  reason  of  this  will  be  understood  by 
referring  to  the  subjects  of  resolution  of  forces 
and  of  bent  levers,  in  former  parts  of  the  work. 
For  the  same  reason,  if  such  a  wheel  were  used 
in  lifting  weights,  a  man  turning  it  could  lift  as 

much  more  attached  at  the  point  e  than  at  the  point  d}  as  the  line  d  c  is 
longer  than  e  b.  A  man  turning  -this  wheel  in  the  direction  from  e  to  a, 
with  a  weight  hanging  at  e,  would  be  lifting  that  weight  exactly  as  if  he 
were  rolling  it  up  the  inclined  plane  or  curve  e  a.  This  figure  is  useful 

in  explaining  the  varying  intensity  of  the  ac- 
tion of  a  crank  or  winch,  in  different  parts  of 
its  revolution,  and  of  the  combination  of  levers 
used  in  the  Stanhope  printing-press^  in  their 
different  positions  :  it  explains  also  the  degrees 
of  strength  and  support  afforded  by  oblique 
stays  in  buildings  and  in  ships'  rigging,  and 
many  other  kindred  matters. 

The  arrangement  of  cross-jointed  wires,  re- 
presented here,  connects  different  velocities,  and 
therefore  is  really  a  mechanic  power.  It  has 


Fig.  60. 


SIMPLE     MACHINES.  Ill 

been  applied  to  some  curious  purposes,  but  to  none  of  much  utility.  By 
pressing  the  ends  a  and  b  towards  each  other,  the  wires,  from  being  in  the 
position  represented  in  the  upper  figure,  immediately  assume  the  position 
represented  in  the  lower ;  so  that  the  end  c  darts  outward  much  farther  than 
the  ends  a  and  b  approximate. 

Different  intensities  of  force  are  balanced,  although  not  simultaneously,  by 
the  following  means ;  which,  therefore,  according  to  the  old  idea,  have 
some  claim  to  the  name  of  mechanic  powers. 

A  man  may  have  a  purpose  to  effect,  which  a  forcible  downward  push 
would  accomplish :  but  his  body  being  too  weak  to  give  that  push  directly, 
he  may  employ  a  certain  time  in  carrying  a  weight  to  such  an  elevation, 
above  his  work,  that  when  let  fall  its  momentum  may  do  what  is  required. 
Here  the  continued  effort  of  the  man  in  lifting  the  weight,  to  a  height  of 
perhaps  thirty  feet,  may  be  just  sufficient  to  produce  a  blow  which  will 
cause  a  stake  or  pile  to  sink  into  the  earth  one  inch  ]  and  the  contrivance 
has  therefore  balanced  forces,  of  which  the  relation  as  to  intensity  is  marked 
by  the  spaces  thirty  feet  and  an  inch. 

So  also  hammerSj  clubs,  battering-rams,  slings,  &c.,  are  machines  which 
enable  a  continued  moderate  effort  to  overcome  a  great  but  short  resistance. 

The  fly-wheel,  which  by  persons  ignorant  of  natural  philosophy,  has  often 
been  accounted  a  positive  power,  in  common  cases  merely  equalizes  the 
effect  of  an  irregular  force. 

In  using  a  winch  to  turn  a  mill,  for  instance,  a  man  does  not  act  with 
equal  force  all  around  the  circle  :  but  a  heavy  wheel  fixed  on  the  axis  mode- 
rates acceleration,  and  receives  or  absorbs  momentum,  while  his  action  is 
above  par,  and  returns  it  again,  giving  to  the  machine,  while  his  action  is 
below  par,  thus  equalizing  the  movement.  And  in  the  common  instances 
of  circular  motion  produced  by  a  crank,  as  when,  by  the  pressure  of  the  foot 
on  a  treadle,  we  turn  a  lathe  or  grind-stone,  or  spinning-wheel,  the  force  is 
only  applied  during  a  small  part  of  the  revolution,  or  in  the  form  of  inter- 
rupted pushes ;  yet  the  motion  goes  on  steadily,  because  the  turning  grind- 
stone, or  wheel,  or  lathe,  becomes  a  fly  and  reservoir,  equalizing  the  effect 
of  the  force.  In  a  steam-engine  which  moves  machinery  by  a  crank,  the 
upward  and  downward  pushes  of  the  piston  are  converted,  by  means  of  a 
heavy  fly-wheel  into  a  very  steady  rotatory  motion. 

A  heavy  wheel,  however,  has  sometimes  been  used  as  a  concentrator  of 
force  or  a  machanic  power.  By  means  of  a  winch,  or  a  weight,  or  otherwise, 
motion  or  momentum  being  gradually  accumulated  in  the  wheel,  is  then 
made  to  expend  itself  in  producing  some  sudden  and  proportionally  great 
effect.  Thus,  a  man  may  lift  a  very  heavy  weight  by  first,  in  any  way 
giving  motion  to  a  fly-wheel,  and  then  suddenly  hooking  a  rope  from  the 
weight  to  the  axle  of  the  wheel,  which  rope  being  wound  upon  the  axle, 
lifts  the  weight. 

A  fly-wheel  moved  in  the  same  manner,  and  containing  the  result  of  a 
man's  action  during  perhaps  one  hundred  seconds,  if  made  to  impel  a  screw- 
press,  will,  with  one  blow  or  punch,  stamp  a  perfect  medal,  or  from  a  rough 
flat 'plate  of  silver  will  form  a  finished  spoon,  or  other  utensil. 

A  spring,  in  the  same  sense,  may  become  a  mechanical  power.  A  person 
may  expend  some  minutes  in  bending  it,  and  may  then  let  fly  its  accumulated 
energy  in  an  instantaneous  blow.  A  gun-lock  shows  this  phenomenon  on  a 
small  scale.  The  slow  bending  of  a  bow,  which  afterwards  shoots  its  arrow 
with  such  velocity,  is  another  instance. 


112  MECHANICS. 

These,  then,  are  the  principal  means  which  the  solid  state  of  bodies  affords 
us  of  balancing  forces  of  different  intensities.  We  s*hall  find  other  such  means 
or  mechanic  powers  belonging  to  liquids  and  airs.  All  of  them  are  of  inesti- 
mable value  to  man,  by  enabling  him  to  accommodate  the  forces  which  he 
can  command  to  any  kind  of  work  which  he  has  to  perform.  Thus  he  makes 
his  millstone  turn  with  the  same  velocity,  whether  it  be  moved  by  the  slow 
exertion  of  a  horse  or  bullock,  walking  in  a  ring,  or  by  the  quicker  motion 
of  a  river  gliding  under  the  wheel,  or  by  the  rapid  gush  of  a  water-fall,  or 
by  the  invisible  swiftness  of  the  wind.  And  again,  each  of  these  forces  he 
can  equally  apply  to  turn  the  heavy  millstone  or  to  twist  a  cotton  thread. 

The  wants  of  men  seem  first  to  have  led  them  to  use  the  simple  machines, 
for  the  purpose  of  raising  great  weights,  or  overcoming  great  resistances 
and  hence  the  name  long  used  of  mechanic  powers, — particularly  for  the 
Lever,  Wheel  and  Axle,  Plane,  Wedge,  Screw,  and  Pulley  :  but  the  term 
conveys  to  the  uninformed  a  false  idea  of  their  real  nature,  and  has  begotten 
the  common  prejudice  with  respect  to  them,  that  they  generate  force,  or 
have  a  sort  of  innate  power  for  saving  labour.  Now  so  far  is  this  from 
being  true,  that  in  using  them  in  any  case,  even  more  labour  or  bodily 
exertion  is  expended  than  would  suffice  to  do  the  work  without  them.  This 
assertion  is  intentionally  rendered  paradoxical  to  arrest,  attention,  but  its 
truth  will  appear  from  the  following  considerations. 

One  man  may  be  able,  with  a  tackle  of  pulleys  having  ten  plies  of  the 
rope,  to  raise  a  weight  which  it  would  require  ten  men  to  raise  at  once  with- 
out pulleys.  But  if  the  weight  is  to  be  raised  a  yard,  the  ten  men  will  raise 
it  by  pulling  at  a  single  rope  and  walking  one  yard,  while  the  one  man  at  his 
tackle  must  walk  until  he  has  shortened  all  the  ten  plies  of  rope  of  one  yard 
each  j  that  is,  he  must  walk  ten  yards,  or  ten  times  as  far  as  the  ten  men 
did.  In  both  cases,  therefore,  to  accomplish  the  same  end,  we  have  just 
the  same  quantity  of  man's* work  expended,  in  the  first,  performed  by  ten 
men  in  one  minute,  in  the  second,  by  one  man  in  ten  minutes ;  and  if  the 
work  were  of  a  nature  to  continue  longer,  let  us  say  a  whole  day  for  the  ten 
men,  it  would  last  ten  days  for  the  single  man,  and  there  would  be  ten  days7 
wages  of  a  man  to  pay  in  both  cases.  There  is,  therefore,  no  direct  saving  of 
human  effort  from  using  pulleys  j  indeed,  there  is  a  loss,  because  of  the 
great  friction  which  has  to  be  overcome.  Now  exactly  the  same  is  true  of 
all  other  simple  machines,  or  mechanic  powers ;  none  of  them  save  labour, 
in  a  strict  sense  of  the  phrase  ;  they  only  allow  a  small  force  to  take  its  time 
to  produce  any  requisite  magnitude  of  effect,  at  the  expense  of  additionality 
overcoming  a  certain  amount  of  friction  or  other  such  risistance. 

The  real  advantage  of  these  machines  are  such  as  the  following  : 

That  one  man's  effort,  or  any  small  power,  which  is  always  at  command, 
by  working  proportionally  longer,  will  answer  the  purpose  of  the  sudden 
effort  of  many  men,  even  of  hundreds  or  thousands,  whom  it  might  be  most 
inconvenient  and  expensive,  or  even  impossible,  to  bring  together. 

A  ship's  company  of  a  few  individuals  easily  weighs  a  heavy  anchor  by 
means  of  the  capstan. 

A  solitary  workman,  with  his  screw  or  other  engine,  can  press  a  sheet  of 
paper  against  types,  so  as  to  take  off  a  clear  impression  ;  to  do  which  without 
the  press,  the  direct  push  of  fifty  men  would  scarcely  be  sufficient :  and 
these  fifty  men  would  be  idle  and  superfluous  except  just  at  the  instants  of 
pressing,  which  occur  only  now  and  then.  In  this  way  the  screw  may  be 
said  to  do  the  work  of  fifty  men,  for  it  is  as  useful. 

A  man  with  a  crow-bar  may  move  a  great  log  of  wood  to  a  convenient 


SIMPLE     MACHINES.  113 

place,  where  twenty  men  would  have  been  required  to  move  it  without  the 
crow-bar  ;  and  although  the  single  man  takes  twenty  minutes,  perhaps,  to 
do  what  the  many  men  would  have  done  in  one  minute,  as  the  twenty  might 
not  have  been  wanted  again  for  the  rest  of  the  day,  the  crow-bar  may  really 
be  as  useful  as  the  twenty  men. 

It  is  so  important  to  have  correct  notions  on  the  subject  of  the  simple 
machines  or  mechanical  powers,  that  more  space  has  been  here  allotted  to 
the  explanation  of  the  general  principle,  than  has  been  usual  in  such  works. 
After  the  examination  which  it  has  now  undergone,  however,  the  author 
hopes  that  none  of  his  readers  will  have  difficulty  in  conceiving  clearly,  that 
"whatever,  through  a  machine,  is  gained  in  power,  is  lost  in 'speed  or  in 
time,  and  vice  versa" — or  will  have  difficulty  in  detecting  immediately  any 
common  fallacy  connected  with  the  subject ; — as  that  of  supposing,  for 
instance;  that  a  lever,  or  great  pendulum,  or  spring,  or  heavy  fly-wheel,  &c., 
can  never  exert  more  force  than  has  passed  into  it  from  some  source  of 
motion. 

11  By  solid  connecting  parts,  also,  the  direction  of  any  existing  motion  or 
force  may  be  changed.  Hence  the  endless  variety  of  COMPLEX  MACHINES." 
(Read  the  Analysis  at  p.  84.  ) 

It  is  this  power  of  changing  the  direction  of  motion,  added  to  the  power 
of  connecting  and  adjusting  various  intensities  of  force  and  resistance  by  the 
simple  machines  last  described,  which  has  enabled  man  to  make  complex 
machines,  rivaling  in  their  performances  the  nicest  work  of  human  hands. 
It  would  be  endless  to  attempt  the  enumeration  of  the  modes  in  which  the 
directions  of  motions  may  thus  be  changed,  for  it  would  be  to  enumerate 
and  describe  the  whole  apparatus  of  the  arts  and  sciences ;  but  we  shall 
advert  to  a  few  as  specimens. 

Straight  motion  changed  into  rotatory. — The  straight  motion  of  wind  or 
water  becomes  rotatory  in  wind  or  water-wheels. — The  straight  downward 
pressure  of  the  human  foot,  acting  at  intervals  on  a  treadle  and  crank,  turns 
round  the  grindstone,  and  common  lathe,  and  spinning-wheel.  The  alter- 
nate rising  and  falling  of  the  piston  of  a  steam-engine  is  made,  by  means  of 
a  crank,  to  turn  the  great  fly-wheel  and  any  other  wheels  which  a  steam- 
engine  may  move. 

Rotatory  motion  into  straight. — An  axle  in  turning  will  wind  up  a  rope, 
and  lift  a  weight  in  a  straight  line. — A  crank  on  a  turning  axle,  if  connected 
with  a  pump  rod,  will  work  the  piston  up  and  down;  or  it  will  work  a  saw. 
Pallets  or  teeth  on  a  turning-wheel  act  on  the  handle  of  a  great  forge  ham- 
mer, so  that  every  one  in  passing  lifts  the  hammer  and  produces  a  blow. 

We  need  not  multiply  instances.  By  a  visit  to  great  manufacturing  towns, 
or,  indeed,  by  simply  directing  the  eyes  to  what  is  passing  around,  in  any 
part  of  the  civilized  world,  we  discover  miracles  of  mechanic  art: — machines 
driven  by  wind,  water  or  steam  for  grinding  corn; — machines  for  sawing 
wood  and  giving  it  various  forms ; — machines  in  which  rods  of  metal  are 
seized  between  great  rollers,  and  are  flattened  at  once  into  thin  plates,  as  if 
they  were  of  clay,  and  these  plates  again  are  slit  into  bars  or  ribbons — 
spinning  machines,  which  perform  their  delicate  office  even  more  uniformly 
than  human  hands,  forming  thousands  of  threads  at  once,  in  obedience  to  the 
impulse  of  a  single  ste  im-engine; — weaving  machines,  which  accomplish  their 
difficult  task  with  the  most  admirable  perfection; — paper-making  engines, 
which  convert  worn-out  and  apparently  useless  remnants  of  our  apparel,  into 


114  MECHANICS. 

the  uniform  and  beautiful  texture  of  paper,  a  texture  which,  with  the  farther 
assistance  of  the  pen,  or  types,  or  engraved  plate,  becomes  a  magic  conserva- 
tory of  mind,  shutting  up  among  its  folds  the  brightest  effusions  of  genius, 
and  ready,  at  any  instant,  to  disclose  them  again  to  the  delighted  student, 
nothing  changed  after  revolving  centuries  ; — coining  machinery,  which  form 
a  bar  or  plate  of  metal  cuts  out  and  stamps  thousands  of  beautiful  medals  in 
an  hour,  and  keeps  an  exact  record  of  its  work; — cranes, — pile  engines, — 
turning-lathes, — time-pieces, — all  the  implements  of  agriculture,  of  mining, 
of  navigation,  &c.,  &c.  If  Aristotle  deemed  the  title  or  definition  of  tool- 
using  animal  appropriate  to  man  two  thousand  years  ago,  what  title  should 
be  given  now  ? 

In  many  of  the  complex  machines,  several  of  the  simple  ones  are  found 
as  elements;  and  in  the  same  machine  maybe  comprised  many  of  the  means 
of  changing  the  direction  of  motion. 

"  Friction."     (Read  the  Analysis,  p.  84.  ) 

In  estimating  the  effects  of  mechanical  contrivances,  by  the  rule  of  compara- 
tive velocities  of  the  power  and  resistance,  there  is  an  important  correction 
to  be  made,  on  account  of  the  mutual  friction  of  the  moving  parts.  In 
the  steam-engine,  where  the  rubbing  parts  are  numerous,  the  loss  of 
power,  from  friction  often  amounts  to  one-third  of  the  whole. 

Impediment  from  friction  seems  to  be  owing  to  two  causes;  1st,  a  degree  of 
cohesive  attraction  between  the  touching  substances;  2nd,  the  roughness  of 
these  surfaces,  even  where,  to  the  naked  eye,  they  appear  smooth. 

It  is  supposed  to  be,  because  the  roughness,  or  little  projections  and  cavi- 
ties, in  pieces  of  the  same  or  of  homogeneous  substances  mutually  fit  each 
other,  as  the  teeth  of  similar  saws  would,  so  as  to  allow  the  bodies,  in  a 
degree  to  enter  into  each  other,  that  the  friction  is  greater  between  such  than 
between  pieces  of  different  or  of  heterogeneous  substances  with  dissimilar 
grain. 

The  friction  of  one  piece  of  iron,  wood,  brick,  stone,  &c.,  on  another  piece 
of  the  same  substance,  has  been  measured  by  using  the  second  piece  as  an 
inclined  plane,  and  then  gradually  lifting  one  end  of  it  until  the  upper  mass 
began  to  slide, — the  inclination  of  the  plane,  just  before  the  sliding  com- 
mences, is  called  the  angle  of  repose.  This  angle,  different  for  different  sub- 
stances, is  found  to  be,  for  metals,  generally  such  as  to  mark  that  the  force 
required  to  overcome  the  friction  between  small  pieces  of  them  is  equal  to 
about  a  fourth  of  the  weight  of  the  moving  piece,  and  for  woods  it  is  about 
a  half.  But  for  large  pieces  or  great  pressures,  the  friction  is  proportionably 
much  less. 

It  is  this  angle  in  the  substances  concerned,  which  determines  the  degrees 
of  acclivity  which  can  exist  in  the  sides  of  hills  composed  of  sand,  gravel, 
earth,  &c.,  in  the  banks  of  canals,  rivers,  &c 

If  the  thread  of  a  screw  winds  round  the  spindle  with  an  angle  less  than 
this,  the  screw  can  never  recoil  or  slide  back  from  force  acting  against  its 
point. 

But  for  friction,  men  walking  on  the  ground  or  pavement  would  always  be 
as  if  walking  on  ice;  and  our  rivers,  that  now  flow  so  calmly,  would  all  be 
frightful  torrents.  Friction  is,  therefore,  in  these  cases  of  great  use  to  men. 

Friction  is  useful,  also,  when  it  enables  men,  out  of  the  comparatively 
short  fibres  of  cotton,  flax,  or  hemp,  to  form  their  lengthened  webs  and 
cordage, — for  it  is  friction  alone,  consequent  upon  the  interweaving  and 


SIMPLE     MACHINES.  115 

twisting  of  the  fibres  and  threads,  which  keeps  the  material  of  these  fabrics 
together. 

The  following  means'are  used  to  dimmish  friction  between  rubbing  sur- 
faces ;  and  they  are  used  singly  or  in  combination,  according  to  the  circum- 
stances. 

1.  Making  the  rubbing  surfaces  smooth; — but  this  must  be  done  within 
certain  limits,  for  great  smoothness  allows  the  bodies  to  approach  so  near 
that  a  degree  of  cohesion  takes  place. 

2.  Letting  the  substances  which  are  to  rub  on  each  other  be  of  different 
kinds.     Axles  are  made  of  steel,  for  instance,  and  the  parts  on  which  they 
bear  are  made  of  brass ;  in  small  machines  as  time-keepers,  the  steel  axles 
often  play  in  agate  or  diamond.     The  swiftness  of  a  skater  depends  much 
on  the  great  dissimilarity  between  steel  and  ice. 

8.  Interposing  some  lubricating  substance  between  the  rubbing  parts;  as 
oils  for  the  metals,  soap,  grease,  black-lead,  &c.,  for  the  woods.  There  is  a 
laughable  illustration  of  this  in  the  holiday  sport  of  soaping  a  lively  pig's 
tail,  and  then  offering  him  as  the  prize  of  the  clever  fellow  who  can  catch 
and  hold  him  fast  by  his  slippery  appendix. 

4.  Diminishing  the  extent  of  the  touching  surfaces ;  as  in  making  the 
rubbing  axis  of  a  wheel  very  small. 

5.  Using  wheels  as  in  wheel-carriages,  instead  of  dragging  a  rubbing  load 
along  the  ground.     Casters  on  household  furniture  are  miniature  wheels. 

6.  Using  what  is  called  friction-wheels  ; — which 

still  farther  diminish  the  friction  even  of  a  smooth  Fig.  61. 

axis,  by  allowing  it  to  rest  on  their  circumferences, 
which  turn  with  it.  Here  a  represent  the  end  of 
an  axis,  resting  on  the  exteriors  of  two  friction- 
wheels,  b  and  c. 

7.  Placing  the  thing  to  be  moved  on  rollers  or 
balls  as  when  a  log  of  wood  is  drawn  along  the 
ground  upon  rounded  pieces  of  wood;  or  when  a 

cannon  with  a  flat  circular  base  to  its  carriage,  turns  round  by  rolling  on 
cannon-balls  laid  on  a  hard  level  bed.  In  these  two  cases  there  is  hardly 
any  friction,  and  the  resistance  is  merely  from  the  obstacles  which  the  rollers 
or  balls  may  have  to  pass  over. 

Of  all  rubbing  parts  the  joints  of  animals,  considering  the  strength,  fre- 
quency and  rapidity  of  their  movements,  are  those  which  have  the  least 
friction.  The  rubbing  surfaces  in  these  are  covered,  first,  with  a  layer  of 
elastic  cartilage,  and  then  with  an  exceedingly  smooth  membrane,  over  which 
there  is  constantly  poured  from  the  glands  around,  a  fluid  called  synovia, 
more  emollient  and  lubricating  than  any  oil.  and  which  is  renewed  constantly 
as  may  be  required.  We  study  and  admire  the  perfection  of  animal  joints, 
without  being  able  very  closely  to  imitate  it. 

Wheel-carriages  merit  notice  here,  as  illustrating  many  of  the  circum- 
stances connected  with  friction ;  and  moreover  as  being  among  the  most 
common  of  machines. 

Wheel-carriages  have  three  advantages  over  the  sledges  for  which  they 
are  the  substitutes : 

1.  The  rubbing  or  friction,  instead  of  being  between  an  iron  shoe  and  the 
stones  and  irregularities  of  the  road,  is  between  the  axle  and  its  bush,  of 
which  the  surfaces  are  smoothed  and  fitted  to  each  other,  and  well  lubricated. 


116 


MECHANICS. 


2.  While  the  carriage  moves  forward,  perhaps  fifteen  feet,  by  one  revolu- 
tion of  its  wheel,  the  rubbing  "part,  viz.,  the  axle,  passes  over  only  a  few 
inches  of  the  internal  surface  of  its  smooth  greased  bush. 

3.  The  wheel  surmounts  any  abrupt  obstacle  on  the  road  by  the  axle 

describing  a  gently  rising  slope 

Fig.  62.  or  curve,  —  as  shown  in  this 

/?,.  figure,  where  a  represents  an 

obstacle,  and  where  the  curve 
from  c,  of  which  the  beginning 
has  the  direction  shown  by  the 
line  c  e,  represents  the  path 
of  the  axle  in  surmounting  it. 
The  wheel  is  as  if  rising  on  an 
inclined  plane,  and  gives  to'the 
drawing  animal  the  relief  which 
such  a  plane  would  bring.  This 
kind  of  advantage  is  greater  in  a  large  wheel,  for  evidently  the  smaller  wheel 
here  represented,  in  having  to  surmount  the  same  size  of  obstacles,  has  to 
rise  in  the  steeper  curve  beginning  at  d, — but  the  difference  of  advantage,  in 
this  respect,  is  not  so  great  as  the  difference  of  size.  It  is  true  again,  that 
a  small  wheel  would  sink  to  the  bottom  of  a  hole,  where  a  larger  one  would 
rest  on  the  edges  as  a  bridge,  and  would  sink  less.  The  fore  wheels  of  car- 
riages are  usually  made  small,  because  such  construction,  by  allowing  the 
wheel  to  go  under  the  body  of  the  carriage,  facilitates  the  turning  of  the 
carnage.  It  is  not  true,  however,  according  to  the  popular  prejudice,  that 
the  large  hind  wheels  of  coaches,  wagons,  &c.,  help  to  push  on  the  little 
wheels  before  them,  as  if  the  carriage  were  on  an  inclined  plane  resting  on 
the  wheels ;  but  there  is  the  accidental  advantage,  that  in  ascending  a  hill, 
when  the  horses  have  to  put  forth  their  strength,  the  load  rests  chiefly  on 
the  hind  wheels,  and  in  descending,  when  an  increased  resistance  is 
desirable,  the  load  falls  chiefly  on  the  fore-wheels. 

From  the  causes  mentioned  in  the  last  paragraphs,  the  difference  in  per- 
forming the  same  journey  of  a  mile,  by  a  sledge  and  by  a  wheel  carriage,  is 
that  while  the  former  has  to  rub  over  e"very  roughness  in  the  road  and  to  be 
jolted  by  every  irregularity,  the  rubbing  part  of  the  latter,  the  axle,  glides 
very  slowly  over  about  thirty  yards  of  a  smooth  oiled  surface,  in  a  gently 
waving  line.  Thus,  by  wheels,  the  resistance  is  reduced  to  about  the  hun- 
dredth part  of  what  it  is  for  a  sledge. 

On  hilly  roads,  in  descending,  it  is  common  to  lock  or  fix  one  of  the  wheels 
of  a  carriage,  and  the  horses  have  then  to  pull  nearly  as  much  as  on  a  level 
road  with  the  wheel  free ;  showing  the  effect  of  a  little  increase  of  friction. 
The  wheel  of  a  carriage,  simple  as  from  our  extreme  familiarity  with  it, 
it  now  appears  to  us,  is  a  thing  of  very  nice  workmanship,  and  which  has 
exercised  much  ingenuity. — It  acquires  astonishing  strength,  indeed  that  of 
the  arch,  from  what  is  called  its  dished  form,  seen 
here  in  the  wheel  c,  as  contrasted  with  the  flat  wheel 
a.  In  a  wheel  of  this  form,  the  extremity  of  a  spoke 
cannot  be  displaced  inwards,  or  towards  the  car- 
riage, unless  the  rim  of  the  wheel  be  enlarged,  or 
all  the  other  spokes  yield  at  the  same  time,  and  it 
cannot  be  displaced  outwards,  or  away  from  the  car- 
riage, unless  the  rim  be  diminished,  or  the  other 
spokes  yield  in  the  opposite  direction: — now  the 


Fig.  63. 


FRICTION.  117 

rim  being  strongly  bound  by  a  ring,  or  tire  of  iron,  cannot  suffer  either 
increase  or  diminution,  and  the  strength  of  all  the  spokes  is  thus  by  it 
conferred  on  each  individually.  In  &flai  wheel  a  given  degree  of  displace- 
ment outwards  or  inwards  of  the  extremities  of  a  spoke,  would  less  affect  the 
magnitude  of  the  circumference,  and  therefore  the  rim  of  such  a  wheel, 
secures  much  less  firmly.  A  watch-glass  and  a  round  piece  of  egg-shell  are 
stronger  than  flat  pieces  of  like  substances,  for  the  same  reason  that  a  dished 
wheel  is  stronger  than  a  flat  wheel. — The  dished  form  of  a  wheel  is  farther 
useful  by  leaving  more  room  between  the  wheels  for  the  body  of  the  carriage, 
and  is  useful,  also,  in  this,  that  when  the  carriage  is  on  an  inclined  road, 
and  more  of  the  weight  consequently  falls  upon  the  wheel  of  the  lower  side, 
the  inferior  spokes  of  that  wheel  become  nearly  perpendicular,  and  thereby 
support  the  increased  weight  more  safely.  The  strongest  form  of  wheel  is 
the  doubly  dished,  that  is,  a  wheel  having  half  of  the  spokes  passing  from 
within  to  the  rim  as  from  c  to  d,  fig.  63,  and  the  other  half  similarly  from 
without.  This  form  is  adopted  in  the  wheel  recently  constructed  entirely 
of  iron,  in  which  there  is  a  farther  peculiarity  that  the  load  is  supported 
more  by  hanging  by  the  upper  spokes  than  by  resting  on  the  lower.  When 
wheels,  instead  of  standing  upright,  like  b  and  d  shown,  fig.  63,  are  made 
to  incline  outwards,  as  is  common,  owing  to  the  ends  of  the  axletrees  being 
bent  down  a  little  to  give  a  security  against  the  accident  of  the  wheels  falling 
off,  the  pull  to  the  horses  in  deep  or  sandy  roads  is  much  increased ;  for  an 
inclining  wheel  would  naturally  describe  a  curved  and  outward  path,  as  is 
seen  when  a  hoop  or  wheelbarrow  inclines ;  and  the  horses,  therefore,  in 
drawing  straight  forward,  have  constantly  to  overcome  the  deviating  tendency 
in  all  inclining  wheels.  This  cause  of  resistance  is  still  more  remarkable 
when  the  wheels  have  broad  rims.  Such  wheels  must  be  conical,  that  is,  of 
smaller  diameter  at  the  outer  than  at  the  inner  edge,  as  the  end  of  a  cask  is 
smaller  than  its  middle,  and  then,  as  the  iron  hoops  or  tires  which  cover  the 
different  parts  cannot  all,  by  an  equal  number  of  turns,  truly  measure  the 
same  length  of  road,  there  will  be  a  constant  rubbing  or  grinding  forward  of 
the  lesser  rings,  and  a  grinding  backward  of  the  larger,  injuring  the  road, 
rapidly  wearing  the  iron,  and  exhausting  the  strength  of  the  pulling  animals. 
Such  wheels  rolling  free  would  describe  a  circular  path,  as  is  exemplified 
when  a  thimble,  or  drinking  glass,  or  sugar-loaf,  which  also  are  conical,  is 
pushed  forward  on  any  plane  surface. 

The  application  of  springs  to  carriages,  which  is  an  improvement  of  com- 
paratively recent  date,  not  only  renders  them  soft  moving  vehicles  on  rough 
roads,  but  much  lessens  the  pull  to  the  horses.  When  there  is  no  spring, 
the  whole  load  must  rise  with  every  rising  of  the  road,  and  if  time  be  given, 
must  sink  with  every  depression,  and  the  depression  costs  as  much  labour 
as  the  rising,  because  the  wheel  must  be  drawn  up  again  from  the  bottom  of 
it ;  but  in  a  spring-carriage  moving  rapidly  along,  only  the  parts  below  the 
springs  are  moved  in  correspondence  with  the  road-surface,  while  all  above, 
by  the  inertia  of  the  matter,  have  a  soft  and  even  advance.  Hence  arises 
the  superiority  of  those  modern  carriages,  furnished  with  what  are  called 
under-springs,  which  insulate  from  the  effect  of  shocks,  all  the  parts  except- 
ing the  wheels  and  axletrees  themselves.  When  only  the  body  of  the  car- 
riage is  on  springs,  the  horses  have  still  to  rattle  the  heavy  frame-work 
below  it  over  all  irregularities,  and  then  the  wheels  as  well  as  the  structure 
generally  require  to  be  of  much  greater  strength  and  weight  to  bear  the 
consequent  shocks. 


118  MECHANICS. 

The  subject  of  wheel  carriages  is  interesting  to  medical  men,  from  their 
having  often  to  direct  in  transporting  the  sick  or  wounded. 

It  is  perhaps  difficult  to  conceive  anything  more  elegant  and  perfect  than 
the  carriages  of  modern  refinement;  and,  therefore,  a  man,  who  sees  them 
gliding  swiftly  along  the  prepared  levels  and  slopes  of  our  present  landscapes, 
and  thinks  of  the  clumsy  vehicles  on  the  bad  roads  of  former  times,  may 
readily  imagine  that  absolute  perfection  is  at  last  attained.  Yet  we  are  per- 
haps now  on  the  eve  of  a  farther  change  which,  for  many  purposes,  will  be 
of  greater  importance  than  all  that  has  yet  been  achieved — viz.,  the  general 
adoption  of  rail-roads,  with  new-fashioned  carriages  to  suit  them.  To  all 
who  study  such  subjects,  it  is  now  known,  that  to  draw  a  loaded  wagon  up 
one  inconsiderable  hill,  costs  more  force  than  to  send  it  thirty  or  forty  miles 
along  a  level  rail- way ;  and  the  conclusion  is  obvious,  that  although  the  origi- 
nal expense  of  forming  the  level  line  might  considerably  exceed  that  of  mak- 
ing an  ordinary  road,  still,  in  situations  of  great  traffic,  the  difference  would 
soon  be  paid  for  by  the  savings,  and  when  once  paid,  the  savings  would  be 
as  a  profit  for  ever.  To  readers  conversant  with  political  economy,  it 
would  be  superfluous  to  speak  here  of  the  advantages  of  any  great  facility 
of  intercourse,  but  to  those  who  are  not,  the  following  reflections  may  be 
interesting. 

In  reviewing  the  history  of  the  human  race,  we  find  that  every  remarkable 
increase  in  civilization  has  taken  place  very  much  in  proportion  to  the  facili- 
ties of  intercourse  offered  in  the  particular  situation.  First,  therefore,  civili- 
zation, grew  along  the  banks  of  great  rivers,  as  the  Nile,  the  Euphrates  and 
the  Ganges ;  or  along  the  shores  of  inland  seas  or  archipelagoes,  as  in  the 
Mediterranean  and  the  numerous  islands  of  Greece  ;  or  over  fertile  and  ex- 
tended plains,  as  in  many  parts  of  India.  When  the  situation  thus  bound  a 
great  number  of  individuals  into  one  body,  the  useful  new  thought  or  action 
of  any  one  unusually  gifted,  and  which,  in  the  insulated  state,  would  soon 
have  been  forgotten  and  lost,  extended  its  influence  immediately  to  the  whole 
body,  and  became  the  thought  or  action  of  all  who  could  benefit'by  it,  besides 
that  it  was  recorded  for  ever,  as  part  of  the  growing  science  of  art  of  the  com- 
munity. And  in  a  numerous  society,  such  useful  thoughts  and  acts  would 
naturally  be  more  frequent,  because  persons  feeling  that  they  had  the  eyes  of 
a  multitude  upon  them,  and  that  the  rewards  of  excellence  would  be  propor- 
tionally great,  would  be  excited  to  emulation  in  all  the  pursuits  that  could 
contribute  to  the  well-being  of  the  society.  Men  soon  learned  to  estimate 
aright  these  and  many  other  advantages  of  easy  intercourse,  and  after  having 
possessed  themselves  with  avidity  of  the  stations  naturally  fitted  for  their 
purposes,  they  began  to  improve  the  old  and  to  make  new  stations.  They 
created  rivers  and  shores,  and  plains  of  their  own,  that  is,  they  constructed 
canals  and  basins,  and  roads ;  and  so  connected,  artificially,  regions  which 
nature  seemed  to  have  separated  forever. — In  the  British  isles,  whose  fa- 
voured children  have  taken  so  remarkable  a  lead  in  showing  what  prodigies 
a  wise  policy  may  effect,  the  advantages  arising  from  certain  lines  of  canal 
and  road  first  executed,  soon  led  to  numberless  similar  enterprises,  and  within 
half  a  century  the  empire  has  been  thus  bound  together  in  all  directions :  and 
it  seems  as  if  the  noble  work  was  now  to  be  crowned  by  the  substitution  of 
level  railways  for  many  of  the  common  roads  and  canals.*  Several  rail- 

*  These  observations  were  first  published  (the  substance  had  been  written  long 
before,)  soon  after  the  Darlington  rail-road,  the  first  of  any  note  intended  for  pas- 
sengers", was  opened.  The  Manchester  and  Liverpool  rail-road  has  since  then 
admirably  verified  the  anticipations. 


STRENGTH    OF    MATERIAL.  119 

roads  of  short  extent  have  already  been  established,  and  although  they  and 
the  carriages  upon  them  are  far  from  having  the  perfection  which  philosophy 
says  they  will  admit,  the  results  have  been  very  satisfactory.  If  we  sup- 
pose the  progress  to  continue,  and  the  price  of  transporting  things  and  per- 
sons to  be  thus  reduced  to  a  fourth  of  the  present  charge — and  in  many  cases 
it  may  be  less — and  if  we  suppose  the  time  of  journeying  with  safety  also  to 
be  reduced  in  some  considerable  degree, — of  which  there  can  be  as  little 
doubt — the  general  adoption  of  such  roads  would  operate  an  extraordinary 
revolution  and  improvement  in  the  state  of  society.  Without  in  reality 
changing  the  distances  of  places,  it  would  in  effect  bring  all  places  nearer  to 
each  other,  and  would  give  to  every  spot  in  the  kingdom  the  conveniences 
of  the  whole, — of  town  and  country,  of  sea-coast  and  of  highland  district. 
A  man,  wherever  residing,  might  consider  himself  virtually  near  to  any  other 
part,  when,  at  the  expense  of  time  and  money  now  expended  in  travelling  a 
short  way,  he  might  travel  very  far,  and  he  would  thus  find  remarkably 
extended,  the  sphere  both  of  his  business  and  of  his  pleasures.  The 
over-crowded  and  unhealthy  parts  of  towns  would  scatter  their  inhabitants 
into  the  country  •  for  the  man  of  business  could  be  as  conveniently  at  his 
post  from  a  distance  of  several  miles,  as  he  is  now  from  an  adjoining  street. 
The  present  heavy  charges  for  bringing  distant  produce  to  market  being 
nearly  saved,  the  buyer  everywhere  would  purchase  cheaper,  and  the  pro- 
ducer would  be  still  better  remunerated. 

In  a  word,  such  a  change  would  be  affected,  as  if  by  magic  the  whole  of 
Britain  had  been  compressed  into  a  circle  of  a  few  miles  in  diameter,  yet 
without  any  part  losing  aught  of  its  magnitude  or  beauties.  All  this  may 
appear  visionary ;  but  it  is  less  so  than  seventy  years  ago  it  would  have 
been  to  anticipate  much  of  what,  in  respect  to  travelling,  has  really  come  to 
pass, — as?  that  the  common  time  of  passing  from  London  to  Edinburgh 
would  be  forty-six  hours.  At  the  recent  opening  (in  1825)  of  the  railroad 
near  Darlington,  a  train  of  loaded  carriages  was  dragged  by  one  little  steam- 
engine  a  distance  of  twenty-five  miles  within  two  hours ;  and  in  some  parts 
of  the  journey  the  speed  was  more  than  twenty  miles  an  hour  :  the  load 
was  equal  to  a  regiment  of  soldiers,  and  the  coal  expended  was  not  of  the 
value  of  a  crown.  An  island  with  such  roads  would  be  an  impregnable 
fortress ;  for  in  less  time  than  an  enemy  would  require  to  disembark  on 
any  part  of  the  coast,  the  forces  of  the  country  might  be  concentrated  to 
defend  it. 

"  Strength  depends  on  the  magnitude,  form  and  position  of  bodies,  as  well 
as  on  the  degree  of  cohesion  in  the  material."  (Head  the  Analysis, 
page  84.) 

The  minute  details  connected  with  this  branch  of  the  subject  belong  to 
the  practical  engineer,  but  there  are  some  of  the  general  truths  which  should 
be  familiar  to  every  body. 

Of  similar  bodies  the  largest  is  proportionally  the  weakest. 

Suppose  two  blocks  of  stone  left  projecting  from  a  hewn  rock,  of  which 
blocks  one,  as  dr  p.  120,  is  twice  as  long,  and  deep,  and  broad,  as  the  other, 
b.  The  larger  one  will  by  no  means  support  at  its  end  as  much  more  weight 
than  the  smaller,  as  its  mass  is  greater,  and  for  two  reasons.  1st.  Iii  the 
larger,  each  particle  of  the  surface  of  attachment  at  c,  in  helping  to  bear 


120  MECHANICS. 

the  weight  of  the  block  itself,  has  to  support  hy 
Fig.  64.  its  cohesion  twice  as  many  particles  beyond  it  in 

the  double  extent  of  projection,  as  a  particle  has 
to  support  in  the  shorter  block  at  a;  and  2ndly, 
both  the  additional  substance,  and  anj-thing  ap- 
pended at  its  outer  extremity,  are  acting  with  a 
double  lever  advantage  to  break  it,  that  is,  to  de- 
stroy the  cohesion  at  c.  Hence,  if  any  such  mass 
be  made  to  project  very  far,  it  will  be  broken  off 
or  will  fall  by  its  own  weight  alone.  And  what 
is  thus  true  of  a  block  supported  at  one  end,  is 
equally  true  of  a  block  supported  at  both  ends,  and 
indeed  of  all  masses,  however  supported,  and  of 
whatever  forms,  if  they  have  projecting  parts.  It 
is  ,to  be  observed,  also,  that  masses,  like  an  abso- 
lutely perpendicular  cliff,  which  have  no  projecting 

or  overhanging  parts,  are  still  limited,  as  to  size,  by  the  degree  of  cohesive 
force  among  their  particles,  for  the  upper  part  of  such  a  mass  tends  to 
crush  or  break  down  the  lower.  A  lofty  pillar  cannot  be  formed  of  soft  clay. 
That  a  large  body,  therefore,  may  have  proportionate  strength  to  a 
smaller,  it  must  be  still  thicker  and  more  clumsy  than  it  is  longer :  and 
beyond  a  certain  limit,  no  proportions  whatever  will  keep  it  together,  in 
opposition  merely  to  the  force  of  its  own  weight. 

This  great  truth  limits  the  size  and  modifies  the  shape  of  most  productions 
of  nature  and  art; — of  hills,  trees,  animals,  architectural  or  mechanical 
structures,  &c. 

Hills.  Yery  strong  or  cohesive  material  may  constitute  hills  of  sublime 
elevation,  with  very  projecting  cliffs  and  very  lofty  perpendicular  precipices; 
and  such  accordingly  are  seen  where  the  hard  granite  protrudes  from  the 
bowels  of  the  earth,  as  in  the  Andes  of  America,  the  Alps  of  Europe,  the 
Himalayas  of  Asia,  and  the  Mountains  of  the  Moon  in  Central  Africa. 
But  material  of  inferior  strength  exhibits  more  humble  risings  and  more 
rounded  surfaces.  The  gradation  is  so  striking  and  constant,  from  granite 
mountains,  down  to  those  of  chalk,  or  gravel,  or  sand,  that  the  geologist  can 
often  tell  the  substance  of  which  a  hill  is  composed  by  observing  the  pecu- 
liarities of  its  shape. 

Even  in  granite  itself,  which  is  the  strongest  of  rocks,  there  is  a  limit  to 
height  and  projection ;  and  if  an  instance  of  either,  much  more  remarkable 
than  now  remains  on  earth,  were  by  any  chance  to  be  produced  again,  the 
law  which  we  are  considering  would  prune  the  monstrosity.  The  grotesque 
figures  of  rocks  and  mountains,  seen  in  the  paintings  of  the  Chinese — or 
actually  formed  in  minature  for  the  gardens,  to  express  their  notions  of 
perfect  sublimity  and  beauty — are  caricatures  of  nature  for  which  originals 
can  never  have  existed.  Some  of  the  smaller  islands  in  the  Eastern  Ocean, 
however,  and  some  of  the  mountains  of  the  chains  seen  in  the  voyage 
towards  China,  along  the  coasts  of  Borneo  and  Palawan,  exhibit,  perhaps, 
the  very  limits  of  possibility  in  singular  shapes.  In  the  moon,  where  the 
weight  or  gravity  of  bodies  is  less  than  on  earth,  on  account  of  her  smaller 
size,  mountains  of  a  given  material  might  be  many  times  higher  than  on 
earth  —  and  observation  proves  that  the  lunar  mountains  are  in  fact  very 
high. 


STRENGTH    OF    MATERIAL.  121 

By  the  action  of  winds,  rains,  currents,  and  frosts,  upon  the  mineral  masses 
around  us,  there  is  unceasingly  going  on  an  undermining  and  wasting  of 
supports,  so  that  every  now  and  then  immense  rocks,  or  almost  hills,  are 
torn  by  gravity  from  the  station  which  they  have  held  since  the  earth  re- 
ceived its  present  form,  and  fall  in  obedience  to  the  law  now  explained. 

The  size  of  vegetables,  of  course,  is  obedient  to  the  same  law.  We  have 
no  trees  reaching  a  height  of  three  hundred  feet,  even  when  perfectly  perpen- 
dicular, and  sheltered  in  forests  that  have  been  unmolested  from  tho 
beginning  of  time :  and  oblique  or  horizontal  branches  are  kept  within 
comparatively  narrow  limits  by  the  great  strength  required  to  support  them. 
The  truth,  that  to  have  proper  strength,  the  breadth  or  diameter  of  bodies 
must  increase  more  quickly  than  the  length,  is  well  illustrated  by  the  con- 
trast existing  between  the  delicate  and  slender  proportions  of  a  young  oak  or 
elm,  yet  in  the  seedman's  nursery,  and  the  sturdy  form  of  one  which  has 
braved  for  centuries  all  the  winds  of  heaven,  and  has  become  the  monarch 
of  the  park  or  forest. 

Animals  furnish  other  interesting  illustrations  of  this  law. 
How  massive  and  clumsy  are  the  limbs  of  the  elephant,  the  rhinoceros, 
the  heavy  ox,  compared  with  the  slender  forms  of  the  stag,  antelope,  and 
greyhound !  And  unless  the  bones  were  made  of  stronger  material  than  now, 
an  animal  much  larger  than  the  elephant  would  fall  to  pieces  owing  to  its 
weight  alone.  The  whale  is  the  largest  of  animals,  but  feels  not  its  enor- 
mous weight  because  lying  constantly  in  the  liquid  support  of  the  ocean.  A 
cat  may  fall  with  impunity  from  a  greater  height  than  would  suffice  to  dash 
the  bones  of  an  elephant  or  an  ox  to  pieces. 

For  the  reason  which  we  are  now  considering,  the  giants  of  the  heathen 
mythology  could  not  have  existed  upon  this  earth;  although,  on  our  moon, 
where,  as  already  stated,  weight  is  much  less,  such  beings  might  be.  In  the 
planet  Jupiter,  again,  which  is  many  times  larger  than  the  earth,  an  ordi- 
nary man  from  hence  would  be  carrying  in  the  simple  weight  of  his  body  a 
load  sufficient  to  crush  the  limbs  which  supported  him.  The  phrase  a  little 
compact  man,  points  to  the  fact  that  such  a  person  is  stronger  in  proportion 
to  his  size  than  a  taller  man. 

The  same  law  limits  the  heighth  and  breadth  of  architectural  structures. 
In  the  houses  of  fourteen  stories,  which  formerly  stood  for  protection  close 
under  the  Castle  of  Edinburgh,  there  was  danger  of  the  superincumbent 
wall  crushing  the  foundation. 

Roofs.  Westminster  hall  approaches  the  limit  of  width  that  is  possible 
without  either  very  inconvenient  proportions  or  central  supports;  and  the 
dome  of  the  church  of  St.  Peter  in  Rome  is  in  the  same  predicament. 

Arches  of  a  bridge.  A  stone  arch,  much  larger  than  those  of  the  magni- 
ficent bridges  in  London,  would  be  in  danger  of  crushing  or  splintering  its 
material. 

Ships.  The  ribs  or  timbers  of  a  boat  have  scarcely  a  hundredth  part  of 
the  bulk  of  the  timbers  of  a  ship  only  ten  times  longer  than  the  boat.  A 
ship's  yard  of  ninety  feet  contains,  perhaps,  twenty  times  as  much  wood  as  a 
yard  of  thirty  feet,  and  even  then  is  not  so  strong  in  proportion.  If  ten  men 
may  do  the  work  of  a  three-hundred-ton  ship,  many  more  than  three  times 
that  number  will  be  required  to  manage  a  ship  three  times  as  large.  Very 
large  ships,  such  as  the  two  built  in  Canada,  in  the  year  1825,  which  carried 
each  nearly  ten  thousand  tons  of  timber,  are  weak  from  their  size  alone ;  and 
the  loss  of  these  first  two  specimens  of  gigantic  magnitude  will  not  encourage 
the  building  of  others. 


122 


MECHANICS. 


a 


II d, 


The  degree  in  which  the  strength  of  structure  is  dependent  on  the/orm  and 
position  of  their  parts,  will  be  illustrated  by  considering  the  two  cases  of 
longitudinal  and  transverse  compression.  And  the  rule  for  giving  strength 
to  any  structure  will  be  found  to  be,  to  cause  the  force  tending  to  destroy 
it,  to  act,  as  equally  as  possible,  on  the  whole  resisting  mass  at  once,  and 
with  as  little  mechanical  advantage  as  possible. 

In  longitudinal  compression,  as  produced  by  a  body  a,  on  the  atoms  of  the 
support  6,  the  weight,  while  the  support  remains  straight,  can  only  destroy 
the  support,  by  crushing  it  in  opposition  to  the  repulsion  and  impenetrability 

of  all  its  atoms.     Hence  a  very  small  pillar,  if 
Fig.  65.  kept  perfectly  straight,  supports  a  very  great 

/^     ^  weight  j    but   a  pillar   originally  crooked,  or 

P *1        beginning  to  bend,  resists  with  only  part  of  its 

strength  j  for,  as  seen  in*  c  d,  the  whole  weight 
above  is  supported  chiefly  on  the  atoms  of  the 
concave  side,  which  are  therefore  in  greater 
danger  of  being  oppressed  and  crushed,  while 
those  on  the  convex  side,  separated  from  their 
natural  helpmates,  are  in  the  opposite  danger  of 
being  torn  asunder.  The  atoms  near  the  centre 
in  such  a  case  are  almost  neutral,  and  might  be 
absent  without  the  strength  of  the  pillar  being 
much  lessened. 

Long  pillars  or  supports  are  weaker  than  short 
pillars  of  the  same  diameter,  because  they  are 
more  easily  bent;  and  they  are  more  easily  bent 

L- — '      because  a  very  inconsiderable,  and  therefore 

easily  effected  yielding  between  each  adjoining 
two  of  their  many  atoms,  makes  a  considerable 

bend  in  the  whole ;  while  in  a  very  short  pillar  there  cannot  be  much  bend- 
ing without  a  great  change  in  the  relation 
Fig.  66.  of  proximate  atoms,  and  such  as  can  be 

effected  only  by  great  force.     The  weight 
resting  on  any  pillar,  and  bending  it,  may 
be  considered  as  acting  (with  obliquity 
dependent  on  the  degree  of  bending  )  at 
the   end  of  a  long   lever  which  reaches 
from  the  extremity  to  the  centre  of  the 
\       pillar,  against  the  strength  resisting  always 
\      directly  at  a  short  lever  reaching  from  the 
/      side  d  to  the  centre  j  the  strength  of  the 
•        pillar,  therefore,  has  relation  to  the  differ- 
ence between  these  levers  and  to  the  degree 
of  bending.     Shortness,  then,  or  any  stay 
or  projection  as  a  e  b,  which,  by  making 
the  resisting  lever  longer,  opposes  bending, 
really  increases  the  strength  of  a  pillar. 

A  column  with  ridges  projecting  from 
it,  is  on  this  account  stronger  than  one 
that  is  perfectly  smooth. 

A  hollow  tube  of  metal  is  stronger  than  the  same  quantity  of  metal  as  a 
solid  rod,  because  its  substance  standing  farther  from  the  centre  resists  bond- 


I 
j 
e.  \--c- 


\ 


STRENGTH    OF    MATERIAL.  123 

ing  with  a  longer  lever.  Hence  pillars  of  cast-iron  are  generally  made 
hollow,  that  they  may  have  strength  with  as  little  metal  as  possible. 

In  the  most  perfect  weighing-beams  for  delicate  purposes,  that  there  may 
"be  the  least  possible  weight  with  the  required  strength,  the  arms,  instead 
of  being  of  solid  metal,  are  hollow  cones,  of  which  the  substance  is  not  much 
thicker  than  writing  paper. 

Masts  and  yards  for  ships  have  been  made  hollow  in  accordance  with  the 
same  principle. 

In  Nature's  work  we  have  to  admire  numerous  illustrations  of  the  same 
kind. 

The  stems  of  many  vegetables,  instead  of  being  round  externally,  are  ribbed 
or  angular  and  fluted,  that  they  may  have  strength  to  resist  bending.  Many 
also  are  hollow,  as  corn-stalks,  the  elder,  the  bamboo  of  tropical  climates, 
&c.,  thereby  combining  lightness  with  their  strength." — A  person  who  has 
visited  the  countries  where  the  bamboo  grows,  cannot  but  admire  the  almost 
endless  uses  to  the  inhabitants,  which  its  straightness,  lightness  and  hollow- 
ness,  fit  it  to  serve.  Being  found  of  all  sizes,  it  has  merely  to  be  cut  into 
pieces  of  the  lengths  required  for  any  purposes,  and  nature  has  already  been 
the  turner,  and  the  polisher,  and  the  borer,  &c.  In  many  of  the  Eastern 
Islands  it  is  the  chief  material,  both  of  the  dwellings,  and  of  the  furniture ; 
there  are  the  bamboo  huts  and  bungallows,  and  then  the  fanciful  chairs, 
couches,  beds,  &c. ;  flutes  and  other  wind  instruments  there,  are  merely 
pieces  of  the  reed  with  holes  bored  at  the  requisite  distance  :  conduits  for 
water  are  pipes  of  bamboo ;  bottles  and  casks  for  preserving  liquids  are 
single  joints  of  larger  bamboo  with  the  natural  partitions  remaining;  and 
bamboo  split  into  threads  is  twisted  jnto  rope,  &c. 

From  the  animal  kingdom  also  we  have  illustrations  of  our  present  sub- 
ject : — as  in  the  hollow  stiffness  of  the  quills  of  birds;  the  hollow  bones  of 
birds ;  the  bones  of  animals  generally — strong  and  hard,  and  often  angular 
externally,  with  light  cellular  texture  within,  &c. 

Transverse  Pressure. 

When  a  horizontal  beam  is  sup- 
ported at  its  extremities,  as  at  a  and  6, 
its  weight  bends  its  middle  down  more 
or  less,  as  here  shown,  the  particles 
on  the  upper  side  being  compressed, 
while  the  parts  below  are  distended; 
and  the  bending  and  tendency  to 

break  are  greater,  according  as  the  beam  is  longer  and  its  thickness  and 
depth  is  less. 

The  danger  of  breaking  in  a  beam,  so  situated  is  judged  of,  by  consider- 
ing the  destroying  force  as  acting  by  a  long  lever  reaching  from  an  end  of 
the  beam  to  the  centre,  and  the  resisting  force  or  strength  as  acting  only  by 
a  short  lever  from  the  side  d  to  the  centre  :  while  only  a  little  of  the  sub- 
stance of  a  beam  on  the  under  side  is  allowed  to  resist  at  all.  This  last  cir- 
cumstance is  so  remarkable,  that  the  scratch  of  a  pin  on  the  under  side  of  a 
plank  resting,  as  here  supposed,  will  sometimes  suffice  to  begin  the  fracture. 

Because  the  resisting  lever  is  small  in  proportion  as  the  beam  is  thinner,  a 
plank  bends  and  breaks  more  readily  than  a  beam,  and  a  beam  resting  on  its 
side  bears  less  weight  than  if  resting  on  its  edge.  Where  a  single  beam 


124  MECHANICS. 

cannot  be  found  deep  or  broad  enough  to  have  the  strength  required  in  any 
particular  case,  as  for  supporting  the  roof  of  a  house,  several  beams  are 
joined  together,  and  in  a  great  variety  of  ways,  as  is  seen  in  house-rafters, 
&c.,  w.hich  although  consisting  of  three  or  more  pieces,  may  be  considered 
as  one  very  broad  beam,  with  those  parts  cut  out  which  would  contribute 
least  to  the  strength. 

The  arched  form,  resting  against  immoveable  abutments,  bears  transverse 
pressure  so  admirably  because  by  means  of  it  the  force  that  would  destroy, 
is  made  to  compress  not  one  side  only,  but  all  the  atoms  or  parts  of  both 

sides  nearly  in  the  same  degree.  By 

Fig-  68.  comparing  this  figure  with  the  last, 

d  we  see  that  the  atoms  on  the  under 

side  of  an  arch,  must  be  compressed 
about  as  much  as  those  on  the  upper 
side,  and  are  therefore  in  no  danger  of 
being  torn,  or  overcome  separately. 
The  whole  substance  of  the  arch  therefore  resists,  nearly  like  that  of  a 
straight  pillar  under  a  weight,  and  is  nearly  as  strong. 

An  error,  which  has  been  frequently  committed  by  bridge-builders,  is  the 
neglecting  to  consider  sufficiently  the  effect  of  the  horizontal  thrust  of  the 
arch  on  its  piers.  Each  arch  is  an  engine  of  oblique  force  (see  page  56,) 
pushing  the  pier  away  from  it.  In  some  instances,  one  arch  of  a  bridge 
falling,  has  allowed  the  adjoining  piers  to  be  pushed  down  to.wards  it,  by  the 
thrust,  no  longer  balanced,  of  the  arches  beyond,  and  the  whole  structure 
has  given  way  at  once  like  a  child's  house  or  bridge  built  of  cards. 

It  is  not  known  at  what  time  the  arch  was  invented,  but  it  was  in  com- 
paratively modern  times.  The  hint  may  have  been  taken  from  nature, 
for  there  are  instances  in  Alpine  countries  of  natural  arches,  where  rocks 
have  fallen. bet  ween  rocks,  and  have  there  been  arrested  and  suspended,  or 
where  burrowing  water  has  at  last  formed  a  wide  passage  under  masses  of  rock, 
and  has  left  them  balanced  among  themselves  as  an  arch  above  the  stream. 
Nothing  can  surpass  the  strength  and  beauty  of  some  modern  stone  bridges ; 
— those,  for  instance,  which  spans  the  Thames  as  it' winds  through  London. 

Iron  bridges  have  been  made  with  arches  twice  as  large  as  those  of  stone  ; 
the  material  being  more  tenacious  and  easily  moulded,  is  calculated  to  form 
a  lighter  whole.  The  bridge  of  three  fine  arches  lately  built  between  the 
city  of  London  and  Southwark,  is  a  noble  specimen,  and  compared  with 
those  erected  in  the  preceding  century,  appears  almost  a  fairy  structure  of 
lightness  and  grace. 

The  great  domes  of  churches,  as  those  of  St.  Peter's  in  Rome  and  St. 
Paul's  in  London,  have  strength  on  the  same  principle  as  simple  arches. 
They  are  in  general  strongly  bound  at  the  bottom  with  chains  and  iron- 
bars,  to  aid  the  masonry  in  counteracting  the  horizontal  thrust  of  the 
superstructure. 

The  jGotbic  arch  is  a  pointed  arch,  and  is  calculated  to  bear  the  chief 
weight  on  its  summit  or  key-stone.  Its  use,  therefore,  is  not  properly  to 
span  rivers  as  a  bridge,  but  to  enter  into  the  composition  of  varied  pieces  of 
architecture.  With  what  effect  it  does  this,  is  seen  in  the  truly  sublime 
Gothic  structures  which  still  adorn  so  many  parts  of  Europe. 

The  following  are  instances,  in  smaller  bodies,  of  strength  obtained  by  the 
arched  form. — A  thin  watch-glass  bears  a  very  hard  push ;  a  dished  or 
arched  wheel  for  a  carriage  is  many  times  stronger  to  resist  all  kinds  of  shocks 
than  a  perfectly  flat  wheel; — a  full  cask  may  fall  with  impunity,  where  a 


STRENGTH  OF  MATERIAL.  125 

strong  square  box  would  be  dashed  to  pieces ; — a  very  thin  globular  flask 
or  glass,  corked  and  sent  down  many  fathoms  into  the  sea,  will  resist  the 
pressure  of  water  around  it,  where  a  square  bottle,  with  sides  of  almost  any 
thickness,  would  be  crushed  to  pieces. 

We  have,  from  the  animal  frame,  an  illustration  of  the  arched  form  giving 
strength,  in  the  cranium  or  skull,  and  particularly  in  the  skull  of  man, 
which  is  the  largest  in  proportion  to  its  thickness  : — the  brain  required  the 
most  perfect  security,  and  in  the  arched  form  of  the  skull  has  obtained  it 
with  little  weight. — The  common  egg-shell  is  another  example  of  the  same 
class  :  what  hard  blows  of  the  spoon  or  knife  are  often  required  to  penetrate 
this  wonderful  defence  of  a  dormant  life  !  The  weakness  of  a  similar  sub- 
stance not  arched,  is  seen  in  a  scale  from  a  piece  of  freestone  so  readily 
crumbling  between  the  fingers. 

To  determine,  for  particular  cases,  the  best  forms  and  positions  of  beams 
and  joists,  and  of  arches,  domes,  &c.,  is  the  business  of  strict  calculation, 
and  belongs  therefore  to  mathematics,  or  the  science  of  measures. 

It  was  a  beautiful  problem  of  this  kind,  which  Mr.  Sineaton,  the  English 
engineer,  solved  so  perfectly,  in  the  construction  of  the  far-famed  Eddy- 
stone  .light-house.  He  had  to  determine  the  form  and  dimensions  of  a 
building,  which  would  stand  firm  on  a  sunken  rock,  in  the  channel  of  a 
swift  ocean  tide,  and  exposed  to  the  fury  of  tempests  from  every  quarter. 
Only  the  man  who  has  himself  been  driven  before  the  irresistible  storm  in 
the  darkness  of  night,  and  in  the  midst  of  dangers,  and  whose  eyes  have 
watched  the  steady  ray  from  the  light-house  which  saved  him,  can  appre- 
ciate fully  the  importance  of  the  studies  which  bring  such  useful  results ;  or 
can  feel  how  happy  he  is  to  have  fellow-men,  whose  talents,  although  ex- 
erted usually  for  individual  good,  are  yet,  by  God's  providence,  made  to 
accomplish  the  most  philanthrophic  ends,  and  to  bind  the  whole  of  human 
kind  into  one  great  society  of  helping  brotherhood. 

[For  Animal  and  Medical  Mechanics,  see  Part  V.  Sect.  1.] 


126  HYDROSTATICS. 

PAET   III. 

THE    PHENOMENA    OF    FLUIDS.* 
SECTION  L— HYDROSTATICS. 

ANALYSIS   OF   THE   SECTION. 

The  particles  of  a  fluid  mass  are  freely  moveable  among  one  another,  so  as 
to  yield  to  the  least  disturbing  force  ;  and  if  bearing  force  at  all,  can  be 
at  rest  only  when  equally  forced  in  all  directions.  Hence  : 

1.  In  a  mass  of  fluid  submitted  to  compression,  the  whole  is  equally  affected^ 
and  equally  in  all  directions.     A  given  pressure,  for  instance,    made  by 
a  plug  forced  inwards  upon  a  square  inch  of  the  surf  ace  of  a  fluid  filling 
a  vessel,  is   suddenly  communicated  to   every  square   inch  of  the  vessel's 
surface,    however  large,  and  to  every  inch   of  the  surface  of  any  body 
immersed  in  the  fluid. 

2.  In  any  fluid,  the  particles  that  are  below  bear  the  weight  of  those  that 
are  above,  and  there  is,  therefore,  within  the  mass,  a  pressure  increasing 
exactly  with  the  perpendicular  depth,  and  not  influenced  by  the   size,  or 
shape,  or  position  of  the  containing  vessel. 

3.  The  open  surface  of  a  fluid  is  level;  and  if  various    pipes  or  vessels 
communicate  with  each  other,  any  fluid  admitted  to  them  will  rise  to  the 
same  level  in  all. 

4.  A  body  immersed  in  a  fluid  displaces  exactly  its  own  bulk  of  it,    which 
quantity  having  been  just  supported  by  the  fluid  around,    the  body   is 
pressed  upwards,  or   supported,  with  a  force  exactly  equal  to  the   weight 
of  the  fluid  displaced,  and  must  sink  or  swim  according  as  its  own  weight 
is  greater  or  less  than    this.     By  comparing,  therefore,  the  weight  of  a 
body  with  the  force  which  holds  it  up  in,  a  fluid,  the  comparative  weight 
or  specific  gravities  are  found. 

«  Fluid." 

It  was  explained  in  Part  I.,  that  the  same  atoms  may  exist  in  the  form 
of  a  solid  or  of  a  fluid ;  and  as  a  fluid,  they  may  either  constitute  a  dense 
liquid  like  water,  or  a  light  elastic  mass  like  air.  A  pound  of  ice,  or  a 
pound  of  water,  or  a  pound  of  steam,  differs  only  in  the  particles  being 
more  or  less  distant  from  each  other,  owing  to  the  different  quantities  of 
heat  among  them.  In  the  ice,  they  are  comparatively  near,  and  are  held 
together  by  attraction,  as  if  they  were  spitted  or  glued  to  each  other ;  in 
the  water,  the  repulsion  of  heat  seems  nearly  to  balance  attraction,  and  to 
leave  the  particles  at  liberty  to  glide  about  among  each  other  almost  without 

*  Read  again  the  Synopsis,  page  20. 


FLUIDS.  127 

friction  ;  and  in  the  steam,  the  repulsion  altogether  overcomes  the  attraction, 
and  the  particles  separate  to  a  great  distance,  as  if  held  apart  by  some  bulky 
elastic  medium.  The  few  facts  not  evidently  reconcilable  with  the  simple 
and  satisfactory  explanation  of  so  many  phenomena, — as  that  water  in  freez- 
ing, and  even  in  cooling  down  from  forty '  degress  to  the  freezing  point, 
increases  in  volume,  instead  of  contracting,  like  things  in  general,  and  like 
itself  in  cooling  at  other  temperatures, — and  that  baked  clay,  in  proportion 
as  it  is  more  heated,  contracts  instead  of  dilating, — are  treated  of  in  other 
parts  of  our  work. 

Whether  matter  be  in  the  solid  or  fluid  form,  the  properties  of  the  indi- 
vidual atoms  remain  unchanged,  that  is,  the  atoms  always  exist  in  accord- 
ance with  the  "  general  truths ;"  but  as,  in  the  chapter  on  Mechanics,  we 
found  so  many  important  modifications  of  effect  produced  by  the  circum- 
stance of  the  attraction  being  in  the  degree  which  produces  solid  cohesion 
among  the  particles,  in  this  chapter  on  fluids  we  shall  find  as  many 
important  results  springing  from  the  circumstances  of  non-cohesion  or 
fluidity. 

In  a  liquid  the  particles,  although  comparatively  near  to  one  another, 
seem  not  to  be  in  actual  contact  j  for  the  mass  may  be  condensed  indefinitely 
by  pressure.  The  force  required,  however,  to  change  the  volume  of  a  liquid 
in  any  sensible  degree,  is  so  great,  that  until  improved  means  of  experiment, 
recently  contrived,  liquids  were  accounted  absolutely  incompressible.  In 
aeriform  fluids,  on  the  contrary  each  particle,  under  common  circumstances, 
has  about  two  thousand  times  as  much  space  to  itself  as  when  forming  part 
of  a  liquid  or  solid  ;  and  hence  it  is  that  these  fluids  are  so  extensively  com- 
pressible and  dilatable — or  elastic,  as  they  are  called.  On  account  of  this 
elasticity,  they  exhibit  so  many  important  phenomena,  in  addition  to  those 
of  mere  fluidity,  that  the  consideration  of  them  requires  to  be  gone  into 
apart,  and  forms  the  branch  of  the  subject  called  pneumatics,  from  a  Greek 
word,  signifying  "spirit"  or  "breath  I" 

"  In  a  quantity  of  fluid  submitted  to  compression,  the  whole  mass  is  equally 
affected,  and  similarly  in  all  directions.  A  given  pressure,  therefore, 
made  upon  an  inch  of  the  surface  of  a  fluid  confined  in  a  vessel,  as  by  a 
plug  forced  inwards,  is  suddenly  borne  by  every  inch  of  the  surface  of 
the  vessel,  however  large,  and  by  every  inch  of  the  surface  of  any  body 
immersed  in  the  fluid.'' 

This  truth  is  of  great  importance,  both  from  its  explaining  so  many  re- 
markable phenomena  of  nature,  and  from  the  useful  applications  of  it  in  the 
construction  of  machinery. 

When  a  man  compresses  in  his  hands  a  bladder  full  of  air,  he  readily 
conceives  that  the  air  immediately  under  his  fingers  is  not  at  all  more 
compressed  than  that  in  every  other  part  of  the  bladder ;  and  of  course 
that  every  part  of  the  bladders' s  surface  must  be  pressing  the  air  as  much 
as  those  parts  of  it  on  which  his  fingers  rest,  and  must  be  bearing  a  reaction 
or  resistance  of  the  air  in  an  equal  degree ;  and  that  every  single  particle 
of  air  must  be  acted  upon  equally  on  every  side,  so  that  if  a  small  opening 
were  made  in  the  bladder  anywhere,  the  air  would  issue  from  it  with 
equal  readiness.  This  is  in  accordance  with  the  characteristic  of  fluidity, 
"  that  the  particles  glide  about  among  one  another  almost  without  friction, 
so  that  a  particle  can  never  be  at  rest  unless  when  equally  urged  in  all  di- 
rections." 


128 


HYDROSTATICS. 


In  like  manner,  if  a  close  vessel  B  be  filled  with  water,  and  into  the  top  of 
it  a  tube  a  c  be  screwed,  and  if  then,  by  means  of 


Fig.  69. 


1 


a  cork  or  moveable  plug  in  the  tube  at  c,  the  surface 
of  the  water  in  the  vessel  be  pressed  upon  with  a 
force  of  one  pound,  the  water  throughout  the  whole 
will  be  squeezed  or  condensed  in  proportion  to  the 
pressure,  and  every  other  portion  of  the"  vessel  B, 
of  equal  surface  with  c,  will  be  keeping  up  the  con- 
densation just  as  much  as  c,  and  will  be  bearing  the 
resistance  or  elasticity  of  the  water  to  the  extent  of 
one  pound.  And  if  there  were  another  similar  tube, 
b,  also  with  a  plug,  screwed  into  the  top  of  the  box  B, 
the  force  of  one  pound  depressing  the  plug  c  would 
be  pushing  up  the  plug  b,  with  the  same  force.  And  if  there  were  many 
other  similar  tubes  and  plugs,  by  acting  on  one,  all  would  be  equ- illy  affected  ; 
and  a  plug  or  piston  of  double  size  would  be  twice  as  much  affected  as  the 
smaller  one  ;  and  a  plug  d,  of  ten  times  the  size,  would  bu  lifted  with  a 
force  of  ten  pounds.  Hence  it  appears  that,  -through  the  medium  of  con- 
fined fluid,  a  force  of  one  pound,  acting  upon  an  inch  square  of  the  fluid  sur- 
face in  a  vessel,  may  become  a  bursting  force  of  ten,  or  a  hundred,  or  a  thous- 
and pounds,  according  to  the  size  of  the  vessel,  or  may  be  used  as  a  mechani- 
cal power  to  overcome  a  force  much  more  intense  than  itself.  It  will  be 
explained  below  that  the  well-known  hydrostatic  press  is  merely  a  large  plug 
or  piston  as  here  described,  forced  up  against  the  substance  to  be  pressed  by 
the  action  of  a  smaller  piston  in  another  barrel. 

If,  in  the  above  figure,  the  tube  a  were  such  as  to  contain  just  one  pound 
of  water,  on  the  plug  c  being  withdrawn  from  it,  and  water  being  poured  in 
to  fill  it,  the  .same  pressure  or  condensation  would  take  place  in  the  box  B 
as  when  the  plug  was  pressed  with  the  force  of  one  pound }  and  of  course 
exactly  the  same  effects  would  follow  on  the  sides  of  the  vessel  and  on  the 
other  pistons  ;  and  if,  in  the  other  tubes  also,  water  were  substituted  for  the 
pistons,  it  is  evident  that,  to  effect  a  balance  in  all,  it  would  require  to  stand 
as  high  in  every  one  as  in  the  tube  a  c,  producing  the  same  level  in  all, 
whatever  their  size 

•  The  fact  that  the  weight  of  one  pound  of  water,  or  any 
other  force  of  one  pound  similarly  applied,  may  be  made, 
through  the  medium  of  extended  fluid  surface  to  produce  a 
pressure  of  hundreds  or  of  thousands  of  pounds,  has  been 
called  the  "  hydrostatic  paradox,"  yet  there  is  nothing  in 
reality  more  paradoxical  in  it  than  that  one  pound  at  the  long 
end  of  the  lever  should  balance  ten  pounds  at  the  short  end  : 
indeed  it  is  but  another  means,  like  the  contrivances  usually 
called  mechanical  powers,  and  described  in  the  last  chapter, 
of  balancing  different  intensities  of  force,  by  applying  them 
to  parts  of  an  apparatus  moving  with  different  velocities. 
Here  the  tube  a  being  ten  times  smaller  than  the  tube  e,  the 
piston  a  must  descend  ten  inches  to  raise  the  greater  piston 
in  e  one  inch. 

This  law  of  fluid  pressure  is  rendered  very  striking  in  the 
experiment  of  bursting  a  strong  cask  by  the  weight  or  action 
of  a  few  ounces  of  water.  Suppose  a  cask  a  already  filled 
with  water,  and  that  a  long  small  tube  I  c  is  screwed  tightly 
into  its  top,  which  tube  will  contain  only  a  few  ounces  of 


Fig.   70, 

r 

b 


FLUIDS. 


129 


Fig.  71. 


water ;  by  pouring  these  few  ounces  into  the  tube,  the  cask  will  be  burst. 
In  explanation  it  is  unnecessary  to  say  more  than  that  if  the  tube  have  an 
area  of  a  fortieth  of  an  inch,  and  contain,  when  filled,  half  a  pound  of  water, 
that  water  would  produce  a  pressure  of  half  a  pound  upon  every  fortieth  of 
an  inch  all  over  the  interior  of  the  cask,  or  nearly  2,000  Ibs.  on-  the  square 
foot, — a  pressure  greater  than  a  common  cask  can  bear. 

A  similar  effect  is  seen  in  what  is  called  the  hydrostatic  bellows.  This 
consists  of  a  long  small  tube  a  I,  into  which  water  is  poured  to  enter  the 
body  of  the  apparatus  at  c,  which  resembles  the  common  bellows,  in  having 
wooden  boards  above  and  below,  and  strong  leather  connecting  them.  If 
the  tube  a  b  holds  an  ounce  of  water,  and  has  itself  only  one-thousandth  of 
the  area  of  the  top  of  the  bellows,  an  ounce  of  water  in  it  will  balance 
weights  of  a  thousand  ounces  placed  on  the  top  of  the  bellows  at.  d.  If 
mercury  were  substituted  in  this  machine  for  water,  the  effect  would  be 
fourteen  times  greater,  because  mercury  is  fourteen 
times  heavier  in  the  same  bulk.  And  if  a  man  stand 
on  a  large  bellows  of  the  kind,  he  may  raise  himself 
by  blowing  into  the  tube  with  his  mouth. 

The  annexed  cut  will  give  an  idea  of  Mr.  Braham's 
singularly  powerful  and  useful  hydrostatic  or  hydrau- 
lic press;  which,  if  compared  with  the  bellows,  exhi- 
bits merely  a  strong  forcing  pump  instead  of  the 
lofty  tube,  and  a  barrel  with  its  piston  instead  of  the 
leather  and  boards.  The  letter  e  points  out  the  pis- 
ton of  the  forcing  pump  worked  by  the  handle  d,  and 
driving  water  along  the  horizontal  tube  into  the  space 
f  under  the  large  solid  piston  c,  which  last,  with  its 
spreading  top,  is  urged  against  the  object  to  be  com- 
pressed. If  the  small  pump  have  only  one-thousandth 
of  the  area  of  the  large  barrel,  and  if  a  man  by  means 

of  its  lever-handle  d,  press  its  piston  down  with  a  force  of  five  hundred 
pounds,  the  piston  of  the  great  barrel  will 
rise  with  a  force  of  one  thousand  times  five 
hundred  pounds,  or  more  than  two  hundred 
tons.  Scarcely  any  resistance  can  withstand 
the  power  of  such  a  press ;  with  it  the  hand 
of  an  infant  can  break  a  strong  iron  bar ;  and 
it  is  used  to  condense  substances,  as  cotton 
or  hay  for  sea  voyages,  to  raise  great  weights, 
to  uproot  trees,  to  tear  things  asunder,  &c. 

The  Dilater  is  a  surgical  instrument  of  extensive  applicability,  of  which 
the  action  depends  on  the  principle  of  the  communication  of  fluid  pressure. 
It  was  proposed  by  the  author  some  years  ago,  and  was  brought  to  great 
practical  perfection  by  his  brother  Dr.  James  Arnott,  (now  superintendent 
surgeon  in  the  service  of  the  Hon.  East  India  Company,)  in  whose  publica- 
tion it  is  minutely  treated  of.  Many  professional  men  in  this  country 
doubted  of  its  power,  from  not  being  aware  of  the  nature  of  fluid  nature ; 
but  it  is  in  reality  a  kind  of  hydraulic  press,  allowing  the  operator  to  act 
with  the  most  gentle  or  most  energetic  force.  Further  remarks  are  made 
upon  it  in  the  medical  Section  which  follows  this  chapter. 


Fig.  72. 


130  HYDROSTATICS. 

"  In  any  fluid,  the  particles  that  are  below,  bear  the  weight  of  those  that  are 
above,  and  there  is  therefore  a  pressure  among  them  increasing  in  exact 
proportion  to  the  perpendicular  depth,  and  not  .influenced  by  the  size,  or 
shape,  or  position  of  the  containing  vessels." 

The  atoms  of  matter  having  gravity,  it  is  evident  that  the  upper  layer  of 
any  mass  of  fluid  must  be  supported  by  the  second,  and  this  with  its  load  by 
the  third,  and  the  third  with  its  double  load  by  the  fourth,  and  so  on.  This 
truth  is  experimentally  proved  by  putting  different  heights  of  liquid  into  an 
upright  tube,  of  which  the  bottom  is  closed  by  a  flap  having  a  spring  or  lever 
to  support  it,  and  to  indicate  the  force  acting  on  it.  And  what  is  true  of  the 
entire  column  of  water  in  the  tube,  may  be  considered  true  of  any  single  line 
of  atoms  ;  just  as  it  would  be  true  of  a  line  of  bricks  piled  one  above  another. 

A  tube  of  which  the  area  is  an  inch  square,  holds,  in  two  feet  of  its  length, 
nearly  a  pound  of  water ;  hence,  the  general  truth,  well  worth  recollecting, 
that  the  pressure  of  water,  at  any  depth,  whether  on  the  side  of  a  vessel  or 
on  its  bottom,  or  on  any  body  immersed,  is  nearly  one  pound  on  the  square 
inch  for  every  two  feet  of  depth. 

The  striking  effects  from  the  increase  of  pressure  in  a  fluid,  at  great  depths, 
are,  of  course,  most  commonly  exhibited  at  sea.  The  following  instances  will 
illustrate  them. 

If  a  strong  square  glass  bottle,  empty,  and  firmly  corked,  be  sunk  in 
water,  its  sides  are  generally  crushed  inwards  by  the  pressure  before  it 
reaches  a  depth  of  ten  fathoms. 

'A  chamber  of  air  similarly  let  down  with  a  man  it,  would  soon  allow 
him  to  be  drowned  by  the  water  bursting  in  upon  him ;  as  really  happened 
to  an  ignorant  projector. 

When  a  ship  founders  in  shallow  water,  the  wreck,  on  breaking  to  pieces, 
generally  comes  to  the  surface,  and  is  cast  upon  the  beach  ;  but  when  the 
accident  happens  in  deep  water,  the  great  pressure  at  the  bottom  forces 
water  into  the  pores  of  the  wood,  and  makes  it  so  heavy  that  no  part  can 
ever  rise  again  to  reveal  her  fate. 

A  bubble  of  air  or  of  steam,  set  at  liberty  far  below  the  surface  of  water, 
is  small  at  first,  and  gradually  enlargens  as  it  rises. 

A  man  who  dives  deep,  suffers  much  from  the  compression  of  his  client, 
as  the  elastic  air  within  him  yields  under  the  strong  pressure.  This  limits 
the  depth  to  which  divers  can  safely  go. 

It  is  not  known  whether  there  is  a  limit  to  the  pressure  which  fishes  can 
bear  to  impunity,  but  they  are  chiefly  found  living  in  the  shallower  waters 
on  coasts,  or  on  banks  in  the  midst  of  the  ocean,  such  as  the  banks  of 
Newfoundland,  the  Dogger-bank,  and  other  fishing  stations  out  at  sea.  In 
rounding  the  Cape  of  (rood  Hope,  at  a  considerable  distance  from  land, 
ships  pass  over  the  bank  of  Lagullas,  where  a  hook  let  down  with  a  bit  of 
red  rag,  or  almost  any  thing  as  a  bait,  immediately  secures  its  codfish. 

By  sending  a  vessel  prepared  for  the  purpose,  down  into  the  deep  sea, 
we  can  readily  prove  the  compressibility  of  water.  Suppose  the  vessel  to 
be  made  with  only  one  entrance,  and  that  a  small  round  opening,  into  which, 
instead  of  cork,  a  sliding  rod  has  been  closely  fitted.  If,  then,  when  filled 
with  water,  and  having  the  rod  inserted  into  the  opening,  it  be  allowed  to 
sink  into  the  sea,  the  pressure  around  will  push  the*rod  inwards,  in  a  degree 
proportioned  to  the  yielding  or  compression  of  the  water  within ;  and  if 
there  be  on  the  road  a  stiff  sliding-ring,  or  other  contrivance  to  indicate  on  the 
return  of  the  vessel  how  far  the  rod  has  been  driven  inwards,  the  apparatus 


PRESSURE    AS    DEPTH.  131 

will  show  the  degree  of  compression  at  the  greatest  depth  to  which  it  has 
descended.  Water  a  thousand  fathoms  below  the  surface  is  less  bulky  by 
about  one-twentieth  part  than  when  at  the  surface. 

The  following  are  proofs  of  the  pressure  of  weight  in  an  open  fluid,  opera- 
ting in  all  directions,  as  any  pressure  does  in  the  case  of  a  confined  fluid. 

A  bottle-cork  carried  far  under  water,  is  not  flattened  as  if  it  were  pressed 
unequally,  but  is  reduced  in  all  its  dimensions  so  as  to  appear  a  pbial-cork 
of  ^ie  usual  form. 

If  a  corked  empty  bottle  be  sent  down  into  the  sea,  the  cork  is  generally 
'forced  inwards  at  a  given  depth,  and  equally  so  in  whatever  direction  the 
mouth  of  the  bottle  may  happen  to  point. 

If  a  vessel  containing  water  have  an  opening  in  the  side,  covered  by  a 
valve  or  flap  so  contrived  as  to  tell  the  force  required  to  keep  it  shut,  we  find 
that  the  water  tends  to  escape  just  as  powerfully  through  such  an  opening 
as  it  would  through  one  in  the  bottom,  with  the  same  elevation  of  water 
over  its  centre.  And  different  equal  openings  in  the  side  of  a  vessel  require 
to  be  closed  with  forces  exactly  proportioned  to  the  heights  of  liquid  above 
their  centres. 

In  an  open  square  sided  vessel  full  of  water,  the  whole  pressure  on  any- 
upright  side  is  just  half  the  pressure  on  an  equal  extent  of  horizontal  bottom ; 
for  the  centre  of  the  side  being  just  half  as  deep  as  the  bottom,  the  pres- 
sure on  any  point  there  is  only  half  as  great  as  on  a  point  at  the  bottom,  and 
on  points  above  the  level  of  the  centre  is  just  as  much  less  than  half,  as,  at 
corresponding  distances  below,  it  is  more  than  half,  and  so  it  amounts  to  an 
exact  half  in  the  whole.  Considering  that  the  pressure  on  every  point 
below  the  central  level  is  greater  than  on  every  point  above  it,  we  see  the 
reason  why,  to  support  a  sluice  or  flood-gate  by  a  single  stay  on  the  outside, 
the  point  at  which  the  pressure  has  to  be  made  is  below  the  central  level. 
Calculation  discovers  that  this  point,  called  the  centre  of  pressure,  is  attme- 
third  from  the  bottom.  The  knowledge  of  such  facts  furnishes  rules  for  the 
construction  of  large  vessels  for  liquids,  canal  embankments,  &c. 

The  pressure  on  a  given  extent  of  the  side  of  a  narrow- vessel  is  just  as 
great  as  on  the  same  extent  of  the  side  of  a  wide  vessel,  having  the  same 
depth  of  fluid;  because,  as  now  expjained,  it  depends  entirely  on  the  extent 
of  surface  acted  upon  and  the  depth  of  liquid. 

Hence  a  flood-gate  or  sluice  which  shuts  out  the  ocean,  as  in  docks  open- 
ing to  the  sea,  bears  no  more  pressure  than  if  it  stood  only  against  an  equal 
depth  of  lake  or  river ;  or  than  if  it  were  one  of  two  such  flood-gates  become 
the  sides  of  a  very  narrow  vessel,  made  to  contain  only  a  few  hogsheads  of 
water. 

Hence,  again,  the  fear  is  unfounded  which  has  been  expressed  with  refer- 
ence to  the  formation  of  a  canal  between  the  Red  Sea  and  the  Mediterra- 
nean,— that  because  the  former,  owing  to  the  effects  of  easterly  winds  at  its 
mouth,  &c.,  is  twenty  feet  higher  than  the  later,  it  might  burst  through  the 
flood-gates,  and  carry  devastation  along  its  course. 

A  deep  crevice  in  a  rock,  when  filled  by  a  shower,  is  often  the  cause  of 
the  rock  being  torn  asunder,  and  of  part  being  precipitated* 

Extensive  walls  or  faces  of  masonry,  intended  to  confine  banks  of  sand 
or  earth,  if  no  openings  were  left  for  water  to  escape  from  behind  them, 
would  be  burst  after  a  rain  unless  they  had  the  strength  of  flood-gates  of 
the  same  size.  Ignorance  of  this  danger  has  led  to  some  extraordinary 
catastrophies. 


132  HYDROSTATICS. 

Other  examples  of  the  pressure  in  fluids  being  in  all  directions,  and  pro- 
portioned to  the  depth,  are  : — the  swelling  and  bursting  of  leaden  pipes  when 
filled  from  a  very  elevated  source: — the  tearing  up  of  the  coverings  of  sub- 
terranean drains  or  water  courses,  when,  during  a  flood,  any  accident  chokes 
them  near  their  lower  openings  : — the  violence  with  which  water  escapes  by 
an  opening  near  the  bottom  of  any  deep  vessl,  or  enters  by  an  opening  or 
leak  near  the  keel  of  a  deep-floating  ship : — the  great  strength  required  in 
the  lower  hoops  and  securities  of  those  enormous  vessels  of  porter-brewers, 
called  vats,  some  of  which  contain  man}'  thousand  barrels  of  liquid.  » 

In  speaking  of  the  pressure  of  a  fluid  in  all  directions,  some  persons  havc^ 
difficulty  in  conceiving  that  there  is  an  upward  as  well  as  a  downward  and  a* 
lateral  pressure.  But  if,  in  a  fluid  mass,  the  particles  below  had  not  a  ten- 
dency upwards  equal  to  the  weight  or  downward  pressure  of  the  fluid  over 
them,  they  could  not  support  that  fluid,  which  entirely  rests  upon  them. 
Their  tendency  upward  is  owing  to  the  pressure  around  them  from  which 
they  are  trying  to  escape.  Accordingly,  if  a  long  tube,  open  at  both  ends, 
and  with  a  sliding  plug  or  piston  in  it  near  one  end,  be  partially  plunged  into 
water  by  the  plugged  end,  the  water  is  found  to  press  the  plug  upwards  with 
force  proportioned  to  the  depth  to  which  it  is  carried,  and  exactly  equal  to 
the  force  with  which  water  presses  upon  an  equal  extent  of  the  bottom  or 
side  of  any  other  vessel  having  in  it  the  same  depth  •  or,  with  which,  in  the 
same  vessel,  it  would  press  other  plugs  in  other  branches  of  the  tube  pro- 
jecting in  various  directions.  On  removing  such  a  plug  altogether,  the  up- 
ward pressure  is  visibly  proved  and  measured  by  the  column  of  water  pushed 
into  the  tube  from  below,  and  supported  there  to  the  level  of  the  water  around. 

The  pressure  in  a  mass  of  fluid  is  proportioned  to  the  perpendicular  depth, 
and  is  not  at  all  influenced  by  the  size,  shape  or  position  of  the  containing 
vessel. 

A  body  immersed  in  the  water  of  a  lake,  one  foot  under  the  surface  is 
just  as  much  pressed  upon  as  if  it  were  one  foot  under  the  surface  of  the  sea, 
and  no  more  than  if  it  were  one  foot  under  the  surface  of  a  small  cistern. 
Suppose  vessels  differing' from  each  other  in  form  and  capacity,  as  sketched 
here  at  «,  6,  and  c,  but  all  having  flat  bottoms, 
of  exactly. the  same  area ;    if  fluid  be  poured 
into  all  to   the    same  level  or   perpendicular 
height,  as  represented  here  by  the  dotted  lines, 
although  the  quantity  be  very  different  in  each, 
the  pressure  on  the  bottom  will  be  the  same  in 
all.  This  truth  is  easily  proved  experimentally, 
by  having  the  bottoms  inoveable,  and  held  to 
their  places   by  weights  or  springs  capable  of 
measuring  the  pressure :  or  by  letting  the  three 

vessels  all  communicate  with  the  same  vessel  of  water  below  them,  and  then 
observing  that  the  water  in  all  has  still  the  same  level.  These  results  are 
other  exemplifications  of  the  truths,  "pressure  equal  in  all  directions" 
tl pressure  as  depth,"  and  "pressure  as  the  extent  of  surface."  For  as  a 
column  of  the  fluid,  resting  on  the  middle  of  each  bottom,  just  presses  with 
its  whole  weight,  and  therefore,  according  to  its  altitude,  this  column  could 
not  remain  at  rest  if  there  were  any  greater  or  less  pressure  than  its  own 
near  it ;  then  as  the  fluid  really  is  at  rest  in  all  the  cases,  and  in  all  a 
central  column  is  of  the  same  height,  the  pressure  must  be  equal  on  all  the 
bottoms.  The  case  of  the  largest  vessel  a,  is  in  a  degree  illustrated  by 


FLUID    LEVEL.  133 

supposing  the  water  in  it  to  be  suddenly  converted  into  smooth  upright  small 
columns  or  rods  of  ice  or  glass ;  Then  evidently  only  those  pieces  which 
rested  on  the  bottom,  could  press  on  it  while  the  others  would  be  supported 
by  the  oblique  sides  of  the  vessel,  and  by  the  lateral  resistance  of  the  pieces 
around  them. 

"  Level  surface  of  a  Fluid."     (Read  the  Analysis.) 

That  the  surface  of  a  fluid  must  be  level,  follows  from  the  facts  of  all  the 
particles  being  equally  attracted  towards  the  centre  of  the  earth,  and  being 
perfectly  moveable  among  themselves.  The  particles  forming  the  surface 
may  be  regarded  as  the  tops  of  so  many  columns  of  particles,  supported  at 
any  given  level  below,  by  a  uniform  resistance  of  pressure; — for  no  particle 
of  an  inferior  level  can  be  at  rest  unless  equally  urged  in  all  directions,  and 
therefore  all  the  particles  at  such  a  level,  and  which,  by  equally  urging  one 
another,  keep  themselves  at  rest,  must  all  be  bearing  the  weight  of  equal 
columns :  thus  a  higher  column  must  sink  and  a  lower  one  must  rise,  until 
just  balanced  by  those  around ;  that  is,  until  all  become  alike.  Besides, 
just  as  a  ball  rolls  down  a  slope  or  inclined  plane,  so  do  the  particles  of 
a  fluid  slide  or  move  from  any  higher  situation  among  themselves  to  any 
lower  unoccupied  situation  near  them.  The  account  now  given  explains 
why  any  accidental  elevation  or  depression  of  a  fluid  surface,  usually  called 
a  wave,  continues  to  rise  and  fall,  or  to  oscillate,  for  some  time  with  gradu- 
ally diminishing  forces ; — for  when  the  mass  is  raised  above  the  general  level 
it  is  not  quite  supported,  and  therefore'-soon  sinks,  but  in  sinking,  like,  a 
falling  pendulum,  it  acquires  momentum  which  carries  it  below  the  general 
level,  until  opposed  and  arrested  by  a  resistance  greater  than  its  weight,  it 
then  rises  again,  by  acquiring  a  new  momentum  in  its  rise,  it  has  to  fall 
again,  again  to  rise,  and  this  alteration  continues,  until  the  lateral  sliding 
ot  the  particles,  and  the  friction  among  them,  gradually  destroy  it. 

A  perfectly  level  surface  on  earth  really  means  one  in  which  every  particle 
is  equi-distant  from  the  centre  of  the  earth,  and  it  is  therefore  truly  a 
spherical  surface  ;  but  so  large  is  the  sphere,  that  if  a  slice  of  it  of  two  miles 
in  diameter  were  cut  off,  and  laid  on  a  perfect  plane,  the  centre  of  the  slice 
would  only  be  four  inches  higher  than  the  edges.  Any  small  portion  of  it, 
therefore,  for  all  common  purposes,  may  be  accounted  a  perfect  plane. 

So  truly  smooth  does  a  fluid  surface  become,  that  it  forms  a  perfect 
mirror;  that  is,  it  reflects  or 'throws  back  the  rays  of  light,  which  fall 
upon  it  so  exactly  in  the  order  which  they  had  on  leaving  the  object,  that 
an  eye  which  receives  them  may  fancy  the  object  to  be  placed  in  the 
direction  of  the  mirror. — It  was  over  the  glassy  surface  of  the  fountain  or 
the  lake,  that  the  shepherdess  of  the  young  world  bent  themselves,  to  learn 
the  charms  which  nature  had  bestowed  on  them.  And  a  child  contemplates 
with  wonder  and  delight,  through  the  window  of  a  still  pool  or  gliding^ 
stream,  another  sky  below  the  ground,  with  its  cloulds,  and  sun  or  stars; 
and  another  landscape,  with  inverted  wood  and  mountains,  the  supposed 
dwelling  of  fairy  beings. 

In  the  cutting  of  canals,  th^  making  of  railways,  and  in  many  other  ope- 
rations of  engineering,  it  is  of  essential  importance 
to  determine  the  level  or  horizontal  direction  at  any  Fig.  74. 

place  ;  and  this  is  usually  done  by  a  tube  or  glass, 
a  c,  filled  with  spirit  except  one  bubble  of  air   b,      ^       — & —       -^ 

and  called  a  spirit  level.     When  this  tube  is  hori-        " 

zontal,  the  bubble  has  no  tendency  to  move  to  either 


134  HYDROSTATICS. 

end ;  but  if  the  tube  inclines  ever  so  little,  the  bubble  rises  to  the  end  which 
is  highest ;  or  to  speak  more  correctly,  the  denser  spirit  falls  down  to  the 
lower  end,  and  forces  the  light  bubble  away  from  it.  Such  a  tube  properly 
fixed  in  a  frame,  with  a  telescope  attached  to  it,  or  simply  with  sight-holes 
to  look  through,  becoming  the  engineer's  guide  in  many  of  his  most 
important  operations. 

A  hoop  surrounding  the  earth  would  bend  away  from  a  perfectly  straight 
line  four  inches  in  a  mile.  In  cutting  a  level  canal,  therefore,  which  may 
be  considered  as  part  of  a  hoop,  there  must  be  everywhere  a  falling  from 
the  straight  line,  called  by  geometers  a  tangent,  in  the  proportion  now 
described.  All  rivers  also  have  the  curvature  of  hoops  applied  to  the  surface 
of  the  earth. 

Canals  leading  from  sea-ports  to  the  interior  of  countries  have  generally 
to  ascend  ;  but  as  water  cannot  become  stagnant  in  any  channel  which  is  riot 
level,  the  canal  is  divided,  by  gates  or  sluices,  into  portions  at  different 
levels,  like  steps  of  a  stair,  the  rising  .at  the  joinings  being  generally  from 
six  to  twelve  feet.  The  boats  are  raised  or  lowered  from  one  level  to 
another  by  the  contrivance  called  a  lock,  which  is  merely  a  portion  of  the 
canal,  of  sufficient  capacity  f6r  the  boat  to  lie  in,  furnished  with  high  walls, 
and  with  flood-gates  at  both  ends;  so  that  when  the  gates  below  are  shut, 
and  water  is  gradually  admitted  from  above,  it  becomes  part  of  the  high 
level,  ready  as  such  to  deliver  a  boat,  or  receive  one ;  and  when  the  upper 
flood-gates  are  shut,  and  the  water  is  gradually  allowed  to  escape  from  the 
lock,  it  becomes  a  part  of  the  low  level,  and  a  boat  may  enter  it;  or  leave  it 
by  its  lower  gates. 

The  cutting  of  canals  is  one  of  the  great  items  in  the  mass  of  modern 
improvement,  which  both  mark  and  hasten  the  progress  of  civilization. 
Adverting  to  the  importance  of  easy  intercouse,  as  explained  in  a*  former 
section,  we  need  only  say  here,  that  a  horse  which  can  draw  only  one  ton 
on  our  best  roads,  can  draw  thirty  with  the  same  speed  in  a  canal-boat. 

And  what  a  glorious  triumph  to  science  and  art  it  is,  to  be  able  to  conduct 
vessels  of  all  kinds,  even  those  originally  intended  for  the  ocean  surge  alone, 
through  the  quiet  valleys  of -an  interior  country  !  In  Scotland,  now,  along 
the  Caledonian  canal,  a  noble  frigate  may  be  seen,  wandering  as  it  were 
among  the  inland  solitudes,  and  displaying  her  grace  and  majesty  to  the 
astonished  gaze  of  the  mountain  shepherd  ;  and  when  she  has  traversed  the 
kingdom,  and  visited  the  lonely  lakes,  whose  waters  until  lately  had  borne 
only  the  skiff  of  the  hunter,  she  descends  again  by  the  steps  of  her  liquid 
stair,  and  safely  resumes  her  place  among  the  waves. 

It  was  lately  in  contemplation  to  lead  a  ship  canal  across  the  isthmus 
which  joins  North  and  South  America.  The  elevation  to  which  the  canal 
must  reach,  to  surmount  the  central  ridge,  is  considerable,  and  will  increase 
the  difficulty ;  but  such  important  consequences  would  follow  the  accomplish- 
ment of  the  object,  that,  with  the  continuance  of  general  peace,  and  the 
increase  of  political  wisdom,  it  will  probably  be  attained.  If  so,  the  loaded 
vessel,  rising  from  the  Atlantic,  would  soon  be  descried  among  the  mountain 
heights,  and,  a  few  hours  after  would  be  safely  lodged  in  a  port  of  the  oppo- 
site sea;  having  performed,  by  a  near  cut,  a  voyage  which  at  present  costs 
months  of  delay  and  hazard,  in  a  tedious  navigation  round  the  whole  southern 
continent. — And  if  the  Red  Sea  and  Mediterranean  were  joined  in  the  same 
way,  as  has  also  been  proposed,  the  operation  would,  in  effect,  bring  India 
nearer  to  Europe,  and  would  more  and  more  strengthen  the  bonds  of  mutual 
utility  and  brotherhood  among  the  nations  of  the  earth.  Then,  indeed,  might 


FL-U  ID    LEVEL.  135 

it  be  said  with  truth,  that  the  world  is  a  vast  garden,  given  to  man  for 
his  abode,  of  which  every  spot  has  its  peculiar  sweets  and  treasures  j  but 
because  the  cultivator  of  each  may  exchange  a  share  of  its  produce  for  shares 
in  return,  the  same  general  result  follows  as  if  every  field  or  farm  contained 
within  itself  the  climates  and  soils  and  capabilities  of  the  whole. 

In  a  canal,  the  least  deviation  from  the  true  level  would  immediately  cause 
any  water  admitted  into  it  to  flow  towards  the  lower  end.  This  flux  to  a 
lower  situation  is  what  is  going  on  in  the  myriads  of  streams,  which  render 
the  face  of  the  earth  a  scene  of  such  varied  beauty  and  incessant  change. 

As  in  the  animal  body,  from  every  the  minutest  point  a  little  vein  endowed 
with  living  power,  takes  the  blood  which  has  just  brought  life  and  nutriment 
to  the  part,  and  delivers  it  into  a  larger  vein,  whence  it  passes  into  a  larger 
still,  until  at  last,  in  the  great  reservoir  of  the  heart,  it  meets  the  blood  re- 
turned from  every  part  of  the  body,  so,  in  this  terraqueous  globe,  where  the 
magic  moving  power  is  simply  fluid  seeking  its  level,  does  the  rain  which 
falls  to  sustain  vegetable  and  animal  life,  and  to  renovate  nature,  glide  from 
every  point  of  the  surface  into  a  lower  bed,  and  from  thence  into  a  lower  still, 
until  the  countless  streams,  so  formed,  after  every  variety  of  course  combine 
to  form  the  swelling  rivers,  which  return  the  accumulated  waters  into  the 
common  reservoir  of  the  ocean.  In  the  living  body,  the  arteries  carry  back 
the  blood  with  renewed  vitality  to  every  point  whence  the  veins  had  with- 
drawn it,  and  so  complete  the  circulation;  and  in  what  may  be  called  the 
living  universe,  the  circulation  is  completed  by  the  action  of  heat  and.  of  the 
atmosphere,  which,  from  the  extended  face  of  the  ocean  raise  a  constant 
exhalation  of  watery  vapour  of  invisible  purity,  which  the  winds  then  carry 
away  and  deposit  as  rain  or  dew  on  every  spot  of  the  earth. 

A  very  slight  declivity  suffices  to  give  the  running  motion  to  water.  Three 
inches  per  mile,  in  a  smooth  straight  channel,  gives  a  velocity  of  about  three 
miles  per  hour.  The  Ganges,  which  gathers  the  waters  of  the  Himalaya 
mountains,  the  loftiest  in  the  world,  is,  at  eighteen  hundred  miles  from  its 
mouth,  only  eight  hundred  feet  above  the  level  of  the  sea — that  is,  above 
twice  the  height  of  St  Paul's  Church  in  London ;  and  to  fall  these  eight 
hundred  feet,  in  its  long  course,  the  water  takes  nearly  a  month.  The  greater 
river  Magdalena,  in  South  America,  whose  channel,  for  a  thousand  miles,  is 
between  two  ridges  of  the  Andes,  falls  only  five  hundred  feet  in  all  that  dis- 
tance. Above  the  commencement  of  the  thousand  miles,  it  is  seen  descending 
in  rapids  and  cataracts  from  the  mountains.  The  gigantic  Rio  de  la  Plata 
has  so  gentle  a  descent  to  the  ocean,  that,  in  Paraguay,  fifteen  hundred  miles 
from  its  mouth,  large  ships  arrive  which  have  sailed  against  the  current  all 
the  way,  by  the  force  of  the  wind  alone  :  that  is  to  say,  which  on  the  beau- 
tifully inclined  plane  of  the  stream,  have  been  gradually  lifted  by  the  soft 
wind,  and  even  against  the  current,  to  an  elevation  greater  than  that  of  our 
loftiest  spires. 

A  small  lake  or  extensive  mill-pond,  with  uneven  bottom,  if  suddenly 
emptied  by  a  sluice  or  opening  at  its  lowest  part,  would  exhibit  a  number  of 
pits  or  pools  of  various  size  and  shape  left  among  the  inequalities.  But  sup- 
posing rain  to  fall,  and  frequently  to  recur,  the  water  seeking  its  level  would 
soon  effect  a  very  remarkable  change.  In  consequence  of  each  pool  discharg- 
ing over  its  lowest  part,  that  is,  sending  out  a  streamlet  either  into  another 
lower  pool,  or  into  a  channel  leading  directly  to  the  sluice  or  opening,  there 
would  be  a  constant  wearing  down  of  the  part  or  side  of  the  pool  over  which 
the  water  was  running,  that  is  to  say,  a  deepening  of  a  breach  or  channel 
there,  and  the  surface  of  water  in  the  pool  would  be  consequently  becoming 


136  HYDROSTATICS. 

lower,  while,  at  the  same  time,  the  bottom  would  be  rising,  owing  to  the 
deposit  of  sand  or  mud  washed  down  by  the  rain  from  the  elevations  around : 
and  these  two  operations  continuing,  the  pool  would  at  last  altogether  disap- 
pear. And  by  this  change  going  on  in  every  pool  through  the  whole  of  the 
emptied  mili-pond,  the  general  bottom  would  at  last  exhibit  only  a  varied  or 
undulated  surface  of  dry  land,  with  a  beautiful  arrangement  of  ramifying 
water  channels,  all  sloping  with  a  precision  unattainable  by  art,  to  the  general 
mouth  or  estuary. — The  reason  that,  in  the  supposed  case,  and  in  every  other, 
a  water  course  soon  becomes  so  singularly  uniform,  both  as  to  dimensions 
and  descent,  is,  that  any  pits  or  hollows  in  it, are  filled  up  by  the  sand  and 
mud  carried  along  in  the  stream,  and  deposited  where  the  current  is  slack ; 
while  any  elevations  are  worn  away  by  the  action  of  the  more  rapid  current 
which  accompanies  shallowness. 

The  above  paragraph  describes,  in  miniature,  what  has  been  going  on  over 
the  general  face  of  our  earth  ever  since  that  convulsion  of  nature  which 
produced  its  present  form.  In  many  places  the  phenomenon  is  already  com- 
plete ;  in  others  it  is  only  in  progress.  The  whole  of  what  is  now  dry  land, 
has  at  some  period  been  under  water,  and  much  of  it  has  evidently  been  a 
gradual  deposition  from  water.  By  some  extraordinary  convulsion,  there- 
fore, our  present  continents  and  islands  must  have  been  thrown  up  from  the 
bottom  of  an  ocean,  or  an  ocean  must  have  subsided  away  from  them ;  and 
in  either  case  the  land  must  have  merged  as  checkered  and  unsightly  as  the 
bottom  of  the  emptied  lake  above  supposed.  And  it  is  the  gradual  opera- 
tion of  water  seeking  its  level  which  has  at  last  converted  the  earth  into  the 
paradise  which  we  now  behold. 

The  marks  of  the  former  state  of  the  world,  and  of  the  progressive 
change,  are  every  where  most  strikingly  evident  to  the  enlightened  eye  of 
philosophy.  The  present  kingdom  of  Bohemia,  for  instance,  is  the  bottom 
of  one  of  the  great  lakes  formerly  existing  over  Europe.  It  is  a  basin  or 
amphitheatre,  formed  by  a  wall  of  mountains,  and  the  only  gate  or  opening 
to  it,  is  that  remarkable  one  by  which  the  water  now  escapes  from  it,  and 
which  evidently  has  been  gradually  cut  or  formed  by  the  action  of  the  run- 
ning stream.  As  the  bottom  became  uncovered,  owing  to  the  sinking  of  the 
water,  and  the  formation  of  a  regular  sloping  channel  from  every  part,  the 
former  lake  was  converted  into  a  fine  and  fertile  country,  a  fit  habitation  for 
man ;  and  the  continued  drain  from  it  of  the  rains  which  fall  over  its  surface, 
and  either  pass  rapidly  away,  or  sink  into  the  earth,  and  ooze  again  more 
gradually  in  the  form  of  springs,  is  the  beautiful  river  which  we  now  call 
the  Elbe. 

In  Switzerland,  many  of  the  valleys  which  were  formerly  lakes,  have  the 
opening  for  the  exit  of  water  so  narrow,  that,  as  happened  in  one  of  them  a 
few  years  ago,  a  mass  of  snow  or  ice  falling  into  it,  converts  the  valley  once 
more  into  a  lake.  On  the  occasion  alluded  to,  the  accumulation  of  water 
within  was  very  rapid;  and  although,  from  the  danger  foreseen  to  the  coun- 
try below,  if  the  impediment  should  suddenly  give  way,  every  means  was 
tried  to  remove  the  water  gradually,  the  attempt  had  not  succeeded  when  the 
frightful  burst  took  place,  and  involved  the  inferior  country  in  common  ruin. 

The  magnificent  Danube  is  the  drain  of  a  chain  of  basins  or  lakes,  which 
must,  at  one  time,  have  discharged  or  run  over  one  into  another ;  but  owing 
to  the  continued  stream  cutting  a  passage  at  last  low  enough  to  empty  them 
all,  they  are  now  regions  of  fertility,  occupied  by  civilized  man,  instead  of 
the  fishes  which  held  them  formerly.  This  operation  is  still  going  on  in 
all  the  lakes  of  the  earth.  The  lake  of  Geneva,  for  instance,  although  con- 


FLUID    LEVEL.  137 

• 

fined  by  hard  rock,  is  lowering  its  outlet,  and  the  surface  has  consequently 
fallen  within  the  period  of  accurate  observation  and  records  ]  and  as,  at  the 
same  time,  the  wearing  of  the  neighbouring  mountains,  brought  down  by 
the  winter  torrents,  are  filling  up  its  bed,  if  the  town  of  Geneva  last  long 
enough,  its  inhabitants  may  have  to  speak  of  the  river  in  the  neighbouring 
valley,  instead  of  the  picturesque  lake  which  now  fills  it.  Already  several 
towns  and  villages,  which  were  close  upon  the  lake  a  century  ago,  have  fields 
and  gardens  spreading  between  them  and  the  shore. 

Illustrating  this  subject,  it  is  very  interesting  to  observe  the  contrast 
between  the  pure  blue  water  of  the  Rhone  issuing  from  the  lake  of  Geneva, 
and  the  turbid  streams  which  join  its  course  a  little  farther  down.  The 
torrents  which  fall  into  the  lake  all  around,  are  equally  charged  with  the 
debris  or  wearings  of  the  mountains ;  but  having  deposited  all  their  load  in 
the  still  bosom,  of  the  lake,  the  pure  water  alone  escapes  to  form  the  river. 
The  streams,  however,  coining  to  the  Rhone  directly  from  the  Alps,  and 
bringing  with  them  their  charge  of  broken-down  earth,  even  after  they  have 
joined  it,  are  long  distinguishable  by  their  muddy  waters.  It  is  the  mud 
deposited  as  here  described,  which  is  gradually  filling  up  all  lakes,  and  which 
has  formed  the  vast  regions  of  flat  country  seen  about  the:  mouths  of  great 
rivers.  The  greater  part  of  Holland  is  deposition  of  this  kind,  the  whole 
of  lower  Egypt,  a  great  part  of  Bengal,  &c.,  &c. 

There  are  some  lakes  on  the  face  of  the  earth  which  have  no  outlet  towards 
the  sea, — all  the  water  which  falls  into  them,  being  again  carried  off  by 
evaporation  alone — and  such  lakes  are  never  of  fresh  water,  because  every 
substance,  which,  from  the  beginning  of  time,  rain  could  dissolve  in  the  re- 
gions around  them,  has  necessarily  been  carried  towards  them  by  their  feed- 
ing streams,  and  there  has  remained.  The  great  majority  of  lakes,  however, 
being  basins  with  the  water  constantly  running  over  atone  part  towards  the 
sea,  although  all  originally  salt,  have,  in  the  course  of  time,  become  fresh, 
because  their  only  supply  being  directly  from  the  clouds,  or  from  rivers 
and  springs  fed  by  the  clouds,  is  fresh,  while  what  runs  away  from  them 
must  always  be  carrying  with  it  a  proportion  of  any  substance  that  remains 
dissolved  in  them.  We  thus  see  how  the  face  of  the  earth  has  been  gradu- 
ally washed  to  a  state  of  purity  and  freshness  fitting  it  for  the  uses  of  man, 
and  why  the  great  ocean  necessarily  contains  in  solution  all  the  substances 
which  originally  existed  near  the  surface  of  the  earth,  soluble  in  water : — 
viz.,  all  the  saline  substances.  The  city  of  Mexico  stands  in  the  centre  of  a 
vast  and  beautiful  plain,  7,000  feet  above  the  level  of  the  sea,  and  surrounded 
by  sublime  ridges  of  mountains,  many  of  them  snow-capped.  One  side  of 
tne  plain  is  a  little  lower  than  the  other,  and  forms  the  bed  of  the  lake,  which 
is  salt  for  the  reasons  stated  above ; — but  the  lake  will  not  long  be  salt,  for 
it  now  has  an  outlet.  About  150  years  ago,  owing  to  unusual  rains,  an  ex- 
traordinary increase  of  the  water  took  place,  and  covered  the  pavements  of 
the  city.  An  artificial  drain  was  then  cut  from  the  plain,  at  the  distance  of 
about  sixty  miles  from  the  city,  to  the  lower  external  country.  This  soon  freed 
the  city  from  the  water,  and  since  then,  becoming  every  year  deeper  by  the 
wearing  effects  of  the  uninterrupted  stream,  it  is  still  lowering  the  surface 
of  the  lake,  is  daily  rendering  the  water  less  salt,  and  is  converting  the  vast 
salt  marshes,  which  formerly  surrounded  the  city,  into  fresh  and  fertile  fields. 

The  vast  continent  of  Australasia,  or  New  Holland,  (as  large  as  Europe,) 
is  supposed  by  some  to  have  been  formed  at  a  different  time  from  what  is 
called  the  Old  World,  so  different  and  peculiar  are  many  of  its  animal  and 
vegetable  productions  -}  and  the  idea  of  a  later  formation  receives  counte- 


138  HYDROSTATICS. 

• 

nance  from  the  existence  of  immense  tracts  of  marshy  or  imperfectly  drained 
land  discovered  in  the  interior,  into  which  rivers  flow,  but  seem  not  yet  to 
have  worn  down  a  sufficient  outlet  or  discharging  channel  toward  the  ocean. 
Where  the  soil  or  bed  of  a  country  through  which  a  water-track  passes 
is  not  of  a  soft  consistence,  so  as  to  allow  readily  the  wearing  down  of  higher 
parts,  and  the  filling  up  of  hollows  by  deposited  sand,  lakes,  rapids  and  great 
irregularities  of  current  remain.  We  have,  for  instance,  the  line  of  the  lakes 
in  North  America,  the  rapids  of  the  St.  Lawrence,  and  the  stupendous  falls 
of  Niagara,  where  at  one  leap  the  river  gains  a  level  lower  by  a  hundred  and 
sixty  feet.  A  softer  barrier  than  the  rock  over  which  the  river  pours,  would 
soon  be  cut  through,  and  the  line  of  lakes  would  be  emptied. 

The  contemplation  of  the  fact,  that  water  in  seeking  its  level  is  constantly 
wearing  where  it  rubs,  and  carrying  the  abraided  portions  down  to  lower 
level,  and  ultimately  to  the  bed  of  the  ocean,  brings  irresistibly  the  awful 
idea,  that  this  earthly  abode  of  ours,  owing  to  natural  causes  already  in 
operation,  can  have  but  a  limited  existence  in  its  present  state.  No  shower 
falls  that  does  not  send  portions  of  mountains  and  plains  into  the  depths 
of  the  ocean,  and  thus  cause  a  corresponding  encroachment  on  the  shores  by 
the  rising  water ;  and  with  revolving  ages,  unless  new  convulsions  of  nature 
disturb  the  progress,  or  art  succeed,  as  in  Holland  and  elsewhere,  in  shut- 
ting out  the  ocean  from  extensive  low  tracts  by  means  of  sea  dykes  or  em- 
bankments, *the  dry  land  must  at  last  disappear,  and  another  gradual  deluge 
embrace  the  globe. 

There  is,  perhaps,  nothing  which  illustrates  in  a  more  striking  manner 
the  exact  resemblance  among  nature's  phenomena,  or  their  accordance  with 
the  few  general  expressions  or  laws  which  describe  them  all,  than  the  perfect 
level  of  the  ocean  as  a  liquid  surface.  The  sea  never  rises  nor  falls  in  any 
place,  even  one  inch,  but  in  obedience  to  fixed  laws,  and  therefore  its  changes 
may  generally  be  foreseen  and  allowed  for.  For  instance,  the  eastern  trade- 
winds  and  other  causes  force  the  water  of  the  Indian  Ocean  towards  the 
African  coast,  so  as  to  keep  the  Red  Sea  about  twenty  feet  above  the 
general  ocean  level ;  and  the  Mediterranean  is  a  little  below  that  level, 
because  the  evaporation  from*  it  is  greater  than  the  supply  of  its  rivers, 
causing  it  to  receive  an  additional  supply  by  the  Strait  of  Gibralter ; — but 
in  all  such  cases,  the  effect  is  as  constant  as  the  disturbing  cause,  and  there- 
fore can  be  calculated  upon  with  confidence. 

Were  it  not  for  this  perfect  exactness,  in  what  a  precarious  state  would 
the  inhabitants  exist  on  the  sea  shores,  and  on  the  banks  of  low  rivers  !  Few 
of  the  inhabitants  of  London,  perhaps,  reflect,  when  standing  by  the  side  of 
their  noble  river,  and  gazing  on  the  rapid  flood-tide  pouring*  inland  through 
the  bridges,  that  although  sixty  miles  from  the  sea,  the  water  there  is,  at  the 
moment,  lower  than  the  surface  of  the  sea,  which  may  at  the  time  be  heaving 
moreover,  in  lofty  waves,  covered  perhaps  with  wrecks  and  the  drowning. 

The  horrid  destruction  that  would  follow  any  alteration  of  the  level  of 
the  ocean,  may  be  judged  of  by  the  effects  of  occasional  floods,  produced  by 
rains  and  melting  snow  in  the  interior  of  countries,  or  by  these  combined 
with  winds  and  high  tides  on  the  coasts.  The  flood  at  St.  Petersburg,  in 
1825,  was  dreadful,  in  which  strong  westerly  winds  had  retarded  the  flow  of 
the  Neva  so  much,  that  the  water  rose  forty  feet  (the  height  of  an  ordinary 
house)  above  its  usual  mark,  covered  all  the  low  parts  of  the  town,  and 
drowned  thousands  of  the  people. 

In  Holland,  which  is  a  low  flat,  formed  chiefly  by  the  mud  and  sand 
brought  down  by  the  Rhine  and  neighboring  rivers,  much  of  the  country 


FLUID    LEVEL. 


139 


is  really  below  the  level  of  the  common  spring-tides,  and  is  only  protected 
from  daily  inundations  by  artificial  dykes  or  ramparts,  made  strong  enough 
to  resist  the  ocean.  On  one  occasion  the  water  broke  into  such  an  enclosure, 
and  drowned  more  than  sixty  thousand  people.  What  awful  uncertainty 
then  would  hang  over  the  existence  of  the  Dutch,  if  the  level  of  the  sea 
were  subject  to  change ;  for  while  we  know  that  its  waters,  owing  to  the 
centrifugal  force  or  of  the  earth's  rotation,  are  seventeen  miles  higher  at  the 
equator  than  at  the  poles,  if  the  level,  as  now  established,  were  from  any 
cause  to  be  suddenly  changed  but  ten  feet,  millions  of  human  beings  would 
be  the  victims. 

Where  inundation  is  regularly  periodical,  as  in  the  Nile  and  many  other 
rivers,  'the  hurtful  effects  can  be  guarded  against,  and  the  occurrence  may 
even  become  useful,  by  fertilizing  the  soil. 

Tracts  of  land  in  contact  wi^i  rivers,  of  which  land,  the  surface  lies  be- 
tween the  levels  of  ebb  and  flood-tide,  if  surrounded  with  dykes,  may  be 
kept  constantly  covered  with  water,  by  opening  the  sluices  only  at  high  water  j 
or  may  be  kept  constantly  drained,  by  opening  the  sluices  only  at  low  water. 
A  vast  extent  of  rice  fields,  near  the  mouths  of  rivers  in  India  and  China,  is 
managed  in  this  way,  the  admission  or  exclusion  of  water  being  regulated  by 
the  age  of  the  rice  plant.  A  great  part  also  of  the  rich  sugar  plantations 
of  Demerara,  Esequibo,  &c.,  on  the  coast  of  South  America,  are  in  the  same 
predicament;  and  another  advantage  which  these  have  over  the  plantations 
on  the  West-India  Islands,  is  the  saving  of  the  labour  of  transport  effected 
by  the  canals  which  intersect  all  the  fields. 

11  If  various  tubes  and  vessels  communicate  with  one  another,  fluid  admitted 
into  them  will  rise  to  the  same  level  in  all"      (Read  the  Analysis,  p.  84.) 

The  following  sketch  may  represent  a  variety  of  tubes  and  vessels,  fixed 
upon  and  opening  into  the  cistern  or  box  Gr.  Water  poured  into  any  one 
would  fill  the  box,  and  would  then  rise  to  the  same  level  in  all.  The  dotted 
lines  from  a  to/  may  represent  the  surfaces  of  the  fluid  in  the  different 
vessels.  In  the  figure  at  p.  128,  it 
was  seen  why,  in  all  upright  cylin- 
drical vessels,  as  a,  b  and  c,  the  fluid 
rises  to  the  same  level ;  and  the  figure 
at  p.  132  explained  why  shape  of  the 
vessel  cannot  affect  the  level.  Al- 
though in  the  oblique  vessel  c,  re- 
presented here,  there  is  more  water 
than  in  a,  still  there  is  the  same 
pressure  at  the  bottom  of  both,  be-, 
cause  e  supports  part  of  the  weight 
of  its  contained  fluid  on  the  principle  of  the  inclined  plane. 

If  a  tube  twenty  miles  long,  and  rising  and  descending  among  the  ine- 
qualities of.  a  country,  were  filled  with  water,  and  could  have  its  ends 
brought  together  for  comparison,  it  would  exhibit  two  liquid  surfaces  having 
precisely  the  same  level;  and  on  either  end  being  raised,  the  fluid  would 
sink  in  it  to  overflow  from  the  other. 

An  easy  mode  of  determining  a  level  line  at  any  spot  is  to  have  an  open 
tube,  bent  up  at  its  ends  a  and  b,  and  nearly  Fig.  76. 

filled  with  liquid :  by  then  looking  along  the 
two  liquid  surfaces,  or  through  floating  sights 
resting  on  them,  an  observer  looks  in  a  line 
which  is  quite  horizontal  at  the  middle  point  between  them. 


f 


140  HYDROSTATICS. 

If  there  were  two  lakes  on  adjoining  hills  of  different  heights,  a  pipe  of 
communication  descending  across  the  valley  and  connecting  them,  would 
soon  bring  them  to  the  same  level ;  or  if  one  were  much  higher  than  the 
other,  would  empty  that  one  into  the  other. 

A  projector  thought  that  the  vessel  of  his  contrivance,  represented  here, 
was  to  solve  the  renowned  problem  of  the  perpetual  motion.    It  was  goblet- 
shaped,  lessening  gradually  towards  the  bottom  until  it  became  a  tube,  turned 
upwards  at  c,  and  putting  with  an  open  extremity  into  the  goblet  again.    He 
reasoned  thus :  A  pint  of  water  in  the  goblet  a  must  more  than  counterbalance 
an  ounce  which  the  tube  6  will  contain,  and  must  there- 
Fig.  77.  fore  be  constantly  pushing  the  ounce  forward  into  the 
vessel  again,  and  keeping  up  a  stream  or  circulation, 
which  will  cease  only  when  the  water  dries  up.      He 
was   confounded  when^t  trial  showed  him  the  same 
level  always  in  a  and  in  b. 

A  glass  tube  inserted  near  the  bottom  of  a  cask  or 
cistern  of  any  sort,  not  air-tight  above,  which  tube  is 
then  bent  upwards,   to  appear  on  the  outside  like  a 
barometer  tube,  shows  by  the  elevation  of  a  fluid  in  it, 
the  height  of  the  greater  mass  of  fluid  within. 

In  like  manner  a  tube  brought  from  a  river  into  a  neighbouring  cellar  or 
pit,  will  indicate  the  height  of  water  in  the  river. 

A  knowledge  of  the  truth,  that  water  in  pipes  will  always  rise  again  to 
the  height  or  level  of  its  source,  has  enabled  men  in  modern  times  to 
construct  those  admirable  systems  of  iron  pipes,  which  distribute  water  in 
great  towns.  The  water  brought  to  any  elevated  site,  in  or  near  the  town, 
may  be  delivered  from  a  reservoir  there,  by  the  effect  of  gravity  alone,  to 
every  cistern  which  is  under  the  level  of  the  reservoir,  the  result  not  being 
affected  by  the  pipes  having  to  rise  over  heights  and  to  descend  into  valleys 
many  times  in  their  course. 

On  the  hill  north  of  London,  on  which  Pentonville  stands,  there  is  such  a 
reservoir  to  which  water  is  brought  from  Hertfordshire,  by  a  channel  cut  for 
the  purpose,  upwards  of  thirty  miles  in  length,  and  called  the  New  River. 
Another  reservoir  has  lately  been  constructed  by  the  West  Middlesex  Water 
Company,  at  Primrose  Hill,  higher  than  any  house  in  town.  It  is  filled  by 
operation  of  steam-engines  at  thq  Company's  works,  near  Hammersmith, 
five  miles  off.  It  will  supply  water  to  the  summits  of  all  the  houses  con- 
nected with  it,  and  is  exceedingly  useful  in  cases  of  fire. 

Many  persons  have  believed  that  the  ancients  were  ignorant  of  the  law, 
that  fluid  in  pipes  rises  to  the  level  of  its  source,  because,  in  all  the  ruins 
of  their  aqueducts,  the  channel  is  a  regular  slope.  Some  of  the  aqueducts, 
as  works  of  magnitude,  are  not  inferior  to  the  great  wall  of  China,  or  the 
Egyptian  Pyramids;  yet,  at  the  present  day,  a  single  pipe  of  cast-iron  is 
made  to  answer  the  same  purpose,  and  even  more  perfectly.  It  is  now 
ascertained,  however,  that  it  was  not  ignorance  of  the  principle,  but  want  of 
fit  material  for  making  the  pipes,  which  cost  our  forefathers  such  enormous 
labour. 

The  supply  and  distribution  of  water  in  a  large  city,  particularly  since  the 
steam-engine  has  been  added  to  the  apparatus,  approach  closely  to  the  per- 
fection of  nature's  own  work  in  the  circulation  of  blood  through  the  animal 
body.  From  the  great  pumps  or  a  high  reservoir,  main  pipes  issue  to  the 
chief  divisions  of  the  town ;  these  then  send  suitable  branches  to  the  streets, 
which  branches  again  divide  for  the  lanes  and  alleys;  and  at  lust  subdivide 


FLUID    LEVEL.  141 

until  every  house  has  its  small  leaden  conduit  carrying  its  precious  freight, 
if  required,  even  into  the  separate  apartments,  and  yielding  it  anywhere  to 
the  turning  of  a  cock  A  corresponding  arrangement  of  drains  and  sewers, 
most  carefully  constructed  in  obedience  to  the  law  of  level,  receives  the 
water  again  when  it  has  answered  its  purposes,  and  sends  it  to  be  purified 
in  the  great  laboratory  of  the  ocean.  And  so  admirably  complete  and 
perfect  is  this  counter-system  of  sloping  channels,  that  a  heavy  shower  may 
fall,  and  after  washing  and  purifying  every  superficial  spot  of  the  city,  and 
sweeping  out  all  the  subterranean  passages,  may,  within  the  space  of  an 
hour,  form  part  of  the  river  passing  by.  It  is  the  recurrence  of  this  almost 
miracle,  of  extensive,  sudden,  and  perfect  purification,  which  makes  modern 
London  the  most  healthy,  while  it  is  the  largest  city  in  the  world. 

English  citizens  have  now  become  so  habituated  to  the  blessing  of  a 
supply  of  pure  water,  more  than  sufficient  for  all  their  purposes,  that  it  no 
more  surprises  them  than  the  regularly  returning  light  of  day  or  warmth  of 
summer.  But  a  retrospect  into  past  times  may  still  awaken  them  to  a 
sense  of  their  obligation  to  advancing  art.  How  much  of  the  anxiety  and 
labour  of  men  in  former  times  had  relation  to  the  supply  of  this  precious 
element !  How  often,  formerly,  has  periodical  pestilence  arisen  from  the 
deficiency  of  water  j  and  how  often  has  fire  devoured  whole  cities,  which  a 
timely  supply  of  water  might  have  saved  !  Kings  have  received  almost 
divine  honours  for  constructing  aqueducts,  to  lead  the  pure  streams  from 
the  mountains  into  the  peopled  towns.  In  the  present  day,  it  is  he  who  has 
travelled  on  the  sandy  plains  of  Asia  or  Africa,  where  a  well  is  more  prized 
than  mines  of  gold,  or  who  has  spent  months  on  ship-board,  where  the  fresh 
water  is  often  doled  out  with  more  caution  than  the  most  precious  product 
of  the  still,  or  who,  in  reading  history,  has  vividly  sympathized  with  the 
victims  of  siege  or  shipwreck,  spreading  out  their  garments  to  catch  the  rain 
from  heaven,  and  then  with  mad  eagerness,  sucking  the  delicious  moisture 
— it  is  he  who  can  appreciate  fully  the  blessing  of  that  abundant  supply 
which  most  of  us  now  so  thoughtlessly  enjoy.  The  author  of  this  work 
will  long  remember  the  intense  momentary  regret  with  which,  at  once 
approaching  a  beautiful  land  after  months  spent  at  sea,  he  saw  a  stream  of 
fresh  water  gliding  over  a  rock  into  the  salt  waves — it  appeared  to  him  as 
if  a  most  precious  essence,  by  some  accident  were  pouring  out  to  waste. 

The  subject  of  fluid  level  leads  to  a  consideration  of  springs  or  wells,  and 
of  the  operation  of  boring  for  water. 

The  water  which  falls  from  the  clouds,  and  which  must  all  ultimately 
return  to  the  sea,  may  find  its  way  to  the  rivers,  either  by  running  directly 
along  the  surface  of  soils  which  refuse  it  admittance }  or  by  first  sinking  into 
porous  earth,  and  again  oozing  out  at  lower  situations  in  the  form  of  springs. 
If  a  spring  be  as  low  as  the  bottom  of  the  porous  earth  from  which  it  issues, 
that  is  to  say,  as  low  as  the  surface  of  the  impermeable  clay  or  rock  on 
which  at  some  depth  all  such  earth  rests,  it  may  drain  the  whole ;  but  if  not, 
the  water  will  stand  at  a  certain  level  among  the  earth  as  it  would  among 
bullets  in  a  water-tight  vessel.  If  a  hole  or  pit  be  then  dug  in  such  earth, 
reaching  below  the  level  of  the  water  lying  in  it,  the  pit  will  soon  be  filled 
with  water  up  to  the  level,  and  will  be  called  a  well.  In  many  places  this 
water-level  is  very  far  below  the  surface  of  the  ground ;  and  in  some  places, 
by  reason  of  the  water  having  an  easy  drainage  towards  the  sea,  or  of  the 
superficial  soil  being  altogether  impermeable  to  it,  there  is  none  to  be  found 
within  an  accessible  depth. 


142  HYDROSTATICS. 

A  remarkable  illustration  of  this  subject  occurred  a  few  years  ago,  in  Kent, 
on  the  occasion  of  cutting  between  Rochester  and  Gravesend  the  canal  called 
the  Thames  and  Medway  canal.  This  canal  consists  of  but  one  cut  or 
level,  seven  miles  long,  of  which  two  are  in  a  tunnel  through  the  hill — which 
level  is  that  of  high  water  in  the  connected  rivers ;  the  intention  having 
been  to  let  the  canal  be  filled  always  from  the  rivers  at  high  water ; — but  as 
the  level  of  the  subterranean  water  in  the  surrounding  land,  and  therefore  of 
all  the  inhabitants'  wells  there,  is,  as  might  be  anticipated,  half-way  between 
the  levels  of  high  and  low  tides,  the  salt  water  from  the  rivers  was  no 
sooner  admitted  to  the  canal  than  it  spread  into  the  land  on  either  side, 
where  the  resisting  internal  water-level  was  lower,  and  destroyed  all  the 
wells.  If  the  canal  had  been  dug  a  few  feet  lower,  the  mischief  would  not 
have  occurred,  and  the  company  would  have  escaped  paying  the  heavy 
damages,  which  rendered  their  undertaking  a  very  ungainful  speculation. 

All  the  wells  and  springs  in  the. world  are  merely  the  rain  water  which  has 
sunk  into  the  earth,  appearing  again,  and  gradually  escaping  at  lower  places  : 
nature  thus  admirably  making  the  bowels  of  the  earth  an  ever-stored  reservoir 
of  the  substance  most  indispensable  to  the  comfort  and  existence  of  man,  and 
of  all  living  creatures.  It  is  worthy  of  remark  here,  that  high  cultivation  or 
agricultural  improvement  of  a  country  has  a  great  effect  on  the  quantity  of 
spring  water  in  it.  While  the  face  of  a  country  is  rough  the  rain-water 
remains  long  among  its  inequalities,  slowly  sinking  into  the  earth  to  feed  the 
springs,  or  slowly  running  away  from  the  surface  as  from  bogs  and  marshes 
towards  the  rivers.  The  rivers,  hence,  have  a  comparatively  uniform  and 
regular  supply,  even  when  rain  has  not  fallen  for  a  long  time : — but  in  a 
well-drained  country,  the  rain,  by  a  thousand  prepared  channels,  finds  its 
way  to  the  brooks  and  rivers  almost  immediately,  producing  often  dan- 
gerous floods  or  inundations  of  the  neighboring  low  grounds.  A  friend  of 
the  author  had  a  waterfall  and  mill  in  Surrey,  which  he  formerly  let  for  a 
rent  of  £1,200  a  year;  but  after  agricultural  improvements  in  the  district 
from  which  the  water  came,  the  supply  of  water  was  generally  either 
superabundant  or  deficient,  and  the  value  of  the  mill  was  reduced  to  one-half. 

The  surface  of  our  globe  is  formed  of  different  strata  or  layers,  as  of  clay, 
chalk,  sand,  gravel,  £c.,  &c.,  which  appear  all  to  have  been  at  former  periods 
horizontal,  formed  under  water,  and  to  have  been  afterwards  thrown  up,  by 
some  convulsion  or  convulsions  of  nature,  into  every  variety  of  position.  In 
particular  situations,  the  upper  surface  is  now  concave  or  basin-shaped,  the 
different  strata  or  layers,  when  water-tight,  being  like  cups  or  basins  placed 
one  within  another ;  and  as  water  poured  in,  to  fill  the  space  between  two 
basins  so  placed,  would  spring  out  to  the  height  of  its  upper  or  level  surface, 
through  any  hole  made  in  the  side  of  either,  so  on  boring  for  water,  through 
an  innermost  or  superior  water-tight  stratum  or  basin  of  earth,  the  water 
often  springs  out  and  rises  far  above  the  surface  of  the  ground.  London 
stands  in  a  hollow  of  which  the  first-met  layer  is  a  basin  of  clay,  placed 
over  chalk,  and  on  boring  through  the  clay  (sometimes  of  three  hundred  feet 
thickness,)  the  water  issues  and  in  many  places  will  form  a  jet  considerably 
above  the  surface  of  the  ground  ]  showing  that  there  is  a  higher  source  or 
level  somewhere — as  among  the  hills  of  Surrey,  or  those  north  of  London. 
,  When  fluids  of  different  kinds  and  of  different  weights  under  the  same 
bulk,  are  made  to  oppose  or  to  balance  each  other  in  communicating  vessels 
— as  water,  for  instance,  in  one  leg  of  the  bent  tube  b  d  c,and  oil  in  the  other 
— the  surfaces  will  not  all  rest  or  settle  at  the  same  height  or  level,  but  that 
of  the  lighter  fluid  will  be  just  as  much  higher  than  that  of  the  other  as  it  is 


FLUID    SUPPORT. 


143 


Fig.   78. 


'IV 


lighter.  Thus  a  column  of  oil  must  be  of  a 
length  as  d  o,  to  balance  a  column  of  water 
d  w :  and  alcohol,  because  lighter  than  oil, 
to  balance  the  same  water,  would  have  to 
stand  higher  still,  as  at  a  ;  while  mercury  > 
because  thirteen  times  weightier  than  water, 
would  stand  only  about  m.  The  shape,  size, 
or  position  of  the  vessels  in  which  the  oppos- 
ing fluids  might  stand,  would  have  no  in- 
fluence on  the  relative  heights  of  the  surfaces ; 
for  if  we  suppose  a  larger  vessel,  such  as  is 
represented  here  by  the  dotted  lines  between 
the  letters  e  f  m,  to  be  substituted  for  the 
leg  c  d  of  the  tube,  the  various  -fluids  to 
balance  the  water  in  b  d,  would  have  to  stand 
just  as  high  in  it  as  in  the  smaller  tube. 

"  A  body  immersed  in  a  fluid,  displaces  exactly  its  own  bulk  of  it,  which 
quantify  having  been  just  supported  by  the  fluid  around  the  body  is  held 
up  with  force  exactly  equal  to  the  weight  of  the  fluid  displaced,  and  must 
sink  or  swim  according  as  its  own  weight  is  greater  or  less  than  this" 

A  bladder  full  of  air,  and  maintaining  the  bulk  of  a  pound  of  water, 
requires  a  force  of  one  pound  (except  a  few  grains,  the  weight  of  the  air,) 
to  plunge  it  under  the  water.  The  same  bulk  of  gold  is  held  up  in  water 
with  exactly  the  same  force  ;  so  that,  if  previously  balanced  at  the  end  of  a 
weighing  beam,  it  appears  on  immersion  to  have  lost  one  pound  of  its 
weight. 

And  a  piece  of  wood,  ivory,  or  any  other  substance,  having  exactly  the 
same  bulk,  is  opposed  on  entering  the  fluid  by  the  same  resistance. 

The  reason  of  thi^i  is  obvious,  for  the  immersed  body  takes  the  place  of 
water  which  weighed  one  pound  and  yet  was  supported,  and  whose  pressure 
was  necessary  for  the  equilibrium  of  the  rest.     In  a  vessel  of  water  repre- 
sented here  by  the  figure  a  b,  let  us  append  to  any 
portion  of  the  water,  a  single  column  of  particles, 
for  instance,  represented   by  the  line  c  d :  we 
know  that  each  column  is  steadily  supported  in 
its   place,    because   the   particle   of  the   liquid 
immediately   under   it  is    tending   upwards   to 
escape  from  the  surrounding  pressures,  with  force 
exactly  equal  to  the  weight  of  the  column ',  and 
what  is  true  of  a  column  of  single  particles,  is 

true  of  any  other  portion,  such  as  the  larger  column  represented  by  the 
figure/  h  g.  If  such  portion  weighed  exactly  a  pound,  the  surface  under  it 
would  be  tending  upward  with  the  force  of  a  pound ;  and  if  the  portion, 
without  changing  its  bulk  or  form,  were  to  become  ice,  it  would  still  be 
exactly  supported  by  the  surface  below  pressing  upwards  with  a  force  of  a 
pound ;  and  farther,  if  a  similar  column  of  wood,  or  stone,  or  metal,  were  there, 
the  surrounding  pressure  would  still  be  the  same.  Again,  if  we  suppose 
only  half  the  column  to  be  solidified,  the  portion  h  g  for  instance,  it,  would 
be  pressed  upwards  with  a  force  of  one  pourfd  at  g  ;  but  its  own  weight  of 
half  a  pound,  and  the  weight  of  the  half  pound  of  water  above  it,  would 
produce  an  exact  balance  and  maintain  rest. 


Fig.  79. 


J? 


cL 


144  HYDROSTATICS. 

It  is  very  important  to  have  clear  notions  on  this  subject ;  and  as  different 
minds  apprehend  such  matters  with  different  degrees  of  facility,  and  ill 
different  ways,  we  shall  state  the  same  general  truth  in  other  words. 

Let  us  consider  a  mass  of  fluid  as  consisting  of  a  vast  number  of  extremely 
minute  columns  of  single  particles  standing  side  by  side,  where  every 
particle  supports  those  above  it  by  tendency  upwards  which  it  requires 
through  the  pressure  of  the  fluid  surrounding  it.  Now  if  we  suppose  the 
particles  of  a  portion  of  a  fluid  mass,  of  any  shape,  to  stick  together,  or  to 
become  ice  without  change  of  bulk  or  weight,  that  portion  when  solid  would 
still  be  between  the  same  forces  as  when  fluid,  and  therefore  would  be  equally 
supported,  and  would  remain  at  rest.  And  if  gold,  or  silver,  or  glass,  or 
wood,  having  the  same  bulk,  were  substituted  for  the  supposed  ice,  such 
new  substance  would  still  be  sustained  with  the  same  force ;  so  that  a 
substance  of  exactly  the  same  weight  as  the  ice  or  water  displaced,  would 
have  no  tendency  either  to  rise  or  to  fall  more  than  the  water  itself  had  j 
but  a  substance  heavier  would  sink,  and  one  lighter  would  swim,  and  in 
either  case  with  force  exactly  proportioned  to  the  difference  between  its 
weight  and  that  of  an  equal  bulk  of  water. 

Few  persons,  in  now  reading  the  statement  of  this  truth — in  appearance 
so  simple  and  obvious — would  imagine  that  it  had  remained  so  long  unknown 
and  that  the  discovery  of  it  may  be  accounted  one  of  the  most  important 
which  human  sagacity  ever  made, — but  such  is  the  case.  We  owe  the  dis- 
covery to  one  of  the  master-minds  of  antiquity — that  of  Archimedes.  He 
caught  the  idea  one  day  while  his  limbs  were  resting  on  the  liquid  support 
of  a  bath  :  and  as  his  God-like  intellect  darted  into  futurity,  and  perceived 
many  of  the  important  uses  to  which  the  knowledge  was  applicable,  he  is 
said  to  have  become  so  moved  with  admiration  and  delight,  that  he  leapt 
from  the  water,  and  unconscious  of  his  nakedness,  pursued  his  way  home- 
wards, calling  out  "  evgyxa,  wgyxa,"  I  have  found  it.  He  was  thinking  chiefly 
of  the  ready  means,  thus  obtained,  of  ascertaining  in  all  cases  what  has  since 
been  called  the  specific  gravity  of  bodies,  viz.,  the  comparative  weights  of 
equal  bulks  of  different  substances ;  as  of  gold,  or  silver,  or  copper,  or  iron, 
compared  with  water  ;  and  in"  the  case  of  mixtures,  as  of  gold  with  silver  for 
instance,  of  declaring  at  once  the  proportion  present  of  each — important 
problems,  which,  until  then,  could  not  be  correctly  solved. 

The  hydrostatic  law  now  explained,  has  since  led  to  great  advances  in 
various  arts.  It  may  be  regarded  as  a  chief  foundation  of  chemistry,  for  by 
it  the  chemist  distinguishes  one  substance  from  another,  distinguishes  a 
pure  from  an  impure  substance,  and  discovers  the  nature  of  many  mixtures 
or  compounds.  The  merchant  often  judges  by  it  of  the  worth  of  his  merchan- 
dize. In  any  case  it  enables  an  inquirer  to  ascertain  at  once  the  exact  size 
or  solid  bulk  of  a  mass,  however  irregular — even  of  a  bundle  of  twigs.  It 
has  become  the  cause  of  improvements  in  navigation,  in  marine  architecture, 
and  in  many  other  arts. 

We  shall  now  discuss  more  particularly  the  subject  of  comparative 
weights  or  specific  gravity. 

11  The  force  with  which  a  body  is  held  up  in  a  fluid,  being  the  exact  weight 
of  its  bulk  of  that  fluid,  by  ascertaining  this  force  and  comparing  it  with 
the  weight  of  the  body  itself  the  comparative  weights  or  SPECIFIC  GRAVI- 
TIES are  found."  (Read  the  Analysis,  p.  126.) 

If  any  b6dy,  c,  a  mass  of  gold,  for  instance,  be  suspended  by  a  thread  or 


FLUID    SUPPORT.  —  SPECIFIC    GRAVITY. 


145 


Fig.  80. 


hair  from  the  bottom  of  one  scale  b 
of  a  weighing-beam,  and  be  balanced 
by  weights  put  into  the  other  scale  a, 
and  if  a  vessel  of  water  be  then  lifted 
under  it  so  that  the  water  shall  sur- 
round it,  the  body  is  pushed  up  or 
supported  by  the  water  with  force 
equal  to  the  weight  of  the  water  which 
it  displaces;  the  weights,  therefore, 
then  required  in  the  scale  b  to  restore 
the  balance,  show  truly  the  exact 
weight  of  the  water  displaced  ;  or  of 
water  equal  in  bulk  to  the  body ;  and 
the  weights  in  the  two  opposite  scales 
show  the  comparative  weights  of  the 
body  and  of  its  bulk  of  water.  In  the 
supposed  case,  whatever  weight  the 
gold  had  in  the  air,  it  would  seem  to  lose  when  the  water  surrounded  it, 
about  a  nineteenth  part  of  such  weight ;  that  is,  the  water  would  support  it 
with  this  force ;  and  gold  would  thus  be  proved  to  be  about  nineteen  times 
as  heavy  as  water. 

In  making- a  table  of  specific  gravities,  it  was  necessary  to  select  a  common 
standard  with  which  all  other  substances  should  be  compared,  and  this  has 
been  done  in  choosing  water ;  the  reason  of  preference  being,  that  water  can 
be  so  easily  procured  in  a  state  of  purity,  and  therefore  of  uniformity,  in  all 
situations.  When  we  say,  therefore,  that  gold  is  of  the  specific  gravity  19, 
and  copper  9,  and  cork  ^,  we  mean  that  these  substances  are  just  so  much 
heavier  or  lighter  than  their  bulk  of  pure  water  in  its  densest  state,  viz.,  at 
the  temperature  of  40  degrees  of  Fahrenheit's  thermometer. 

As  the  substances  in  nature  differ  as  to  form  and  other  qualities,  cor- 
responding differences  have  to  be  made  in  the  manner  of  ascertaining  their 
specific  gravities  :  the  following  cases  are  most  important. 

Solid  bodies  insoluble  in  water  and  heavier  than  it — as  the  metals,  &c., 
are  merely  suspended  by  a  thread  or  hair,  having  nearly  the  specific  gravity 
of  water,  to  one  scale  of  the  hydrostatic  balance  (simply  a  good  weighing- 
beam  with  a  water- vessel  below  one  of  the  scales ;)  and  the  body  being  first 
balanced  or  weighed  in  the  air,  and  then  in  water,  as  already  described,  the 
weight  and  the  loss,  represented,  if  the  operator  chooses,  by  the  weights  in 
the  opposite  scales,  are  the  weights  of  equal  bulks  of  the  two  substances ; 
and  by  finding,  through  the  arithmetical  operation  of  division,  how  often 
the  weight  of  the  water  is  contained  in  the  weight  of  the  solid,  we  find  the 
specific  gravity  of  the  solid,  or  how  much  it  is  weightier  than  its  bulk  of 
water. — It  is  almost  superfluous  to  remark,  that  putting  weights  into  the 
scale,  b  or  taking  them  out  of  the  scale  a,  are  equivalent  operations.  We 
shall  explain  afterwards,  that  for  very  delicate  purposes  bodies  must  be 
weighed  first  in  a  vacuum,  instead  of  in  air,  or  a  suitable  allowance  must  be 
made ;  for  air  itself  supports  a  little  any  body  immersed  in  it. 

Solids  lighter  than  water,  as  cork,  are  weighed  in  it  by  attaching  to 
them  a  mass  of  metal  or  glass  heavy  enough  to  sink  them,  and  already 
balanced  in  water  for  the  purpose ;  or  by  making  the  line  which  connect 
them  with  the  weighing  beams  pass  under  a  small  pully  fixed  at  the  bottom 
of  the  vessel,  so  that  the  rising  of  the  end  of  the  beam  to  which  they  are 
attached  shall  draw  them  down. 

10 


146  HYDROSTATICS. 

A  solid  soluble  in  water,  as  a  chrystal  of  any  salt,  may  be  protected 
during  the  operation  of  weighing  in  water,  by  previously  dipping  it  in 
melted  wax,  so  as  to  leave  a  thin  covering  on  it ;  or  it  may  be  weighed  in 
some  liquid  which  does  not  dissolve  it,  allowance  being  afterwards  made  for 
the  difference  between  the  weight  of  such  liquid  and  of  water. 

Powders  insoluble  in  water,  such  as  gold  dust,  are  weighed  in  a  glass  cup 
which  has  previously  been  balanced  in  water  for  that  purpose. 
Powders  soluble  in  water,  must  be  weighed  in  some  other  liquid. 
Mr.  Leslie,  the  highly  endowed  professor  of  natural  philosophy  in -the 
University  of  Edinburgh,  has  lately  suggested  a  novel  and  ingenious  mode 
of  acertaining  the  specific  gravity  of  pulverized  or  porous  bodies  j  but  as  it 
can   be   understood   only   by  persons   acquainted   with   the   doctrines   of 
pneumatics,  the  consideration  of  it  must  come  under  that  head. 

Other  liquids  may  be  compared  with  water  in  several  ways.  1st.  If  a 
phial  be  made  to  hold  exactly  one  thousand  grains  of  distilled  water,  at 
the  temperature  of  40°,  the  weight  of  the  same  measure  of  any  other  liquid 
is  found,  by  simply  filling  the  phial,  and  weighing  it.  Of  sulphuric  acid, 
for  instance,  such  a  phial  will  contain  nearly  nineteen  hundred  grains,  while 
of  alchohol  it  will  receive  only  about  eight  hundred.  2d.  A  bulb  of  glass, 
which  loses  one  thousand  grains  when  weighed  in  water,  (which  thousand 
grains  is  therefore  the  weight  of  its  bulk  in  water,)  may  be  weighed  in  other 
liquids,  and  the  difference  of  loss  marks  the  specific  gravity,* as  in  the  last 
case.  The  bulb  for  this  purpose  may  be  of  any  size,  but  one  which  loses 
in  water  exactly  one  thousand  grains,  is  preferable,  from  the  simplicity 
thereby  given  to  the  calculations  : — This  remark  applies  also  to  the  phial 
last  mentioned.  3d.  A  contrivance  which  renders  the  beam  and  scales  alto- 
gether unnecessary,  is  a  hollow  floating  bulb  of  glass  or  metal  a,  with  a 
slender  stalk  rising  from  it  to  support  the  little  scale  or  dish  b,  and  with 
another  stalk  descending  to  carry  the  weight  or  weights  at  c,  which  serve  as 
ballast  to  it.  The  whole  is  so  adjusted  that  when  displacing  one  thousand 
grains,  or  other  known  quantity  of  pure  water,  it  shall  float  with  a  certain 
mark  upon  the  upper  stalk  just  at  the  surface  of  the  water.  By  then  im- 
mersing it  in  other  liquids  ancl  finding  how  much  weight  must  be  added  to, 
or  taken  from  it  above  or  below,  to  make  it  float  in  them 
at  the  same  elevation,  the  comparative  weights  of  these 
other  liquids  and  of  water  are  found  : — or  the  difference  of 
weight  which  makes  it  float  at  different  elevations  in 
water,  having  been  previously  ascertained,  it  will  only  be 
necessary,  in  any  other  case,  to  note  exactly  its  elevation ; 
an  inch  of  the  slender  stalk  may  be  equivalent  to  a 
difference  of  ten  grains.  This  instrument  is  called  an 
hydrometer.  There  are  generally  printed  tables  and 
directions,  accompanying  all  forms  of  it,  telling  the  exact 
import  of  the  several  indications,  and  the  allowances  to 
be  made  for  temperature,  &c.  It  may  be  used  for  weigh- 
ing solids  as  well  as  liquids,  for  if  any  mass  be  put  into  the  saucer  b,  weights 
exactly  equal  to  the  mass  must  be  taken  out  of  the  saucer  b,  or  from  below 
at  c,  to  restore  the  equilibrium  of  the  instrument.  The  mass  may  be  after- 
wards placed  at  c,  and  weighed  in  water.  4th.  The  shortest  mode  of  ascer- 
taining the  specific  gravities  of  liquids,  is  to  have  a  set  or  series  of  small 
glass  bubbles  of  different  specific  gravities,  so  that  when  they  are  thrown 
into  any  liquid,  those  heavier  than  it  will  sink,  and  those  lighter  will  swim, 
while  that  one  which  marks  its  specific  gravity  will  remain  merely  suspended. 


FLUID    SUPPORT.  —  SPECIFIC    GRAVITY.  147 

The  bubbles  must,  of  course  be  numbered,  and  the  specific  gravity  of  each 
be  previously  known. 

A  common  use  of  hydrometers  is  to  ascertain  the  quality  of  the  distilled 
spirits  brought  to  market,  as  of  rum,  brandy,  gin,  &c.  All  these  consist  of 
alcohol  more  or  less  diluted  with  water;  and' duty  or  tax  is  levied  upon  them 
in  proportion  to  their  strength,  or  the  quantity  of  alcohol  which  they  con- 
tain. A  delicate  hydrometer  discovers  this  at  once. 

A  shop-keeper  in  China  sold  to  the  purser  of  a  ship,  a  quantity  of 
distilled  spirit  according  to  a  sample  shown ;  but  not  standing  in  awe  of 
conscience,  he  afterwards,  in  the*privacy  of  his  store-house,  added  a  certain 
quantity  of  water  to  each  cask. 

The  spirit  having  been  delivered  on  board^  and  tried  by  the  hydrometer, 
•was  discovered  to  be  wanting  in  strength.  When  the  vendor  was  charged 
with  the  intended  fraud,  he  at  first  denied  it,  for  he  knew  of  no  human 
means  which  could  have  made  the  discovery  ;  but  on  the  exact  quantity  of 
water  which  had  been  mixed  being  specified,  a  superstitious  dread  seized 
him,  and  having  confessed  his  roguery,  he  made  ample  amends.  On  the 
instrument  of  his  detection  being  afterwards  shown  to  him,  he  offered  any 
price,  for  what  he  foresaw  might  be  turned  to  great  account  in  his  trade. 

The  specific  gravity  of  aeriform  substances  is  ascertainnd  by  means  of 
a  glass  flask  of  known  size,  furnished  with  a  stop-cock.  It  is  first  weighed 
when  emptied  by  the  air-pump,  and  afterwards  when  filled  successively 
with  water  and  with  different  airs  or  gases.  Comparison  of  the  weights 
gives  the  specific  gravities,  as  already  described. 

The  following  table  shows,  in  round  numbers,  the  comparative  weights 
or  specific  gravities  of  some  common  substances.  Water  is  the  standard 
kept  in  view,  and  any  equal  bulk  of  another  substance  is  heavier  or  lighter 
than  water,  according  to  the  numbers  severally  attached  to  them. 


Common  Salt,      .         .         .2 
Brick       ....         2 

Alcohol       .         .         .         •     S 


Cork  . 

Atmospheric  Air 
Hydrogen  Gas     . 


Platinum     .         .         .         .  22  f 

Gold  19i 

Mercury       .         .         .         .  13  £ 

Copper     ....  8f 
Steel  and  Iron        .         .         .8 

Diamond            .         .         .  3J 

Glass 3 

Common  stones          .         .  2$ 

Complete  tables  are  found  in  systems  of  Dictionaries  of  Chemistry. 

A  cubic  foot  of  water  happens  to  weigh  very  nearly  one  thousand  ounces 
avoirdupois,  or  62  ?  pounds.  Hence,  in  the  foregoing  table,  the  figures 
denoting  the  specific  gravities  tell  how  many  times  a  thousand  ounces  of 
the  different  substances  a  cubic  foot  contains.  Of  gold,  for  instance, 
a  cubic  foot  contains  more  than  nineteen  thousand  ounces,  being  worth  in 
money  about  £63,000  sterling.  A  cubic  foot  of  common  air  contains  only 
a  little  more  than  one  ounce ;  and  of  hydrogen  gas,  the  lightest  of  ponder- 
able things,  a  cubic  foot  contains  less  than  a  drachm. 

The  following  facts  are  also  illustrations  of  the  truth,  that  a  body  immersed 
in  a  fluid  is  held  up,  or  has  its  entrance  resisted,  with  force  equal  to  the 
weight  of  the  quantity  of  fluid  which  it  displaces. 

A  stone  which  on  land  requires  the  strength  of  two  men  to  lift  it,  may  be 
lifted  and  carried  in  water  by  one  man.  There  are  cases,  therefore,  where 


148  HYDROSTATICS. 

the  support  of  water  thus  rendered  useful,  is  equivalent  to  the  assistance  of 
additional  hands.  A  boy  will  often  wonder  why  he  can  lift  a  certain  stone 
to  the  surface  of  water,  but  no  farther. 

The  invention  of  the  diving-bell  in  modern  tirade,  having  enabled  men,  in 
the  building  of  piers,  bridges,  &c.,  to  work  under  water  almost  as  freely  as 
above,  many  have  experience  of  this  influence  of  water:  but  workmen  are- 
generally  surprised  at  first,  to  find  that  below  they  can  move  much  larger 
and  heavier  stones  than  they  can  in  the  air.  Some  had  supposed  the  fact 
accounted  for  by  saying  that  the  denser  air  of  the  diving-bell,  when  received 
into  the  lungs  gave  greater  strength.  In  recovering  property  from  a  sunken 
ship  by  the  diving-bell,  everything  is  found  to  be  lighter  in  the  proportion 
now  stated. 

This  law  explains  also  why  stones,  gravel,  sand  and  mud,  are  so  easily 
moved  by  waves  and  currents.  Many  people  expressed  astonishment,  in 
March,  1825,  to  learn  that  at  the  Plymouth  Breakwater,  the  storm  had  dis- 
placed blocks  of  stone  of  many  tons  weight ;  but  we  now  see  that  the  moving 
water  had  only  to  overcome  about  half  the  weight  of  the  stone. 

When  a  person  lies  in  a  bath,  the  limbs  are  so  nearly  supported  by  the 
water  as  to  require  scarcely  any  exertion  on  the  part  of  the  individual.  When 
this  softest  of  all  beds  has  been  indulged  in  for  half  an  hour  or  more,  the 
person,  on  first  lifting  a  limb  out  of  the  water,  feels  surprise  at  its  great 
apparent  weight.  The  workers  about  diving-bells  always  experience  the 
sensation  now  spoken  of,  on  returning  to  the  air. 

The  bodies  of  most  fishes  are  nea'rly  of  the  specific  gravity  of  water,  and, 
therefore,  if  lying  in  it  without  making  exertion,  they  neither  sink  nor  rise 
very  quickly.  When  this  subject  was  less  understood,  many  persons 
believed  that  fishes  had  no  weight  in  water;  and  it  is  related  as  a  joke  at 
the  expanse  of  philosophers,  that  a  king -having  once  proposed  to  his  men 
of  science  to  explain  this  extraordinary  fact,  many  profound  disquisitions 
came  forth,  but  not  one  of  the  competitors  thought  of  trying  what  really 
was  the  fact.  It  was  beneath  the  dignity  of  science  in  those  days  to  make 
an  experiment.  At  last  a  simple  man  balanced  a  vessel  of  water  in  scales, 
and  on  putting  a  fish  into  the  water,  showed  its  scale  preponderating  just  as 
much  as  if  the  fish  had  been  weighed  alone. 

In  the  sense  now  explained,  water  is  said  to  have  no  weight  in  water. 
The  least  force  will  raise  a  bucket  of  water  from  the  bottom  of  a  well  to  the 
surface;  but  if  the  bucket  be  lifted  at  all  farther,  its  weight  is  felt  just  in 
proportion  to  the  part  of  it  which  is  above  the  surface. 

"  A  body  lighter  than  its  bulk  of  water  will  float,  and  with  force  propor- 
tioned to  the  difference."     (Read  the  Analysis,  p.  126.) 

*  The  reason"of  this  is  clear.  If  any  body,  the  cylinder  abed  for  instance, 
be  partially  immersed  in  water,  we  know  that  tHe 
upward  pressure  of  the  water  on  the  bottom  c  d,  ia 
exactly  what  served  to  support  the  water  displaced 
by  the  body,  viz.,  water  of  the  bulk,  efc  d.  The 
body,  therefore,  that  it  may  remain  out  as  far  as 
here  represented,  must  have  exactly  the  weight 
of  the  water  which  the  immersed  part  of  it  dis- 
places ;  and  if  it  be  lighter  than  this,  it  will  rise 
farther ;  if  heavier,  it  will  sink  farther  until  the 
exact  balance  be  produced. 


FLUID    SUPPORT.  —  SWIMMING.  149 

Hence  of  any  body  which  floats  in  water,  a  pound  weight  displaces  just  a 
pound  of  water,  whether  the  body  be  very  light  in  proportion  to  its  bulk,  as 
cork,  or  heavier,  as  a  piece  of  dense  wood.  This  is  experimentally  shown 
by  putting  such  bodies  to  float  in  a  vessel  originally  full  of  water.  The 
water  displaced  by  each  must  run  over  the.  sides  of  the  vessel,  and  may  be 
caught  and  measured. 

Hence  a  porcelain  basin  weighing  four  ounces  will  sink  in  water  only  as 
far  as  a  similar  wooden  basin  or  bowl  of  the  same  weight ;  and  the  weight  of 
either  basin  may  be  in  the  substance  of  which  it  is  formed,  or  in  anything 
else  put  into  it  as  a  load. 

Hence  a  boat  made  of  iron  floats  just  as  high  out  of  water  as  a  boat  of  simi- 
lar form  and  size  made  of  wood,  provided  the  iron  be  proportionately  thinner 
than  the  wood,  and  therefore  not  heavier  on  the  whole.  An  empty  metallic 
pot  or  kettle  is  often  seen  floating  with  a  great  part  of  it  above  the  surface  of 
the  water. — Prejudice  for  a  long  time  prevented  iron  boats  from  being  used, 
although,  for  various  purposes,  they  are  superior  to  others :  and  there  are  still 
people  who  would  fear  to  go  on  board  of  a  ship  built  of  the  strong  and  singu- 
larly durable  Indian  teaks,  because  it  is  heavier  than  water,  and,  in  the  form 
of  a  log,  therefore,  sinks  in  water.  Many  fine  ships  of  the  line,  however,  and 
East-Indiamen  of  fifteen  hundred  tons  or  more,  are  now  built  of  teak. 

Hence  a  ship  carrying  a  thousand  tons  weight  will  draw  just  as  much 
water,  or  float  to  the  same  depth,  whether  her  cargo  be  of  cotton  or  of  lead : — 
and  the  exact  weight  of  any  ship  and  her  cargo  may  be  determined  by  find- 
ing how  much  water  she  displaces.  In  canal  boats,  which  are  generally  of 
a  simple  form,  this  truth  affords  a  ready  rule  for  ascertaining  the  quantity 
of  their  load. 

The  human  body,  in  an  ordinary  healthy  state  with  the  chest  full  of  air,  is 
lighter  than  water. 

If  this  truth  were  generally  and  familiarly  understood,  it  would  lead  to  the 
saving  of  more  lives,  in  cases  of  shipwreck  and  in  other  accidents,  than  all 
the  mechanical  life-preservers  which  man's  ingenuity  will  ever  contrive. 

The  human  body  with  the  chest  full  of  air  naturally  floats  with  a  bulk  of 
about  half  the  head  above  the  water, — having  then  no  more  tendency  to  sink 
than  a  log  of  fir.  That  a  person  in  water,  therefore,  may  live  and  breathe  it 
is  only  necessary  to  keep  the  face  uppermost.  The  reason  that  in  ordinary 
accidents  so  many  people  are  drowned  who  might  easily  be  saved,  are  chiefly 
the  following : — 

1st.  They  believe  that  the  body  is  heavier  than  water,  and  therefore,  that 
continued  exertion  is  necessary  to  keep  it  from  sinking;  and  hence,  instead 
of  lying  quietly  on  the  back,  with  the  face  upwards,  and  with  the  face  only 
out  of  the  water,  they  generally  assume  the  position  of  a  swimmer,  in  which 
the  face  is  downwards,  and  the  whole  head  has  to  be  kept  out  of  the  water 
to  allow  of  breathing.  Now,  as  a  man  cannot  retain  this  position  but  by  con- 
tinued exertion,  he  is  soon  exhausted,  even  if  a  swimmer,  and  if  he  is  not, 
the  unskilful  attempt  will  scarcely  secure  for  him  even  a  few  respirations. 
The  body  raised  for  a  moment  by  exertion  above  the  natural  level,  sinks  as 
far  below  it  when  the  exertion  ceases;  and  the  plunge,  by  appearing  the 
commencement  of  a  permanent  sinking  terrifies  the  unpractised  individual, 
and  renders  him  an  easier  victim  to  his  fate. — To  convince  a  person  learn- 
ing to  swim  of  the  natural  buoyancy  of  his  body,  it  is  a  good  plan  to  throw 
an  egg  into  water  about  five  feet  deep,  and  then  desire  him  to  bring  it  up 
again.  He  discovers  that  instead  of  his  body  with  the  chest  full  of  air  na- 


150  HYDROSTATICS. 

turally  sinking  towards  the  egg,  he  has  to  force  his  way  downwards,  and  is 
lifted  again  by  the  water  as  soon  as  he  ceases  his  effort. 

2d.  They  fear  that  water  entering  by  the  ears  may  drown,  as  if  it  entered 
by  the  nose  or  mouth,  and  they  make  a  wasteful  exertion  of  strength  to  pre- 
vent it ;  the  truth  being,  however,  that  it  can  only  fill  the  outer  ear,  as  far 
as  the  membrane  of  the  drum,  where  its  presence  is  of  no  consequence. 
Every  diver  and  swimmer  has>  his  ears  thus  filled  with  water,  and  cares  not. 

3d.  Persons  unaccustomed  to  the  water,  and  in  danger  of  being  drowned, 
generally  attempt  in  their  struggle  to  keep  their  hands  above  the  surface, 
from  feeling  as  if  their  hands  were  imprisoned  and  useless  while  below ;  but 
this  act  is  most  hurtful,  because  any  part  of  the  body  held  out  of  the  water, 
in  addition  to  the  face  which  must  be  out,  requires  an  effort  to  support  it, 
which  the  individual  is  supposed  at  the  time  ill  able  to  afford. 

4th.  They  do  not  reflect,  that  when  a  log  of  wood  or  &  human  body  is 
floating  upright,  with  a  small  portion  above  the  surface,  in  rough  water,  as 
at  sea,  every  wave  in  passing  must  cover  it  completely  for  a  little  time,  But 
again  leave  its  top  projecting  in  the  interval.  The  practiced  swimmer  chooses 
this  interval  for  breathing. 

5th.  They  do  not  think  of  the  importance  of  keeping  the  chest  as  full  of 
air  as  possible ;  the  doing  which  has  nearly  the  same  effect  as  tying  a  blad- 
der of  air  to  the  neck,  and  without  other  effort,  will  cause  nearly  the  whole 
head  to  remain  above  the  water.  If  the  chest  be  once  emptied,  while  from 
the  face  being  under  water  the  person  cannot  inhale  again,  the  body  remains 
specifically  heavier  than  water,  and  will  sink. 

When  a  man  dives  far,  the  pressure  of  deep  water  compresses,  or  dimi- 
nishes the  bulk  of  the  air  in  his  chest,  so  that,  without  losing  any  of  that  air, 
he  yet  becomes  really  heavier  than  water,  and  would  not  again  rise,  but  for 
the  exertion  of  swimming.  The  author  of  this  work  once  saw  a  sailor  (  a 
fine-bodied  ^West  India  negro  )  fall  into  the  calm  sea  from  a  yard-arm  eighty 
feet  high.  The  velocity  on  his  reaching  the  water  was  so  great,  that  he  shot 
deep  into  it,  and,  of  course,  his  chest  was  compressed  as  now  explained : 
probably  also  the  shock  stunned  him,  for  although  he  was  an  excellent 
swimmer,  he  only  moved  his 'arms  feebly  once  or  twice,  and  was  then  seen 
gradually  sinking  for  a  long  time  afterwards,  until  he  appeared  only  as  a 
black  and  distant  speck,  descending  towards  the  unknown  regions  of  the 
abyss. 

Every  person  need  not  learn  to  swim ;  but  every  one  who  makes  voyages 
should  have  practiced  the  easy  lesson  of  resting  in  the  water  with  the  face 
out.  The  head,  from  the  large  quantity  of  bone  in  it  is  a  heavy  part  of  the 
body,  yet,  owing  to  its  proximity  to  the  chest,  which  is  comparatively  light, 
a  little  action  of  adjustment  with  the  hands,  easily  keeps  it  uppermost ;  and 
there  is  an  accompanying  motion  of  the  feet,  called  treading  the  water,  not 
diffcult  to  learn,  which  suffices  to  sustain  the  entire  head  above  the*  surface. 
Many  of  the  seventy  passengers  who  were  swallowed  up  on  the  sudden  sink- 
ing of  the  Comet  steam-boat  near  Greenock,  in  November,  1825,  might  have 
been  saved  by  the  boats,  which  so  soon  went  to  their  assistance,  had  they 
known  the  truth  which  we  are  now  explaining. 

A.  man  having  to  swim  far,  may  occasionally  rest  on  his  back  for  a  time, 
and  resume  his  labor  when  he  is  somewhat  refreshed. 

So  little  is  required  to  keep  a  swimmer's  head  above  water,  that  many 
individuals,  although  unacquainted  with  what  regards  swimming  or  floating, 
have  been  saved  after  shipwreck,  by  catching  hold  of  a  few  floating  chips  or 
broken  pieces  of  wood.  An  oar  will  suffice  as  a  support  to  half  a  dozen 


FLUID  SUPPORT.  —  STABILITY.          151 

people,  provided  no  one  of  the  number  attempts  by  it  to  keep  more  than  his 
head  out  of  the  water;  but  often,  in  cases  where  it  might  be  thus  serviceable, 
from  each  person  wishing  to  have  as  much  of  the  security  as  possible,  the 
number  benefitted  is  much  less  than  it  might  be. 

The  most  common  contrivances,  called,  life-preservers,  for  preventing 
drowning,  are  strings  of  cork  put  round  the  chest  or  neck,  or  air-tight  bags 
applied  round  the  upper  part  of  the  body,  and  filled,  when  required,  by  those 
who  wear  them  blowing  into  them  through  valved  pipes. 

On  the  great  rivers  of  China,  where  thousands  of  people  find  it  more  con- 
venient to  live  in  covered  boats  than  in  houses  upon  the  shore,  the  younger 
children  have  a  hollow  ball  of  some  light  material  attached  constantly  to 
their  necks,  so  that,  in  their  frequent  falls  overboard,  they  are  not  in  danger. 

Life-boats  have  a  large  quantity  of  cork  mixed  in  their  structure,  or  of 
air-tight  vessels  of  thin  copper  or  tin  plate  :  so  that,  even  when  the  boats 
are  filled  with  water,  a  considerable  part  still  floats  above  the  general  surface. 

Swimming  is  much  easier  to  quadrupeds  than  to  man,  because  the  ordi- 
nary motion  of  their  legs  in  walking  and  running  is  that  which  best  supports 
them  in  swimming.  Man  is  at  first  the  most  helpless  of  creatures  in  water. 
A  horse  while  swimming  can  carry  his  rider  with  half  the  body  out  of  the 
water.  Dogs  commonly  swim  well  on  the  first  trial. — Swans,  geese,  and 
water-fowls  in  general,  owing  to  the  great  thickness  of  feathers  on  the  under 
part  of  their  bodies,  and  the  great  volume  of  their  lungs,  and  the  hollowness 
of  their  bones,  are  so  bulky  and  light,  that  they  float  upon  the  water  like 
stately  ships,  moving  themselves  about  by  their  webbed  feet  as  oars. 

A  water-fowl  floating  on  plumage  half  as  bulky  as  its  naked  body,  has 
about  half  that  body  above  the  surface  of  the  water ;  and  similarly  a  man 
reclining  on  a  floating  mattrass,  as  in  the  hydrostatic  bed  afterwards  to  be 
described,  has  nearly  as  much  of  his  body  above  the  level  of  the  water- 
surface,  as  he  forces  of  the  mattrass  under  it.  His  position,  therefore, 
depends  on  the  thickness  of  the  mattrass. 

A  man  walking  in  deep  water  may  tread  upon  sharp  flints  or  broken  glass 
with  impunity,  because  his  weight  is  nearly  supported  by  the  water. 

But  many  men  have  been  drowned  in  attempting  to  waae  across  the  fords 
of  rivers,  from  forgetting  that  the  body  is  so  supported  by  the  water,  and 
does  not  press  on  the  bottom  sufficiently  to  give  a  sure  footing  against  a  very 
trifling  current.  A  man,  therefore,  carrying  a  weight  on  his  head,  or  in  his 
hands  held  over  his  head,  as  a  soldier  bearing  his  arms  and  knapsack,  may 
safely  pass  a  river,  where,  without  a  load,  he  would  be  carried  down  the 
stream. 

There  is  a  mode  practised  in  China  of  catching  wild  ducks,  which  requires 
that  the  catcher  be  well  loaded  or  ballasted.  The  light  grain  being  first 
strewed  upon  the  surface  of  the  water  to  temp  them,  a  man  hides  himself 
in  the  midst  of  it,  under  what  appears  a  gourd  or  basket  drifting  with  the 
stream,  and  when  the  flock  approaches  and  surrounds  him,  he  quickly 
obtains  a  rich  booty  by  snatching  the  creatures  down  one  by  one — adroitly 
making  them  disappear  as  if  they  were  diving,  and  then  securing  them 
below.  Each  bird  becomes  as  a  piece  of  cork  attached  to  his  body. 

Fishes  can  change  their  specific  gravity,  by  diminishing  or  increasing  the 
size  of  a  little  air-bag  contained  to  their  body.  It  is  because  this  bag  is 
situated  towards  the  under  side  of  the  body,  that  a  dead  fish  floats  with  the 
belly  uppermost. 

Animal  substances,  in  undergoing  the  process  of  putrefaction,  give  out 
much  aeriform  matter.  Hence  the  bodies  of  persons  drowned  and  remaining 


152  HYDKOSTATICS. 

in  the  water,  generally'swell,  after  a  time,  and  rise  to  the  surface,  again  to 
sink  when  the  still  increasing  quantity  of  air  shall  burst  the  containing  parts. 

A  floating  body  sinks  to  the  same  depth  whether  the  mass  of  fluid 
supporting  it  be  great  or  small : — as  is  seen  when  a  porcelain  basin  is  placed 
first  in  a  pond,  and  then  in  a  second  basin  only  so  much  larger  than  itself 
that  a  spoonful  or  two  of  water  suffices  to  fill  up  the  interval  between  them. 
One  ounce  of  water  in  the  latter  way  may  float  a  thing  weighing  a  pound 
or  more,  exhibiting  another  instance  of  the  hydrostatic  paradox  : — And  if 
the  largest  ship  of  war  were  received  into  a  dock,  or  case,  so  exactly  fitting 
it  that  there  were  only  half  an  inch  of  interval  between  it  and  the  wall  or 
side  of  the  containing  space,  it  would  float  as  completely,  when  the  few 
hogsheads  of  water  required  to  fill  this  little  interval  up  to  its  usual  water- 
mark were  poured  in,  as  if  it  were  on  the  high  sea.  In  some  canal  locks, 
the  boats  just  fit  the  place  in  which  they  have  to  rise  and  fall,  and  thus  the 
expense  of  water  at  the  lock  is  diminished 

The  preceding  examples  of  floating  are  all  illustrations  also  of  the  truth 
that  the  pressure  of  a  fluid  on  any  immersed  body  is  exactly  proportioned 
to  the  depth  and  extent  of  the  surface  pressed  upon.  The  lateral  pressures 
just  balanced  one  another,  and  the  upward  pressure  has  to  be  balanced  by 
the  weight  of  the  body. 

Similar  reasoning  to  that  which  proves  that  the  whole  weight  of  a  body  acts 
as  if  lodged  in  the  point  called  its  centre  of  gravity,  proves  that  the  whole 
buoyancy  of  a  body,  or  the  upward  push  of  the  fluid  in  which  a  body  is 
immersed,  acts  as  if  lodged  in  the  point  which  was  the  centre  of  gravity 
of  the  fluid  displaced.  This  point,  consequently,  is  called  the  "  centre  of 
buoyancy." 

A  floating  body,  to  be  stable  in  its  position,  either  must  have  its  centre 
of  gravity  below  the  centre  of  buoyancy — in  which  case  it  resembles  a 
pendulum  ;  or  it  must  have  a  very  broad  bearing  on  the  water,  so  that  any 
inclination  may  rause  the  centre  of  gravity  to  ascend — in  which  case*it 
resembles  a  cradle  or  rocking-horse. 

Hence  arises,  in  the  stowing  of  a  ship's  cargo,  the  necessity  of  putting 
the  heavy  merchandise  underneath,  and  generally  of  putting  iron  ballast 
under  all  the  merchandise.  Hence,  also,  the  danger  of  having  a  cargo  or 
ballast  which  is  liable  to  shift  its  place.  A  ship  loaded  entirely  with  stones, 
is  sometimes  lost  by  a  wave  making  her  incline  for  a  moment  so  much  that 
the  load  ships  to  one  side,  which  is  then  kept  down.  For  a  similar  reason, 
a  cargo  of  salt  or  sugar  has  a  peculiar  danger  attached  to  it,  for  if  the  ship 
leak,  the  cargo  may  be  dissolved,  and  then  pumped  out  with  the  bilge  water, 
leaving  her  with  altered  trim.  In  a  fleet  coming  home  from  India,  in  1809, 
four  fine  ships  disappeared  during  a  hurricane  off  the  isle  of  France,  and  from 
what  happened  to  the  other  ships  that  were  saved,  the  cause  of  the  destruc- 
tion was  supposed  to  be,  that  the  saltpetre  of  the  cargoes  had  been  dissolved 
and  pumped  out,  and  that  the  ships  in  consequence  became  unmanageable. 

Bladders  used  by  beginners  in  swimming  are  dangerous,  unless  secured 
so  as  not  to  shift  towards  the  lower  part  of  the  body. 

A  great  inventor  (in  his  own  estimation)  published  to  the  world,  that  he 
had  solved  the  important  problem  of  walking  safely  upon  the  water  ;  and  he 
invited  a  crowd  to  witness  his  first  essay.  He  stepped  boldly  upon  the  wave, 
equipned  in  bulky  cork  boots,  which  he  had  previously  tried  in  a  butt  of 
water  at  home ;  but  it  soon  appeared  that  he  had  not  pondered  sufficiently  on 


FLUID    SUPPORT    AMONG    FLUIDS.  153 

the  centres  of  gravity  and  of  floatation,  for  in  the  next  instant  all  that  was 
to  be  seen  of  him  was  a  pair  of  legs  sticking  out  of  the  water,  the  movements 
of  which  showed  that  he  was  by  no  means  at  his  ease.  He  was  picked  up 
by  help  at  hand,  and,  with  his  genius  cooled  and  schooled  by  the  event,  was 
conducted  home. — Some  soldiers  once  finding  a  few  cork/ac&efe,  among  old 
military  stores,  determined  to  try  them  j  but  mistaking  the  shoulder  straps 
for  lower  fastenings,  they  put  them  on  as  drawers^nd  on  then  plunging  in, 
with  the  hope  of  being  able  to  sit  pleasantly  on  th  *water,  their  heavy  heads 
went  down,  and  they  were  nearly  drowned 

When,  on  the  return  of  summer,  the  ice  breaks  up  in  the  polar  regions, 
immense  islands  of  it  are  set  afloat,  rising  high  into  the  air  and  sinking  deep 
into  the  sea.  The  melting  process,  in  most  cases,  does  not  go  on  equally  in 
the  water  and  in  the  air,  and  from  the  mass,  consequently,  changing  form,  its  * 
stability  is  often  lost,  and  one  of  the  grandest  phenomena  in  nature  follows — 
the  overturning  of  a  mountain — the  sudden  subversion  of  an  island — pro- 
ducing a  tumult  in  the  ocean  around,  felt  often  at  the  distance  of  many  leagues. 

The  phenomena  of  pressure,  floating,  &c.,  in  fluids,  vary  in  proportion  to 
the  weight  or  specific  gravity  of  the  fluid. 

A  ship  draws  less  water,  or  swims  lighter,  by  one  thirty-fifth,  in  the  heavy 
salt-water  of  the  sea  than  in  the  fresh  water  of  a  river  :  and  for  the  same  reason 
a  man  swimming  supports  hmiself  more  easily  in  the  sea  than  in  a  river. 

Many  kinds  of  wood  that  float  in  water  will  sink  in  oil. 

A  man  floats  on  mercury  as  the  lightest  cork  floats  on  water,  and  with 
practice  he  might  be  able  to  walk  upon  mercury. 

Had  the  water  of  our  ocean  been  but  a  little  heavier  than  it  is,  men  after 
shipwreck  might  have  died  of  famine  and  cold,  but  would  not  have  been 
drowned 

Oil  floats  on  water,  but  sinks  in  alcohol  or  aether.  The  term  proof  spirit 
means  spirit  light  enough  for  oil  to  sink  in  it.  The  strength  of  spirit  is 
proportioned  to  its  lightness. 

Cream  rises  in  milk,  and  forms  a  covering  to  it. 

Blood,  allowed  to  rest  after  flowing  from  the  living  body,  separates  into 
parts  or  layers,  which  arrange  themselves  according  to  their  specific  gravities. 
The  buffy  coat  of  inflammation  (where  this  exists)  is  uppermost,  forming 
the  surface  of  the  general  coagulum  :  towards  the  lower  part  of  the  coagulum 
there  is  an  accumulation  of  red  globules;  and-the  whole  of  the  solid  part 
floats  in  the  serum,  which  is  therefore  lowest  of  all.  When  the  red  globules 
escape  from  the  coagulum,  they  fall  to  the  bottom  even  of  the  serum. 

Wine,  if  slowly  and  carefully  poured  on  water,  will  float  upon  it.  In  a 
vessel  shaped  like  a  common  sand-glass,  only  with  a  larger 
opening  between  the  chambers  at  c,  if  wine  be  put  into  the  Fig.  83. 
under  chamber,  and  water  into  the  upper,  the  two  liquids 
will  gradually  change  places  :  and  if  the  lower  half  of  the 
glass  be  covered,  so  as  to  leave  the  upper  half  with  the 
appearance  of  a  simple  goblet,  the  water  will  seem  to  have 
been  changed  into  wine.  The  liquids  are  less  mixed,  and 
change  places  sooner,  when  there  is  a  tube  6  to  carry  the 
water  down  to  the  bottom  without  touching  the  wine,  and  a 
tube  a  to  carry  the  wine  directly  to  the  top. 

Mercury,  water,  oil,  air,  and  some  other  fluids  may  all  be 
shaken  together  in  the  same  vessel,  and  on  standing  will 
separate  again  and  arrange  themselves  in  the  order  of  their  specific  gravities. 


154 


HYDROSTATICS. 


When,  in  a  mass  of  water,  part  of  it  is  heated  more  than  the  rest,  that 
part,  by  its  expansion,  becomes  specifically  lighter  than  the  rest,  and  rises 
to  the  surface.  Hence,  wh^n  heat  is  applied  to  the  bottom  of  a  vessel 
containing  water,  a  circulation  is  established,  which  goes  on  from  the  first 
moment  until  the  operation  of  heating  finishes  : — water  is  always  rising 
from  the  hotter  parts  of  the  vessel,  and  descending  over  the  colder  parts. 

In  like  manner,  when  a  tall  glass  containing  hot  water  is  dipped  into  cold 
water,  a  downward  currem  lakes  place  within  the  glass  near  the  sides  all 
round,  and  there  is  an  upward  current  in  the  middle.  This  motion  may  be 
rendered  very  obvious  by  small  portions  of  amber  thrown  into  the  water,  for 
these  being  nearly  of  the  specific  gravity  of  water,  rise  and  descend  with  it. 
On  account  of  the  current  established  in  such  cases,  heat  applied  to  the 
bottom  of  a  vessel  of  liquid  is  soon  equally  diffused  over  it ;  but  heat 
applied  at  the  top  is  there  confined,  because  the  heated  and  lighter  fluid 
does  not  descend.  Water  may  be  made  to  boil  at  its  surface,  while  a  piece 
of  ice  lies  at  the  bottom.  The  converse  is  impossible. 

The  current  in  a  fluid,  produced  by  local  change  of  temperature,  is  an 
important  part  of  the  following  process,  which  the  author  deems  applicable 
to  various  useful  purposes. — Heat  may  be  transferred  from  one  liquid  to 
another,  without  mixing  them,  by  making  the  hot  liquid  descend  in  a  very 
thin  metallic  tube,  through  the  cold  liquid  rising  around  it  in  a  larger  tube, 
Boiling  water  from  the  vessel  e,  for  instance,  may  descend  slowly  by  the 

small  tube  ea  bf,  which  is  sur- 

Fig.  84.  rounded   from  a  to  b  by  cold 

water  ascending  through  the 
tube  c  g.  Then,  as  the  tem- 
perature of  two  liquids,  brought 
so  nearly  into  contact  with  each 
other,  will  not,  after  a  very  short 
time,  differ,  in  any  one  place 
more  than  a  few  degrees,  it  fol- 
lows that  the  water  lately  cold, 
will  on  leaving  the  part  of  the 
tube  g,  which  is  in  contact  with 
the  boiling  water  descending  di- 
rectly from  e,  be  nearly  boiling, 
while  the  water  lately  hot  will, 
on  leaving  the  tube  at  &,  which 
is  in  contact  with  cold  water 
just  arrived  from  A,  be  itself 
nearly  cold  j  and  thus  equal 
quantities  of  hot'and  cold  water  will  have  exchanged  temperatures.  *The  flux 
of  the  hot  water  is  to  be  regulated  by  a  cock  6,  and  that  of  the  cold  water  by 
a  cock  h.  The  water  in  the  part  of  the  tube  c  g  d  rises,  because  it  is  hotter  and 
therefore  specifically  lighter  than  that  in  the  part  h  c. — The  author  believes 
that  an  apparatus  made  on  this  principle,  with  an  arrangement  of  many  thin 
flat  tubes  instead  of  a  single  large  tube,  for  the  descending  fluid,  and  a  spacious 
box  c  g  to  contain  these  and  the  rising  fluid,  would  be  an  excellent  refrigera- 
tor in  a  distilling  apparatus,  and  for  cooling  the  wort  of  brewers ;  or  would 
serve  as  a  means  of  diminishing  the  expense  of  warm  baths,  by  transferring 
the  heat  from  the  water  lately  used  to  pure  water.  In  distilling,  the  wash  or 
low  wines,  about  to  enter  the  still,  might  be  used  as  the  cold  condensing  fluid 
to  surround  the  warm  or  vapor  tubes,  and  thus,  without  expense,  would  be 


FLUID  SUPPORT  AMONG  FLUIDS.        155 

heated  in  its  progress  to  the  still.  Half  the  original  expense  of  a  great 
porter  brewery  is  in  the  construction  of  the  numerous  water-tight  floors  on 
which  the  hot  wort  is  thinly  spread  to  cool.  The  practice  of  warm  bathing, 
so  conductive  to  health,  is  less  common  in  this  country,  because  the  present 
expense  is  so  great. 

It  is  a  general  truth  in  nature,  that  substances  contract  in  size  as  they 
cool.  There  is,  however,  in  water,  a  curious  exception  to  this  rule,  which, 
operating  through  the  principle  of  specific  gravities,  effects  most  important 
purposes  in  the  economy  of  nature.  Water  contracts  only  down  to  the 
temperature  of  40  deg.,  below  which,  towards  32  deg.,  or  the  freezing  point, 
it  goes  on  dilating  again,  and  as  ice  is  much  lighter  than  as  a  fluid.  Ice, 
therefore,  floats  on  the  surface  of  water,  and  being  a  very  slow  conductor  of 
heat,  defends  the  water  underneath  from  the  cold  air,  and  preserves  it  liquid, 
and  a  fit  dwelling  for  the  finny  tribe,  until  the  return  of  the  mild  season. 
And  not  only  is  the  extreme  of  cold  below  thus  prevented,  but  because  very 
cold  water  remains  floating  on  the  surface  of  a  wintry  lake,  as  cream  floats 
on  milk,  it  preserves  underneath  that  warmth  which  is  agreeable  to  the 
fishes,  just  as  very  hot  water  in  summer  remains  uppermost,  preserving 
underneath  an  agreeable  coolness.  By  the  dilation  of  very  cold  water,  then, 
and  the  formation  of  ice,  nature  has  prepared  a  winter  garb  for  the  inhabited 
lakes  and  rivers,  as  complete  and  effectual  as  for  terrestrial  animals,  by  the 
periodical  thickening  of  their  wool  or  fur.  Had  ice  become  heavier  than 
water,  so  that  it  must  have  fallen  to  the  bottom,  and  have  left  the  surface 
•without  protection,  a  deep  lake  in  European  winters,  would  have  been  frozen 
into  a  solid  lifeless  mass,  which  summer  suns  would  no  more  have  melted 
than  they  now  do  the  glaciers  of  Switzerland.  But  for  this  important  ex- 
ception, therefore,  to  a  general  law  of  nature,  many  of  the  now  most  fertile 
and  lovely  portions  of  the  earth's  surface  would  have  remained  for  ever  barren 
and  uninhabited  wastes. 


156  PNEUMATICS. 


PART  III. 


THE  PHENOMENA  OP  PLUIDS 


SECTION  IL--PNEUMATICS. 


ANALYSIS  OF  THE  SECTION. 

In  aeriform  fluids,  that  is,  in  such  as  have  their  particles  held  far  apart 
by  mutual  repulsion,  which  yields,  however,  to  any  force  applied,  so  that 
the  mass  suffers  great  change  of  volume  under  different  degrees  of  com- 
pression.— the  phenomena  are  modified  by  the  GREAT  LIGHTNESS  and 
ELASTICITY  of  the  fluids,  but  are  still  in  strict  accordance  with  the  general 
properties  of  fluids  already  explained,  viz.,  PRESSURE  EQUAL  IN  ALL 

DIRECTIONS — PRESSURE   AS   THE   DEPTH — LEVEL   SURFACE,   and   FLUID 

SUPPORT.  The  pressure  of  air,  in  all  directions,  and  as  the  depth,  may 
be  studied  in  the  effects  of  our  atmosphere — on  solids — on  liquids  : — or 
ichen  it  concurs  with  heat,  in  producing  the  phenomena  of  boiling,  evapo- 
ration, clouds,  rain,  dew,  &c. ;  or  when,  by  varying  in  degree,  it  allows 
certain  substances  to  exist  sometimes  in  the  liquid  and  sometimes  in  the 
aeriform  states.  The  fluid  support  in  air  is  exemplified  by  ballons,  the 
ascent  of  flame,  and  smoke",  winds,  &c. 

WHATa  change  has  taken  place  in  the  degree  of  man's  knowledge  of  nature, 
since  philosophers  thought  that  air  was  one  of  four  primary  elements, 
viz.,  air,  fire,  water,  and  earth,  of  which  all  things  were  composed,  and  each 
of  which  was  for  ever  distinct  from  the  others.  We  now  know  that  air  or 
gas  is  merely  an  accidental  state,  in  which  any  body  may  exist,  according  to 
the  quantity  of  heat  pervading  it:  the  body  being  solid  when  the  absence  of 
heat  allows  its  atoms  to  obey  freely  their  mutual  attraction,  and  to  cohere — 
as  in  ice,  for  instance ;  being  liquid,  when  so  much  heat  is  present  as  nearly 
to  balance  the  attraction,  and  to  let  them  slide  freely  among  each  other — as 
they  do  in  water ;  and  being  aeriform  when  still  more  heat  is  added,  causing 
the  atoms  mutually  to  repel  and  dart  asunder  to  a  great  distance — as  they  do 
in  steam.  But  in  any  one  of  these  three  states,  the  various  substances  are 
as  much  themselves  as  in  the  others,  and  at  the  command  of  the  chemist 
will  assume  any  of  the  forms  which  he  desires,  As  most  substances  in 
nature  have  a  different  relation  to  heat,  there  are  some  which,  at  the  medium 
temperature  of  our  earth  are  solid,  some  which  are  liquid,  and  some  aeriform. 
The  solids,  in  general,  are  the  heaviest  under  a  given  volume,  and  therefore 
sink  down  and  form  the  great  mass  or  centre  of  the  earth  j  the  liquids  follow 
next  in  order,  and  float  upon  this  solid  centre,  filling  up  its  inequalities  with 


AERIFORM    FLUIDS.  157 

a  level  surface,  so  as  to  constitute  the  ocean ;  while  the  airs  are  the  lightest 
of  all,  and  as  a  second  ocean,  rest  above  the  sea  and  above  the  highest  moun- 
tain, to  an  elevation  of  about  fifty  miles.  Among  the  substances  whose 
relation  to  heat  causes  them,  when  not  restrained  in  certain  combinations,  to 
assume  the  form  of  air  at  very  low  temperatures,  there  are  two  in  particular, 
viz.,  oxygen  and  nitrogen,  which  are  very  abundant  in  nature  in  such  uncom- 
bined  state,  and  of  these,  therefore,  the  atmosphere  chiefly  consists ;  but 
smaller  portions  of  almost  every  other  substance  are  found  in  it.  Water, 
among  the  supplementary  matters,  is  much  more  abundant  than  any  of  the 
others,  and  in  various  states  of  cloud,  mist,  rain,  dew  and  snow,  it  answers 
a  thousand  useful  purposes,  and  serves  beautifully  to  vary  the  scenes  of 
nature.  The  atmosphere  is  about  fifty  miles  high  or  deep,  and  therefore, 
in  relation  to  the  bulk  of  the  earth,  is  as  a  covering  of  one-tenth  of  an  inch 
in  thickness  to  a  common  library  globe  of  a  foot  in  diameter. 

The  atmospheric  ocean  is  the  great  laboratory  in  which  most  of  the  actions 
of  life  go  on,  and  on  the  composition  of  which  they  depend.  A  human 
being  requires  for  breathing  a  gallon  of  fresh  air  every  minute,  dying  equally 
if  deprived  of  air,  or  if  confined  to  the  same.  All  other  animals  also  require 
fresh  air,  but  in  various  proportions.  And  in  the  vegetable  creation,  the 
beautiful  green  leaf  and  delicate  flower  are  merely  broad  and  tender  expan- 
sions of  surface  for  the  contact  of  the  vivifying  air.  Animals  give  out  to 
the  atmosphere  a  substance  which  vegetables  absorb,  and  vegetables,  by  the 
absorption,  fit  the  air  again  for  the  use  of  animals ;  so  that,  upon  the  whole, 
in  the  various  changes  of  nature,  there  is  a  perfect  balancing  of  actions, 
which  preserves  the  atmospheric  mass  in  a  uniform  state,  constantly  fit  for 
its  admirable  purposes. 

While  the  ancients  had  that  notion  of  air,  which  made  them  apply  to  it 
vaguely,  and  almost  indifferently,  the  names  of  air,  ether,  spirit,  breath,  life, 
&c.,  they  never*  dreamed  of  making  experiments  upon  it,  with  a  view  to  prove 
its  relation  to  common  matter : — and  one  of  the  most  beautiful  portions  of 
the  modern  history  of  man's  progress  in  knowledge,  is  that  which  exhibits 
the  light  gradually  breaking  in  upon  this  most  interesting  subject.  Galileo 
was  led  to  conclude  that  air  made  a  definite  pressure  upon  things  at  the 
surface  of  the  earth ;  Torricelli  and  Pascal  proved  that  this  was  occasioned 
by  its  weight,  and  hence,  moreover,  they  deduced  the  height  of  the  aerial 
ocean ;  Priestly,  Black,  Lavoisier,  and  others,  discovered  that  air  might  be 
united  with  a  metal,  so  as  to  increase  its  weight,  and  to  produce  a  compound 
of  totally  new  qualities,  for  they  showed  that  many  of  the  ores  of  our  mines 
are  merely  metals  concealed,  by  being  thus  united  with  a  substance  which, 
when  set  free,  ascends  as  one  of  the  ingredients  of  the  atmosphere.  They  at 
last  analyzed  the  atmosphere  itself,  and  exhibited  its  two  ingredients  as 
distinct  substances.  And  within  a  few  years  the  nature  of  air  or  gas  has 
been  so  thoroughly  investigated,  that  we  can  now  take  a  little  of  many  a 
light,  invisible,  impalpable  fluid  such  as  we  breathe,  and  squeezing  the  heat 
out  of  it  by  strong  pressure,  can  make  its  particles  collapse  from  their 
aeriform  distances  to  assume  the  state  of  a  tranquil  fluid;  which  may 
then  be  retained  as  such  for  ever,  or  may  be  decomposed  and  made  solid  in 
combination  with  other  bodies,  or  may  be  again  set  at  liberty. 

The  suspicion  once  excited,  that  air  was  as  much  a  material  fluid  as  water, 
only  much  less  dense,  by  reason  of  a  greater  separation  and  repulsion  of  the 
particles,  it  was  easy  to  follow  out  the  parallel,  and  to  confirm  the  supposition 
by  .reference  to  the  commonest  facts.  Thus,  a  leathern  sack  or  pouch,  opened 
and  dipped  into  water  so  far  as  to  become  full,  if  its  mouth  be  then  carefully 


158  PNEUMATICS. 

closed,  retains  the  water,  and  its  sides  cannot  afterwards  be  pressed  together : 
a  similar  sack  or  bladder,  opened  out,  and  then  closed  in  air,  is  found  to 
remain,  in  a  corresponding  way,  bulky  and  resisting,  and  forms  what  is  called 
an  air-pillow.  The  motion  of  a  flat  board  is  resisted  in  water :  the  motion 
of  a  fan  is  resisted  in  the  air.  Masses  of  wood,  sand,  and  pebbles,  are  rolled 
along  or  floated  by  currents  of  water :  chaff,  feathers,  and  even  rooted  trees 
are  swept  away  by  currents  of  air.  There  are  mills  driven  by  water ;  and 
there  are  mills  driven  by  the  wind.  Oil  set  free  under  the  surface  of  water, 
or  placed  there  in  a  bladder,  rises  to  the  surface :  hot  air  or  hydrogen  gas 
placed  in  a  balloon,  rises  in  the  air.  A  fish  moves  itself  by  its  fins  in  water  : 
a  bird  moves  itself  by  its  wings  in  the  air ;  and  as  on  taking  the  water  from 
a  vessel  in  which  a  fish  swims,  the  creature  falls  to  the  bottom,  gasps  a  few 
moments,  and  dies,  so,  on  exhausting  the  air  from  a  vessel  in  which  birds 
or  butterflies  are  enclosed,  their  useless  wings  may  flap ;  but  they  sink  to 
the  bottom,  and  if  the  cruel  experiment  be  continued,  they  soon  become 
motionless  and  forever. 

We  proceed  now  to  prove  that  air  or  gas,  as  a  fluid,  differs  from  the 
other  fluids,  which  we  call  liquids,  only  in  the  two  circumstances  of  great 
lightness  or  rarity,  and  of  being  very  extensively  elastic,  that  is  to  say,  the 
particles  being  so  related,  that  pressure  brings  them  much  more  nearly  into 
contact,  and  on  ceasing,  allows  them  to  regain  their  former  distance. 

Lightness  of  Air. 

The  lightness  or  rarity  of  atmospheric  air,  as  it  is  found  on  the  general 
gurface  of  the  earth,  is  such,  that  if,  by  the  action  of  a  pump,  a  bag  of  it 
holding  a  cubic  foot  be  emptied  into  the  copper  ball  of  an  air-gun,  the  ball 
weighs  about  an  ounce  and  a  quarter  more  than  before.  The  same  volume 
of  water  weighs  nearly  a  thousand  ounces  j  so  that  common  air  is  about  eight 
hundred  times  lighter  than  water.  Other  gases,  or  substances  in  the  aeri- 
form state,  have  their  various  specific  gravities,  just  as  the  same  substances 
have  when  liquid  or  solid.  Thus  water  in  the  form  of  air,  that  is  to  say, 
when  existing  as  steam,  and  of  the  common  density,  is  little  more  than  half 
as  heavy  as  the  same  bulk  of  common  air  \  hydrogen  is  only  one-fourteenth 
part  as  heavy :  and  carbonic  acid  gas,  which  is  the  air  that  rises  out  of  soda- 
water,  brisk  ale,  champagne  wine,  &c.,  is  so  much  heavier,  that  even  in  the 
atmosphere  it  may  be  poured  out  of  one  open  vessel  into  another,  as  a  liquid 
might,  or,  more  exactly,  as  water  might  be  poured  out  under  oil. 

Elasticity  of  Air. 

A  small  bladder  full  of  air  may  be  pressed  or  squeezed  between  the  hands 
so  as  to  be  much  reduced  in  size,  but  on  being  relieved  from  the 
Fig.  85.      pressure,  it  will  immediately  resume  its  former  bulk. 

d,  If  a  metalic  tube  or  barrel  of  perfectly  uniform  bore  a  b,  be 

fitted  with  a  moveable  plug  or  piston  c,  which  is  covered  with 
leather  and  oiled,  so  as  to  slide  up  and  down  without  allowing 
the  air  to  pass  by  its  sides,  the  air  between  the  piston  and  the 
close  bottom  b  may  be  compressed  to  a  hundredth  or  less  of  its 
usual  bulk ;  but  when  allowed,  will  push  the  piston  back  again 
with  the  same  force  as  it  opposed  to  the  condensation,  and  will 
recover  the  volume  which  it  had  before  the  experiment. 

Again,  if  the  plug,  at  the  commencement  of  the  experiment, 
were  only  an  inch  from  the  bottom,  enclosing  air  of  the  usual 
density,  on  drawing  it  up  to  the  top,  the  inch  of  air  beneath  it 


ELASTICITY    OF    AIR. 


159 


would  expand  so  as  to  occupy  the  whole  tube,  having  become,  of  course, 
proportionally  less  dense. 

To  the  question  why  the  air,  which  admits  of  such  various  density,  is 
found  to  have  that  certain  degree  of  it  met  with  at  the  surface  of  the  earth, 
we  answer,  that  as  the  water,  in  any  place  near  the  bottom  of  the  ocean,  is 
pressed  with  force  exactly  proportioned  to  the  quantity  of  water  above  it,  so 
the  air  at  the  surface  of  the  earth  bears  the  pressure  of  the  superincumbent 
mass  of  air,  and  on  account  of  its  extensive  elasticity,  suffers,  like  the  lower- 
most bags  of  cotton  or  wool  in  a  great  heap,  that  degree  of  compression 
which  the  superincumbent  mass  is  calculated  to  produce.  We  shall  see 
below  that  the  density  of  the  air  near  the  earth  is  changing  with  every 
circumstance  which  affects  the  weight  of  the  atmosphere  above,  as  winds, 
clouds,  rain,  &c.,  and  that  it  bears  relation  to  the  altitude  of  the  place  of 
observation  above  the  level  of  the  sea. 

The  tube  with  its  piston,  described  in  the  last  page,  becomes,  according  to 
the  position  of  its  valves,  either  a  syringe  for  injecting  and  condensing  air, 
or  a  pump  for  exhausting  or  removing  it  from  any  vessel ;  both  opera- 
tions depending  on  the  elasticity  of  air. 

A  barrel  and  piston  is  a  condensing  syringe,  when  in  a  passage  of  com- 
munication between  the  bottom  of  the  syringe  and  a  receiving  vessel,  there 
is  a  flap  or  valve  allowing  air  to  pass  towards  the  receiver  but  not  to  return. 
The  piston,  therefore,  at  each  stroke,  forces  what  the  barrel  contains  of  air 
into  the  receiver.  When  the  piston  is  lifted  again  after  the  stroke,  air 
re-enters  the  barrel  from  the  atmosphere,  either  through  a  valve  in  the 
piston,  or  through  a  small  hole  near  the  top  of  the  barrel.  That  useful 
contrivance,  a  valve,  for  whatever  purpose  used,  and  in  whatever  way  formed, 
is  in  principle  merely  a  moveable  flap,  placed  on  an  opening,  against  which 
it  is  held  by  its  weight,  or  by  some  other  gentle  and  yielding  force.  Such 
a  flap,  it  is  evident,  will  allow  fluid  to  pass  only  in  one  direction,  viz.,  out- 
wards from  the  opening,  for  any  fluid  tending  inwards  must  shut  the  flap, 
and  press  it  the  closer,  the  greater  the  tendency. 

To  convert  a  forcing  syringe  or  pump  into  an  exhausting  syringe  or  pump, 
commonly  called  an  air-pump,  it  is  only  necessary  to  reverse  the  position  of 
the  valves ;  then,  on  the  descent  of  the  piston,  all  the  air  between  it  and  the 
bottom,  instead  of  entering  the  vessel  or  receiver,  as  in  the  last  case,  escapes 

by  a  valve  in  the  piston  itself  towards 
the  atmosphere,  and  on  the  rising  of 
the  piston,  a  perfect  vacuum  would  be 
left  under  it,  but  that  the  valve  below, 
then  opened  by  the  elasticity  of  the  air 
in  the  receiver,  allows  a  part  of  that 
air  to  follow  it.  Thus,  at  each  stroke, 
a  quantity  of  the  air,  proportioned  to 
the  size  of  the  pump,  is  removed  from 
the  receiver.  In  a  good  air-pump, 
there  are  two  similar  pumping  barrels, 
as  a  and  6,  to  quicken  the  operation 
of  exhausting ;  and  both  are  worked 
at  the  same  time  by  the  reciprocating 
winch  or  handle  f}  with  jts  pinion  e, 
acting  on  the  teeth  of  the  piston  rods  d 


Fig.  86. 


160  PNEUMATICS. 

and  c.  This  double  construction  has  the  farther  advantage,  that  the  atmo- 
spheric pressure,  if  fifteen  pounds  per  square  inch  on  the  upper  surface  of 
either  piston,  and  which  for  a  single  piston  would  have  to  be  overcome  by 
the  worker  in  lifting  it,  as  here  balanced  always  by  the  corresponding 
pressure  on  the  other  piston.  Both  pumps  communicate  with  the  tube  g  h, 
which  at  h  rises  tightly  through  the  round  plate  of  the  machine  to  i.  This 
flat  plate  is  so  smooth,  that  a  glass  bell  or  receiver  &,  with  a  smooth 
ground  lip,  when  placed  upon  it,  forms  an  air-tight  joining.  On  working 
the  pump,  such  a  bell  is  exhausted  of  its  air,  and  fitted  for  showing  the 
many  interesting  phenomena  which  the  air-pump  can  display, — and  which 
will  pass  under  review  as  we  proceed.  The  supporting  frame-work  of  the 
pump  is  not  shown  here. 

The  law  of  the  elasticity  of  air  is,  that  its  spring,  or  resistance  to  compres- 
sion, increases  exactly  with  its  density  or  the  quantity  of  it  collected  in  a 
given  space.  Hence,  by  finding  in  any  case  either  the  density  of  the  air, 
or  the  spring,  or  the  compressing  force,  we  know  all  the  three. 

It  has  been  ascertained  by  experiments  described  a  few  pages  farther  on, 
that  in  the  atmospheric  ocean  surrounding  the  earth,  there  are  nearly  fifteen 
pounds  of  air  above  every  square  inch  of  the  surface  of  the  earth  ;  and  that 
the  air  nearest  the  earth,  and  bearing  this  superincumbent  weight  or  pressure, 
has  the  density  of  an  ounce  and  a  quarter  of  weight  to  a  cubic  foot  of  volume. 
We  further  find  that  such  air  is  reduced  to  half  its  bulk,  or  becomes  of  what 
is  called  double  atmospheric  density,  by  an  additional  pressure  of  fifteen 
pounds  on  the  inch,  and  of  triple  density,  by  tripple  pressure,  and  so  forth ; 
and  on  the  other  hand,  that  it  dilates  to  double  bulk,  if  the  pressure  be 
diminished  to  half,  and  to  any  greater  bulk,  even  beyond  a  thousand-fold,  if 
the  pressure  be  diminished  in  a  corresponding  degree ;  and  any  air  bearing 
a  given  force  or  pressure,  is  always  acting  as  a  spring  of  that  force  on 
whatever  it  touches. 

It  is  very  important  to  be  familiar  with  this  truth  or  law,  for  it  holds  very 
nearly  with  respect  to  all  aeriform  fluids  as  well  as  common  air,  and  throws 
light,  therefore,  on  the  action  of  steam-engines,  air-guns,  pneumatic  machines 
generally.  It  also  explains  the  condition  of  our  atmosphere  as  to  density  at 
various  elevations ;  telling  us,  for  instance,  that  when  a  balloon  has  risen 
through  half  of  the  atmospherical  mass,  the  air  around  it  will  be  of  only 
half  the  density  which  exists  at  the  surface  of  the  earth. 

We  know  not  exactly  to  what  extent  the  rarefaction  of  air  may  go  on  the 
removal  of  pressure ;  in  other  words,  at  what  distance  the  gravity  of  the 
particles  becomes  just  a  balance  to  their  mutual  repulsion ;  and  therefore  we 
know  not  exactly  -what  the  degree  of  rarity  is  at  the  top  of  our  atmosphere ; 
but  we  know  that  it  must  be  exceedingly  great,  from  the  fact  that  the  air 
left  in  the  receiver  of  an  air-pump  has  still  spring  or  elasticity  enough  to  lift 
the  valve  of  the  pump,  when  less  remains  than  the  thousandth  part  of  the 
original  quantity.  In  the  most  perfect  air-pumps,  that  the  exhaustion  may 
be  as  complete  as  possible,  the  machine  itself  is  made  to  raise  the  valve. 

The  expansion  of  air  is  well  illustrated  by  a  bladder,  having  a  very  little 
air  in  it,  placed  under  the  receiver  of  an  air-pump.  On  exhausting  the 
receiver,  the  bladder  gradually  swells,  with  force  sufficient  to  lift  a  moderate 
weight  laid  upon  it,  and  at  last  appears  quite  full,  and  may  even  be  burst. 
A  shriveled  apple  treated  in  the  same  way  becomes,plump.  The  explana- 
tion of  such*  phenomena  is,  that  at  first  the  air  in  the  bladder  or  apple  is  in  a 


ELASTICITY    OF    AIR. 


161 


Fig.  87. 


state  of  condensation,  like  all  air,  at  the  surface  of  the  earth  under  the  pres- 
sure of  the  superincumbent  atmosphere;  but  that  its  volume  increases  as  that 
pressure  is  diminished  by  the  air-pump  : — it  is  rarefied  in  the  same  propor- 
tion as  the  air  which  remains  in  the  receiver  surrounding  it. 

The  curious  instrument  called  the  air-guji  has  a  strong  globular  vessel  of 
copper  attached  under  the  lock,  into  which  air  is  usually  forced  to  be  thirty 
or  forty  times  as  dense  as  the  air  in  the  atmosphere  around :  hence  the 
pressure  or  elasticity  tending  outwards  is  thirty  or  forty  times  fifteen  pounds 
on  the  inch,  and  when  the  valve  is  opened  for  an  instant  by  the  action  of 
the  lock,  a  portion  of  the  air  issues  and  propels  the  charge  with  this  force. 
The  effect  of  air  thus  condensed  nearly  equals  that  of  gunpowder,  and  one 
charge  of  the  ball  suffices  for  many  shots,  the  force,  however,  becoming  less 
for  every  successive  discharge. 

If  a  bottle  or  vessel  a  b}  partly  filled  with  water,  have  a 
tube  c  d  passed  tightly  through  the  cork  to  near  the  bottom 
of  the  water ;  and  if  more  air  be  then  forced  through  this 
tube  in  any  way,  so  as  to  accumulate  in  the  upper  part  of  the 
vessel  above  the  water  surface  a  b;  on  turning  the  cock  c, 
which  opens  the  tube,  the  elasticity  of  the  condensed  air 
will  press  the  water  out  as  a  beautiful  jet,  to  a  height  pro- 
portioned to  the  condensation,  and  gradually  diminishing  as 
the  condensation  diminishes.  Or  if  such  a  vessel,  with  air 
of  common  density,  be  placed  under  a  tall  air-pump  receiver, 
on  working  the  pump  so  as  to  diminish  the  density  of  the 
air  in  the  receiver,  the  jet  of  water  will  equally  rise. — A 
table-lamp,,  by  the  force  of  condensed  air,  may  be  supplied 
with  oil  from  a  reservoir  far  below  the  wick :  and  lately  an 
enema  syringe  and  a  shower-bath  have  been  constructed  on 
the  same  principle. 

The  elasticity  of  air  is  rendered  very  serviceable  in  con-- 
nection  with  great  water-pumps,  such  as  those  used  for  the 
supply  of  cities.     A  pump  throws  its  water  by  a  distinct 
gush  at  each  stroke,  while  the  current  through  the  pipe 
towards  the  city  should  be  uniform.     Now  uniformity  is 
attained  by  causing  the  gushes  from  the  pump  a  to  enter  by 
the  passage  b  at  one  side  of  a  large  vessel  c,  of  which  the. 
upper  part  is  full  of  the  condensed  air,  and  from  the  other  side  of  which  at  d  the 
water  issues  on  its  way.     The  air  in  this  vessel 
(called  the  air-vessel)  is  condensed,  as  a  spring,  by 
the  entering  water,  and  its  resisting  elasticity,  both 
immediately,  and  afterwards  during  the  interval  of 
the  strokes,  forces  the  water  along  the  pipe  d.    Each 
entering  gush  has  only  the  effect  of  compressing 
the  air  a  little  more  for  the  time,  while  the  flow  in 
the   great   pipe   continues   nearly  uniform.     The 
pump  itself  is  made  to  take  in  a  little  air  at  each 
stroke,  so  that  not  only  is  the  vessel  always  sup- 
plied, but  some  air  is  constantly  passing  on  with 
the  water,  and  effecting  the  highly  useful  purpose 
of  giving  an  elasticity  to  the  whole  contents  of  the 
pipe  and  its  ramifications. 

The  same  object  is  attained  by  the  same  means  in  the  fire-engine  used  to 
check  conflagration.  In  it  there  are  generally  several  water-pumps  working 

11 


Fig.  88. 


162  PNEUMATICS. 

together,  which  throw  their  interrupted  supply  into  an  air-vessel  whence  it 
passes  in  a  nearly  uniform  jet  to  the  point  desired. 

The  compressibility  and  corresponding  spring  of  air  are  remarkably  exhi- 
bited in  that  singular  contrivance  of  modern  times,  the  diving-bell,  in  which 
men  now  descend  with  safety  to,  considerable  depths  in  the  ocean,  there  to 
reside  and  labor,  attaining  many  objects  of  high  importance  to  them  : — they 
recover  sunken  treasures, — they  are  enabled  to  pursue  works  of  submarine 
architecture,  as  in  constructing  light-houses  and  noble  harbours,  where 
formerly  no  foundations  could  have  been  laid,  &c.  The  diving-bell,  in 
point  of  utility,  has  proved  a  remarkable  contrast  to  its  sister  invention,  the 
balloon,  which,  although  so  wondrously  bearing  man  aloft  to  the  regions  of 
the  clouds,  has  brought  him  as  yet  little  advantage  to  compensate  for  the 
many  fatal  accidents  which  its  use  has  occasioned. 

The  diving-bell  is  a  large  heavy  open-mouthed  vessel,  with  accommoda- 
tion in  it  for  one  or  more  persons.     It  is  let  down  into  the  water  with  its 
mouth  undermost,  from  a  crane  to  which  it  is  suspended,  and  which  rests  .on 
a  suitable  carriage  either  on  the  shore,  or  on  the  deck  of  a  ship,  or  barge 
fitted  for  its  service.     On  first  entering  the  water  it  appears  full  of  air;  but 
air  being  compressible,  according  to  the  law  now  explained,  and  the  pressure 
of  the  water  around  the  descending  bell  increasing  with  the  depth,  the  volume 
of  the  air  gradually  diminishes,  and  at  thirty-four  feet  is  reduced  to  half. 
The  bell,  then,  unless  more  air  be  supplied,  will  of  course  be  half  full  of 
water,  and  a  person  breathing  in  it,  at  each  inspiration  will  receive  twice  as 
much  air  into  the  lungs  as  when  breathing  at  the  surface.     A  constant  supply 
of  fresh  air  is  sent  down  to  the  bell  by  a  forcing-pump  above  :  and  the  heated 
and  contaminated  air,  which  has  served  for  respiration,  and  which  rises  to 
the  top  of  the  bell,  is  allowed  to  escape  by  a  cock  placed  there  for  the  pur- 
pose.    The  men  who  work  at  a  distance  from  the  bell  have  tubes  of  com- 
munication with  it,  by  which  they  inhale  the  air  required  ;  and  they  allow 
the  used  air  to  rise  through  the  water  above  them.     A  man  cannot  breathe 
comfortably  by  such  a  tube  if  he  be  either  much  above  or  much  below  the 
level  of  the  water  in  the  bell ;  for  if  above,  the  air  in  the  bell  is  more  com- 
pressed than  his  chest,  and  is  forced  towards  him  so  as  to  require  an  effort 
to  control  its  admission;    and  if  below,  his  chest  is  bearing  greater  pres- 
sure than  the  air  in  the  bell,  and  he  must  therefore  act  strongly  with  the 
muscles  of  the  ribs  to  draw  the  air  down  to  him.     A  phenomenon  similar 
to  this  takes  place  when  two  bladders  of  air  are  connected  by  a  long  tube, 
and  immersed  in  water  to  unequal  depths,  the  air  being  always  strongly 
forced  from  the  lower  into  the  upper  one,  because  the  lower  one  is  more 
pressed.     The  difficulty  of  pumping  air  down  to  the  diving-bell  increases, 
of  course,  with  the  depth  to  which  it  has  ascended :  for  if  the  bell  be  so  low 
that  the  water  is  pressing  on  the  air  in  it  with  a  force  of  fifteen  pounds  per 
inch,  (  which  would  happen  at  thirty-four  feet, )  it  is  evident  that  a  syringe 
or  pump  can  not  inject  more  air  unless  it  act  with  a  force  greater  than  this. 
Divers  might  often,  if  not  always,  more  conveniently  receive  their  supply 
of  air  through  tubes  from  an  air  vessel  kept  charged  to  the  necessary  density 
in  a  boat  over  them,  or  on  the  shore,  than  from  a  bell  below.     If  they 
would,  moreover,  dress  in  India-rubber  cloth,  and  use  a  hood  of  metal  with 
windows  for  the  head,  they  might  work  under  water  without  wetting  any 
part  but  their  hands. 

It  is  remarkable,  when  the  use  of  the  diving-bell  has  become  so  familiar, 
that  a  kindred  and  still  more  simple  contrivance  of  the  same  class  has  not 
been  introduced  for  certain  purposes,  particularly  of  sudden  emergency,  such 


ELASTICITY    OP    AIR.  163 

as  to  aid  in  the  recovery  of  the  bodies  of  drowning  persons.  A  ten-gallon 
cask,  or  vessel  of  any  kind,  filled  with  air,  and  made  heavy  enough  just  to 
sink  in  water,  with  a  breathing  tube  from  it  like  that  of  a  diving-bell,  would 
be  a  provision  of  air  for  a  man  below  water  for  ten  minutes ;  and  a  man  with 
it  under  his  arm,  might  instantly  descend  from  a  boat  or  walk  from  the  shore, 
into  water  of  any  depth,  to  recover  the  body  of  a  fellow-creature  lately  sunk, 
and  in  time  probably  to  save  the  life,  which  a  few  minutes  wasted  in  waiting 
or  in  unsuccessful  dragging  would  suffer  to  be  lost.  The  author  would  pro- 
pose this  as  an  addition  to  the  apparatus  of  the  Humane  Society  for  the 
recovery  of  persons  apparently  drowned. — It  shows  the  remoteness  from 
common  trains  of  thinking  of  the  truths  connected  with  the  constitution  of 
our  atmosphere  and  sea,  when  a  means  so  simple  and  easily  procured  should 
never  have  been  thought  of  or  tried  in  any  way  by  pearl-fishers,  or  by  per- 
sons who  gain  their  bread  by  diving  to  recover  things  dropped  overboard  in 
harbours  or  anchoring  stations ;  all  of  whom  have  hitherto  been  limited  to 
the  single  gulp  of  air  taken  on  descending.  In  the  case  of  a  man  working 
under  water,  cask  after  cask  of  air  might  be  sent  down,  to  enable  him  to 
remain  as  long  as  necessary. 

There  is  an  exceedingly  beautiful  philosophical  toy,  of  which  the  action 
depends  chiefly  on  the  elasticity  of  air ;  and  as  it  moreover 
illustrates  most  of  the  laws  of  fluidity,  it  is  deemed  worthy  of  Fig.  89. 
description  here.  It  is  a  small  balloon  or  thin  globe  of  glass 
c,  having  an  opening  at  the  bottom,  and  its  little  car  or  basket 
hanging  to  it.  If  put  to  float  in  water  while  the  globe  con- 
tains air  only,  it  is  so  light  that  half  the  globe  remains  above 
the  surface ;  but  water  may  be  introduced  to  adjust  the  specific 
gravity  of  the  whole,  until  it  becomes  only  a  little  less  than 
that  of  water.  If  the  balloon  be  then  placed  in  a  tall  jar  of 
water  a  b,  the  moutn  of  which  is  closely  covered  by  bladder- 
skin  or  India-rubber  tied  upon  it,  on  pressing  such  covering 
with  the  hand,  the  balloon  will  immediately  descend  in  the 
water,  to  rise  again  when  the  pressure  ceases,  and  will  float 
about,  rising,  or  falling,  or  standing  still,  according  to  the 
pressure  made.  The  reason  of  this  is,  that  pressure  on  the 
top  of  the  jar  first  condenses  the  air  between  the  cover  and  the 
water  surface ;  this  pondensation  then  presses  upon  the  water  below,  and  by 
influencing  it  through  its  whole  extent,  compresses  also  the  air  in  the  balloon 
globe,  forcing  as  much  "to ore  water  into  this  as  to  render  the  balloon  heavier 
than  water,  and  therefore  heavy  enough  to  sink.  As  soon  as  the  pressure 
ceases,  the  elasticity  of  the  air  in  the  balloon  repels  the  lately  entered  water, 
and  the  machine,  becoming  as  before,  lighter  than  water,  ascends  to  the  top. 
If  the  balloon  be  adjusted  to  have  a  specific  gravity  too  nearly  that  of  water, 
it  will  not  rise  of  itself  after  once  reaching  the  bottom,  because  the  pressure 
of  the  water  then  above  it  will  perpetuate  the  condensation  of  the  air  which 
caused  it  to  descend.  It  may  even  then,  however,  be  made  to  rise  again  by 
inclining  the  water-jar  to  one  side,  so  that  the  perpendicular  height  of  water 
over  it  shall  be  diminished. 

This  toy  proves  many  things — the  materiality  of  airr  by  the  pressure  of 
the  hand  on  the  top  being  communicated  to  the  water  below  through  the  air 
in  the  upper  part  of  the  j-ar — the  compressibility  of  air,  by  what  happens  in 
the  globe  just  before  it  descends — the  elastic  force  of  air  shown  in  expansion, 
when,  on  the  pressure  ceasing,  the  water  is  again  expelled  from  the  globe — 
the  lightness  of  air;  in  the  buoyancy  of  the  globe  : — -it  shows,  also,  that  in  a 


164 


PNEUMATICS. 


Fig.  90. 


fluid  the  pressure  is  in  all  directions,  because  the  effects  happen  in  whatever 
position  the  jar  be  held — it  shows  that  pressure  is  as  the  depth,  because  less 
pressure  of  the  hand  is  required  the  farther  that  the  globe  has  descended  in 
the  water — and  it  exemplifies  many  circumstances  of  fluid  support.  A 
young  person,  therefore,  familiar  with  this  toy,  has  learned  the  leading 
truths  of  hydrostatics  and  pneumatics,  and  has  had  much  amusement  as 
well  as  instruction. 

On  the  same  principle  as  the  balloon  now  described, 
three  or  four  little  figures  of  men  may  be  formed  of 
glass,  hollow  within,  and  having  each  a  minute  open- 
ing at  the  heel,  by  which  water  may  pass  in  or  out. 
If  these  be  placed  in  a  jar  as  the  balloon  was,  and  be 
adjusted  by  the  quantity  of  water  admitted  into  them, 
so  that  in  specific  gravity,  they  shall  differ  a  little  from 
each  other,  and  if  then,  a  gradually  increased  pressure 
be  made  on  the  cover  of  the  jar,  the  heaviest  figure 
will  descend  first,  and  the  others  will  follow  in  suc- 
cession ,  and  they  will  stop  or  return  to  the  surface 
in  reverse  order  when  the  pressure  ceases.  A  person 
while  exhibiting  these  figures  to  spectators  who  do  not 
understand  them,  may  appear  only  carelessly  to  rest 
his  hand  on  the  cover  of  the  jar  while  he  is  making  the 
required  pressure,  and  he  will  seem  to  have  the  power 
of  ordering  their  movements  by  his  will.  If  the  jar 
containing  the  figures  be  inverted,  and  the  cover  be 
placed  over  a  hole  in  the  table,  through  which,  unob- 
served, the  exhibitor  can  act  by  a  rod  rising  through 
the  hole  and  obeying  his  foot,  he  may  produce  the  most 
amusing  and  surprising  evolutions  among  the  little 
men,  in  perfect  obedience  to  his  word  of  command. 

The  beautiful  fountain,  called  the  fountain  of  Hiero, 
by  which  water  is  made  to  spout  far  above  its  source, 
depends  for  its  action  upon  the  resisting  elasticity 
of  compressed  air.  The  vessel  d  is  first  filled  with 
water,  while  b  and  a  contain  air  only.  On  then 
pouring  water  into  a,  the  water  of  d  darts  upwards 
through  the  jet-pipe  e,to  an  elevation  nearly  equal  to 
the  length  of  the  tube  from  a  to  b.  The  reason  is, 
that  the  water  from  a  descends  by  the  tube  to  b,  and 
compresses  the  air  at  c  ;  which  compression  conveyed 
along  the  other  tube  from  c  to  5,  acts  on  the  water  in 
the  vessel  r/,  and  causes  it  to  jet.  As  the  pressure 
is  produced  by  the  column  of  water  a  b,  the  jet  is 
proportioned  to  the  length  of  that  column. — This 
kind  of  fountain  may  have  its  parts  concealed  under 
a  variety  of  forms  as  here  exemplified,  and  may  thus 
become  a  beautiful  ornament  among  flowers  in  a  sum- 
mer drawing-room.  It  may  be  made  of  size  to  play 
for  an  hour  or  more,  and  it  will  always  recommence 
on  the  water  being  shifted  from  the  low  to  the  high 
reservoir. — The  useful  table-lamp,  consisting  of  a 
simple  column  or  pillar  with  the  oil  rising  to  the 
flame  from  far  below,  is  a  Hiero's  fountain,  only  the 


Fig.  91. 


PRESSURE    AS    THE    DEPTH.  165 

oil,  instead  of  being  allowed  to  jet  out,  rises  in  a  tube  to  the  flame.  The 
contrivance  for  maintaining  the  two  columns  always  of  the  same  length,  not- 
withstanding the  expenditure  of  oil  has  to  be  explained  some  pages  hence. 

Having  now  explained  the  two  peculiarities  which  distinguish  aeriform 
from  other  fluids,  viz.,  their  lightness  and  extensive  elasticity,  we  proceed  to 
show  that  they  have  the  four  other  properties  already  described  under 
hydrostatics,  as  belonging  to  fluids  generally  :  and  first, 

"  Pressure  in  all  directions."     (  Read  the  Analysis,  at  pages  140  and  172.) 

A  quantity  of  air  or  gas  shut  up  in  any  vessel  and  compressed,  is  equally 
affected  throughout,  and  its  tendency  to  escape  from  the  pressure  is  equal  ill 
all  directions,  as  is  proved  by  the  force  necessary  to  keep  similar  valves  close 
wherever  placed.  Hence  the  hydrostatic  press  and  hydrostatic  bellows 
described  in  the  last  section,  which  depend  for  their  action  on  this  law,  may 
be  worked  by  air  or  gas  as  by  a  liquid. 

Owing  to  this  law,  air,  when  allowed,  will  always  rush  from  where  there 
is  more  pressure  to  where  there  is  less.  The  actions  of  the  common  fire- 
bellows,  and  of  the  animal  chest  in  breathing,  blowing,  sucking,  &c.,  are  so 
many  instances. 

The  suddenness  with  which  any  compression  made  on  part  of  a  confined 
aeriform  fluid  is  communicated  through  the  whole,  is  strikingly  seen  in  the 
simultaneous  increase  or  burst  of  all  the  gas-lights  over  an  extensive  build- 
ing or  even  in  a  long  street,  at  any  instant  when  the  force  supplying  the  gaa 
is  augmented. 

Many  very  interesting  illustrations  of  the  fluid  pressure  of  air  being  in, 
all  directions,  will  occur  under  the  next  head,  joined  with  proofs  of  the 
atmospheric  pressure  being  as  the  depth. 

"Pressure  as  the  depth" 

On  first  approaching  this  subject,  a  person  is  naturally  surprised  to  hear 
the  depth  or  height  of  the  atmosphere  spoken  of  as  something  perfectly 
ascertained,  although  nobody  can  ever  have  approached  the  surface  to  mea- 
sure it ;  but  science  often  furnishes  means  of  reaching  precise  truth,  in. 
cases  where  ignorance  would  not'  even  dream  of  the  possibility  of  making 
an  approximation.  It  may  facilitate  the  apprehension  of  this  point  as  regards 
air,  to  describe  first  some  parallel  cases  in  which  water  is  concerned. 

The  bottom  of  a  lake  evidently  supports  all  the  water  in  the  lake,  and 
each  portion  bears  just  the  weight  of  the  water  directly  over  it :  a  means 
then  of  ascertaining  the  weight  or  pressure  of  water  on  any  portion  of  the 
bottom  would  tell  how  much  water  stood  over  that  portion,  and  by  the 
known  rej^,tion  of  the  weight  and  bulk  of  water  would  tell  also  the  depth 
at  that  part.  In  like  manner  the  ocean  of  air  which  surrounds  the  globe 
rests  with  its  whole  weight  upon  the  surface  of  the  globe,  and  each  portion 
of  the  surface  bears  its  share :  if  we  ascertain,  then,  the  pressure  of  the 
atmosphere  on  a  given  extent  of  the  surface,  we  find  how  much  air  is  stand- 
ing directly  over  it;  in  other  words,  the  weight  of  a  column  of  air  resting 
on  such  surface  as  its  base,  and  reaching  to  the  top  of  the  atmosphere. 
Having  then  the  weight  of  the  whole  column,  and  finding  the  weight  of  a 
given  bulk  of  it  at  the  botton  (ascertained  as  described  at  page  158,)  and 
knowing  the  law  of  aerial  elasticity  (explained  at  page  158,  )  we  determine 
the  depth  or  height  of  the  column  by  a  simple  calculation.  Now  accurate 


166  PNEtJM  ATICS. 

experiments  show  that  there  are  nearly  fifteen  pounds  of  air  over  every 
square  inch  of  the  earth's  surface ;  producing  the  same  pressure  as  would 
be  made  by  a  depth  of  water  of  thirty-four  feet,  or  by  a  depth  of  quicksilver 
of  thirty  inches  ;  and  from  this  fact  and  the  ascertained  lightness  and  elasti- 
city of  air,  we  know  that  its  depth  on  earth  must  be  nearly  fifty  miles, 
which,  as  already  stated,  is  about  as  much  in  relation  to  the  size  of  the  garth 
as  the  tenth  of  an  inch  is  to  a  globe  of  one  foot  in  diameter.  The  remaining 
part  of  this  section  has  chiefly  to  trace  the  effects  of  this  mass  of  matter 
resting  upon  the  earth's  surface,  and  as  a  fluid  embracing  and  compressing 
every  object  placed  there. 

Water  is  a  substance  much  more  obvious  to  the  human  senses  than  air, 
and  which  is  constantly  under  observation  ;  yet  many  of  its  most  important 
agencies  escape  the  notice  of  common  observers.  Few  persons,  for  instance, 
of  themselves  discover  the  law  explained  in  the  last  section,  of  the  pressure 
in  water  being  proportioned  to  the  depth  :  but  when  made  to  observe  that  a 
piece  of  cork  plunged  deep  into  it  is  compressed  to  much  smaller  bulk,  and 
that  strong  empty  vessel  of  glass,  or  even  of  metal  under  the  same  circum- 
stances, are  crushed  or  broken  inwards,  and  that  pieces  of  sunken  wood  are, 
at  great  depths,  filled  with  water  through  all  their  pores,  so  as  to  become 
nearly  as  heavy  as  stone,  &c.,  their  minds  are  roused  to  a  sense  of  the  import- 
ant fact  that  a  fluid  presses,  and  in  proportion  to  its  depth.  If  the  truths 
of  hydrostatics  thus  escape  notice,  we  need  not  wonder  that  those  of  pneu- 
matics escape  still  longer. 

If  a  piece  of  bladder-skin  or  a  pane  of  glass  be  laid  at  the  bottom  of  a 
vessel,  holding  water,  the  bladder  or  glass  exhibits  no  sign  of  being  pressed 
upon,  although  it  bears  on  its  upper  side  the  whole  weight  of  the  water 
directly  above  it:  the  reason  being  that  water  beneath  the  bladder  resists 
just  as  strongly  as  the  water  above  presses,  in  the  same  way  that  one  stone 
in  a  pillar  resists  those  above  it :  but  if  the  bladder  be  tied  closely  over  the 
mouth  of  a  common  drinking  glass  or  tumbler  filled  with  water,  and  placed 
at  the  bottom  of  the  vessel,  and  if  then,  by  means  of  a  syringe  or  pump, 
the  water  be  extracted  from  within  the  glass,  the  bladder  itself  has  to  bear 
the  whole  pressure  of  the  water  above  it,  ( independently  of  a  pressure  of 
air,  to  be  explained  afterwards, )  and  will  probably  be  torn  or  burst.  The 
degree  of  pressure,  and  consequently  the  depth  of  the  water,  in  such  a  case, 
might  be  ascertained  by  placing  some  support,  of  which  the  action  could 
be  measured,  under  the  bladder  to  sustain  it  after  the  removal  of  the 
interior  water. — Now  this  case  may  be  closely  copied  in  our  atmosphere  or 
sea  of  air.  A  glass  held  in  the  hand  is  immersed  in  the  fluid  air,  and  is  full 
of  it  as  the  other  glass  was  supposed  full  of  water  :  its  mouth  may  be  covered 
over  with  bladder,  and  no  external  pressure  will  be  apparent,  because  there 
is  a  resistance  of  the  air  within,  just  equal  to  the  pressure  of  the  air  on  the 
outside  : — but  if  the  air  be  extracted  from  under  the  covering  bj^rneans  of 
an  air  pump,  the  bladder  is  first  seen  sinking  down  and  becoming  hollow 
from  the  weight  of  the  air  over  it,  and  at  last  bursting  inwards  with  a  great 
noise  or  crack.  By  placing  a  circular  piece  of  wood  under  the  bladder-skin, 
for  it  to  rest  on,  and  a  spring  of  known  force  to  support  the  wood,  we  may 
ascertain  very  nearly  the  weight  and  pressure  of  the  air  over  it.  This 
mode,  however,  of  ascertaining  the  weight  of  the  atmosphere,  is  not  that 
commonly  used,  but  is  described  here  as  a  good  illustration  of  the  present 
subject;  the  problem  being  solved  much  more  elegantly  and  accurately  by 
means  of  the  barometer  described  farther  on.  The  phenomenon  of  atmos- 
pheric pressure  is  often  exhibited  by  placing  the  hand  on  the  mouth  of  a 


ATMOSPHERIC    PRESSURE    ON    SOLIDS.  167 

glass  so  as  to. cover  it  closely,  and  then  extracting  the  air  from  underneath 
the  hand  :  the  weight  of  the  atmosphere  holds  the  hand  down  on  the  mouth 
of  the  glass  with  the  force  of  fifteen  pounds  to  the  inch. 

As  should  follow,  from  the  pressure  of  fifteen  pounds  per  inch  thus  detected 
at  the  surface  of  the  earth,  being  the  weight  of  our  superincubent  atmo- 
sphere, we  find  that  exactly  as  we  rise  from  the  earth,  and  leave  part  of  the 
atmosphere  beneath  us,  the  pressure  diminishes.  This  fact  now  furnishes 
the  readiest  means  of  ascertaining  the  heights  of  mountains  and  of  balloon 
ascents,  as  will  be  explained  in  considering  the  barometer. 

After  the  many  explanations  here  given  of  fluid  pressure  being  equal  in 
all  directions,  it  is  almost  superfluous  to  remark,  that  the  downward  weight 
of  the  atmosphere  becomes  a  pressure  in  all  directions.  This  is  seen  in  the 
fact  of  the  bladder  seen  above,  being  as  readily  burst  if  turned  sideways 
as  if  turned  directly  upwards.  Every  body  or  substance,  therefore,  on  the 
surface  of  the  earth,  dead  or  living,  solid  or  fluid,  is  compressed  with  this 
force.  In  general  the  pressure  on  one  side  of  a  body,  is  just  balanced  by  the 
equal  pressure  on  the  other,  so  that  no  sensible  effect  follows;  and  it  is  on 
this  account  that  philosophers  were  so  long  in  discovering  it  at  all,  and  that 
half-informed  persons  are  still  disposed  to  doubt  its  existence ;  but  the  proofs 
offered  on  all  sides  to  the  now  awakened  attention  are  irresistible.  We  shall 
speak  first  of 

"  Atmospheric  pressure  on  solids." 

The  atmosphere,  then,  presses  on  the  two  sides  of  a  plate  of  glass  or  metal, 
with  force  of  fifteen  pounds  on  the  inch.  Under  ordinary  circumstances,  no 
sensible  effect  follows,  because  the  opposite  pressures  counterbalance  ;  but  if 
two  plates  of  smooth  glass  or  metal  be  laid  against  each  other,  and  the  air 
be  prevented  from  entering  between  them,  they  cannot  be  separated  by  less 
force  than  fifteen  pounds  per  inch  of  their  surface. 

In  like  manner,  to  draw  down  the  piston  of  a  syringe  from  the  bottom  of 
its  barrel,  while  no  air  is  allowed  to  enter  between  them,  requires  force  of 
fifteen  pounds  to  the  square  inch  of  surface  of  the  piston.  But  if  the 
experiment  be  made  in  the  exhausted  receiver  of  an  air-pump,  the  piston 
falls  by  its  own  weight.  It  is  pushed  back  immediately  on  re-admitting 
the  air.  Wherever  a  vacuum  is  produced  at  the  surface  of  the  earth,  there 
is  an  external  pressure,  of  the  force  stated,  seeking  admittance  all  round. 

An  air-pump  receiver  of  five  inches  diameter  has  nearly  twenty  square 
inches  of  surface  in  its  upper  part  or  roof,  and  bears  a  weight  or  pressure  of 
atmosphere,  of  twenty  times  fifteen,  or  three  hundred  pounds.  WThile  it  has 
air  within  it,  this  pressure  is  exactly  balanced,  and  is  not  sensible ;  but  when 
exhausted  on  the  plate  of  the  air-pump,  it  is  pressed  against  the  plate  with 
this  force.  As  the  atmospheric  pressure  is  in  all  directions,  the  pump-plate, 
of  course,  is  equally  pressed  upwards  against  the  receiver,  and  the  sides  of 
the  receiver  are  pressed  towards  each  other.  This  explains  why  air-pump 
receivers  must  be  made  arched  or  of  dome-shape  to  withstand  the  great 
pressure.  A  flat  piece  of  glass  of  great  thickness,  laid  upon  the  upper 
mouth  of  a  receiver,  so  as  to  form  an  air-tight  cover  to  it,  is  broken 
instantly  by  exhausting  the  air  beneath  ;  and  a  bottle  or  receiver  with  flat 
side,  when  exhausted  suffers  in  the  same  manner. 

Illustrative  of  this  pressure  on  solids  there  is  the  experiment  of  the  Magde- 
burgh  hemispheres,  as  it  is  called.  Two  hollow  half  globes  of  metal  a  and 


168 


PNEUMATICS. 


the  world. 


b,  are  fitted  to  each  other,  so  that  their  lips  wjien  touching 
may  be  air-tight.  While  there  is  air  between  them  or  within, 
resisting  the  pressure  of  the  outward  air,  they  can  be  separated 
from  each  other  without  difficulty ;  but  when  the  air  is 
exhausted  from  within  by  the  air-pump,  a  force  is  required  to 
separate  them  of  as  many  times  fifteen  pounds  as  there  are 
square  inches  in  the  area  of  the  mouth.  The  air  is  extracted 
by  unscrewing  one  of  the  handles  at  b,  and  then  connecting 
the  remaining  stalk  (which  is  hollow  and  has  a  stop-cock) 
with  the  air-pump. — This  experiment  merits  recollection, 
because  it  was  one  of  the  first  which  drew  attention  to  the 
material  nature  and  properties  of  the  air;  and  it  astonished 
Otto  Guericke,  Burgomaster  of  Magdeburgh,  the  inventor,  had 
hemispheres  made  of  three  feet  in  diameter,  and  once  when  he  exhausted 
them,  on  the  occasion  of  a  public  exhibition,  twenty  coach-horses  of  the 
emperor  were  unable  to  pull  them  asunder.  There  being  no  air-pump  when 
Guericke  began  his  experiments,  although  he  himself  invented  it  afterwards, 
he  originally  emptied  the  balls  of  their  air  by  first  filling  them  with  water, 
and  then  extracting  the  water  by  a  common  pump  or  syringe  applied  to  the 
bottom. 

It  is  a  phenomenon  of  the  same  kind  as  the  last  described,  when  a  boy 
with  his  foot  presses  a  circular  piece  of  wet  leather  as  a,  against  a  flat-faced 
stone  as  b,  and  then  lifts  the  stone  by  pulling  at  a  cord  c,  rising  from  the 
centre  of  the  leather.  ,  If  the  leather  be  so  close  in  its  texture  that  air  cannot 
pass  through  it,  and  stiff  enough  not  to  be  puckered  or  drawn  together,  he 
must  exert  a  force  before  detaching  it,  of  as  many  times  fifteen  pounds  as 
there  are  square  inches  of  surface  covered  by  it  for 
such  is  the  weight  or  pressure  of  the  air  over  it,  while 
there  is  no  counterbalancing  pressure  underneath  nearer 
than  on  the  other  side  of  the  stone.  The  weight  of  the 
stone  that  may  be  lifted  is  thus  determined  by  the 
size  of  the  leather.  The  contrivance  has  been  called  a 
sucker,  or  pneumatic  tractor.  A  very  large  sucker 
applie  1  upon  a  rock  or  wall,  would  resist  the  pull  of 
horses  like  the  Magdeburgh  hemispheres. 

This  contrivance  seems  suited  to  some  purposes  of 
surgery.     It  might  assist,  for  instance,  in  raising  de- 
pressed portions  of  a  fractured  skull,  and  might  thus 
sometimes  save  the  operation  of  trepanning : — for  such 
a  purpose  it  would  be  preferable  to  the  small  cupping- 
glass  sometimes  used,  from  its  being  perfectly  inactive, 
except  during  the  instants  when  pulled  at;  whereas  the 
cupping-glass,  by  keeping  up  a  continual  flow  of  blood 
to  the  part,  might  do  injury.     There  is  another  surgical  application  spoken 
of  in  the  last  section  of  the  present  part,  which  the  professional  reader  may 
consult  immediately. 

It  is  from  having  feet  that  act  on  the  principle  of  the  tractor,  that  the 
common  fly  and  other  insects  can  move  along  ceilings,  and  even  polished 
surfaces  of  glass  or  metal  with  their  bodies  hanging  downwards  ;  and  there 
are  many  marine  animals  which  attach  themselves  to  rocks,  or  other  objects 
by  a  similar  action. 

If  two  pneumatic  tractors  be  applied  to  each  other,  men  pulling  opposite 
ways,  to  separate  them,  must  act  with  a  force  of  fifteen  pounds  to  the  square 


Fig.  93. 


ATMOSPHERIC    PRESSURE    ON    LIQUIDS.  169 

inch  of  the  surface  of  contact,  as  if  they  were  separating  the  Madgeburgh 
hemisphere. 

The  case  of  the  pneumatic  tractor  may  be  well  illustrated  by  an  experi- 
ment made  in  a  vessel  containing  a  liquid.  If  a  body  with  a  flat  surface  be 
applied  to  the  bottom  of  the  vessel  so  as  perfectly  to  exclude  the  liquid,  the 
body  bears  the  whole  weight  of  liquid  directly  over  it,  and  cannot  be  detached 
without  force  equal  to  this..  The  case  is  striking  when  a  flat  piece  of  cork  is 
pushed  against  the  smooth  bottom  or  side  of  a  vessel  containing  mercury, 
and  is  found  not  to  rise  again  when  the  hand  is  withdrawn  from  it,  but  to 
be  firmly  held  down  by  the  weight  of  the  mercury.  We  have  to  remark 
that  in  such  experiments  made  in  vessels  open  to  the  air,  the  weight  of  the 
atmosphere  on  the  liquids  adds  a  pressure  of  fifteen  pounds  on  every  inch  of 
the  surface  of  a  body  immersed  in  it. 

"  Atmospheric  pressure  on  liquids" 

The  pressure  of  the  atmosphere  on  liquids  produces  many  important 
effects,  and  now  that  we  comprehend  them,  we  wonder  that  they  should  have 
been  so  long  misunderstood.  We  have  familiar  examples  of  it  in  the  work- 
ing of  pumps  and  syphons.  All  such  phenomena,  in  former  times,  were 
referred  to  what  was  called  nature's  horror  of  a  vacuum,  or  to  an  obscurely 
imagined  principle  of  suction.  It  was  not  until  the  time  of  Galileo  that 
their  true  nature  began  to  be  detected.  The  discovery  has  led  to  many  very 
important  results  in  the  arts. 

Persons  may  at  first  have  a  difficulty  in  conceiving  that  a  fluid  so  rare  and 
subtle  as  air  should  affect  or  resist  u  dense  liquid  like  water  :  but  the  action 
or  resistance  of  air  in  contact  with  water,  is  familiarly  shown  in  the  facts 
that  a  glass  does  not  become  full  of  water  when  plunged,  with  its  open  mouth 
downwards,  from  the  air  into  water;  and  that  when  a  tube,  open  at  bol^i 
ends,  has  been  partially  immersed  in  water,  and,  therefore,  partially  filled, 
the  water  can  be  forced  out  of  it  by  blowing  air  in  at  the  upper  end,  to 
return  only  when  the  blowing  ceases.  Then  it  may  be  recollected  that  a 
hundred  pounds  of  feathers  are  as  great  a  load  as  a  hundred  pounds  of  lead. 

That  there  are  fifteen  pounds  of  air  above  every  square  inch  of  the  earth's 
surface,  is  confirmed  by  the  effects  above  described  of  the  atmospheric  pres- 
sure on  solids;  and  we  now  proceed  to  show  that  many  of  the  phenomena 
among  liquids,  which  long  appeared  so  mysterious,  are  merely  the  necessary 
consequences  of  the  same  pressure  upon  them.  It  will  facilitate  the  com- 
prehension of  these  effects,  if  we  first  view  them  as  they  may  be  produced 
by  more  visible  agents,  viz.,  by  one  liquid  pressing  upon  another ;  and  for 
this  purpose  the  author  has  contrived  the  apparatus  represented  in  the  next 
page,  in  which  a  layer  of  oil  rests  upon  a  layer  of  water,  or  upon  a  layer  of- 
mercury. 

It  has  already  been  shown,  that  an  ocean  of  oil,  spread  over  the  earth,  to 
have  the  same  weight  as  our  atmosphere,  requires  to  be  about  thirty-seven 
feet  .deep  A  vessel,  then,  a  b  c,  with  water  in  it  up  to  the  level  W,  and 
with  thirty-seven  feet  of  oil  above  this,  up  to  the  level  0,  is  fitted  to  illus- 
trate many  of  the  phenomena  of  atmospheric  pressure  on  liquids.  The 
following  are  the  seven  principal  cases. 

1st.  The  weight  of  the  oil  pressing  with  a  force  of  fifteen  Ibs.  per  inch  on 
the  water  at  W,  would  not  at  all  disturb  the  level  surface  of  the  water. 
Neither  does  the  weight  of  the  atmosphere  o*f  fifteen  Ibs.  per  inch  disturb 
any  liquid  surface. 


170 


PNE  UN ATICS  . 


Fig.  94. 


2.  If  the  oil  were  gradually  poured  into 
the  vessel  a  b  c,  over  the  water,  the  water 
would  rise  in  the  tube  i  w,  as  already  ex- 
plained by  the  figure  at  page  143 ;  so  that 
when  there  were  thirty-seven  feet  in  height, 
or  fifteen  pounds  in  weight  of  oil  on  the 
inch,  the  water  i  w  would  stand  thirty-four 
feet  above  its  level  in  the  large  vessel.  If 
these  thirty-four  feet  of  water  were  then 
lifted  out  of  the  tube  by  a  plug  or  piston 
drawn  up  from  the  bottom  of  it  at  i,  a  second 
equal  quantity  would  be  pressed  up  by  the 
oil,  to  be  removed,  if  desired,  in  the  same 
way  as  the  first,  and  the  tube  and  piston 
would  constitute  a  pump.  Now  when  the 
atmosphere,  instead  of  the  oil,  is  allowed 
to  press  upon  a  water  surface  in  such  a 
vessel,  but  is  excluded  from  the  tube,  the 
water  rises  in  the  tube  thirty-four  feet,  as 
in  the  last  case }  and  if  this  quantity  be  lift- 
ed out  of  the  tube  by  a  piston,  a  second  equal 
quantity  is  pressed  up,  and  the  tube  and 
piston  become  a  complete  example  of  the 
common  lifting  or  sucking  pump.  We  have 
to  describe  it  more  particularly  hereafter. 

3d.  If  there  were  a  quantity  of  mercury  or  of  quicksilver  at  the  bottom 
of  the  vessel  ale,  filling  it  up  to  the  level  M,  and  if  a  tube  i  m  issued  from 
under  this  level,  the  mercury  pressed  upon  by  thirty-seven  feet  of  oil  would 
r\se  in  this  short  tube  as  the  water  did  in  the  larger  j  but  by  reason  of  its 
greater  specific  gravity,  it  would  only  reach  a  height  of  thirty  inches  above 
its  level,  the  water  having  stood  at  thirty-four  feet.  Now  thirty  inches  of 
mercury  is  the  height  of  column  which  the  atmospheric  pressure,  acting 
in  the  same  way  really  produces,  as  is  seen  in  a  similar  apparatus  made 
expressly  for  measuring  that  pressure,  and  called  a  barometer  or  measure  ' 
of  weight. 

4th.  If  a  tube  d,  of  an  inch  square  and  open  at  both  ends,  were  plunged 
into  the.  oil,  it  would  of  course  always  be  full  up  to  the  level  of  the  oil  on 
the  outside  of  it ;  and  if  it  were  pushed  low  enough  to  touch  the  water  at 
W,  it  would  just  contain  fifteen  pounds  of  oil  resting  on  an  inch  square  of 
the  water-surface  at  its  mouth  j  which  surface  would  therefore  be  bearing  a 
weight  of  fifteen  pounds  like  every  inch  of  the  surface  around,  but  would 
not  yield,  owing  to  the  force  with  which  it  tended  upwards  to  escape  from 
the  pressure  corresponding  to  its  depth  in  the  oil.  Then  if  the  tube  were 
pushed  a  little  farther  down,  and  if,  by  a  piston  or  plug  in  it,  the  fifteen 
pounds  of  oil  were  lifted  out  of  it,  water  would  rise  into  it  until  enough  had 
entered  to  reproduce  the  pressure  of  fifteen  pounds  on  the  surface  below 
as  before ;  that  is  to  say,  the  water  would  rise  thirty-four  feet,  as  in  the 
external  tube  w  i.  This  internal  tube  and  piston  again  would  form  &pump. 
In  like  manner,  when  a  tube  open  at  both  ends  is  plunged  from  the  air  into 
water,  the  air  presses  on  the  surface  of  the  water  within  the  tube,  as  on  the 
surface  around  it,  with  a  force  of  fifteen  pounds  to  the  inch,  and  the  two 
surfaces  are  not  affected  by  the*  equal  pressures ;  but  if,  by  a  piston,  we  lift 
the  air  out  of  the  tube,  as  we  suppose  the  oil  lifted  in  the  last  experiment, 


ATMOSPHERIC    PRESSURE    Off    LIQUIDS. 


171 


the  »water  will  then  rise,  following  the  piston  to  the  altitude  of  thirty-four 
feet.  This  arrangement  of  parts  is  the  most  usual  for  the  lifting  or  house- 
holt  pump. 

5th.  If  a  common  bottle  or  vessel  of  any  shape,  as  the  bent  tube  e,  were 
filled  with  water,  and  placed  under  the  oil  with  its  mouth  or  mouths  reach- 
ing below  the  water  surface  at  the  level  W,  it  would  remain  full  of  water, 
owing  to  the  pressure  of  the  oil  surrounding  it. — For  a  similar  reason,  any 
such  vessel  or  tube,  surrounded  only  by  air,  when  filled  with  water,  and 
placed  with  its  mouth  or  mouths  under  the  surface  of  water,  remains  full ; 
and  if  such  a  bent  tube  has  one  of  its  ends  in  another  vessel  lower  than  the 
first,  a  current  is  established  in  it; — the  contrivance  being  then  called  a 
syphon. 

6th.  A  fish  in  the  water  below  the  level  W,  would  be  bearing  the  pres- 
sure of  the  oil  from  0  to  W,  as  well  as  the  pressure  of  the  water. — So  a 
fish  in  water  open  to  the  air,  is  bearing  the  atmospheric  pressure  of  fifteen 
pounds  per  inch,  in  addition  to  that  of  the  water  itself.  This  is  proved  by 
extracting  the  air  from  over  water  in  which  a  fish  is  swimming  :  for  then 
the  air-bag  of  the  fish,  situated  near  its  under  side,  as  already  described, 
immediately  dilates  and  turns  the  fish  upon  its  back.  x 

7th.  To  separate  the  Magdeburgh  hemispheres,  or  to  produce  a  vacuum 
in  any  way,  under  the  water  level  W,  would  require  force  proportionate  to 
the  weight  of  oil  above,  in  addition  to  that  required  on  account  of  the 
water ; — and  to  separate  the  Magdeburgh  hemispheres  under  any  water- 
surface  pressed  upon  by  the  atmosphere,  a  force  is  required  of  fifteen  pounds 
per  inch  beyond  what  would  balance  the  effect  of  the  water  itself. 


Fig. 


The  following  remarks  illustrate  more  minutely  some  of  the  objects  which 
we  have  just  been  explaining. 

The  common  lifting-pump  (or  sucking-pump  as  it  used  to  be  called,)  is 
then  merely  a  barrel  a  b,  with  a  close-fitting  moveable  plug  or  piston  in  it  c. 
When  the  lower  end  b}  is  plunged  into  water,  and  the  piston  is  drawn  up 
from  the  bottom,  the  atmosphere  being  prevented  from  pres- 
sing on  the  surface  of  the  water  within  the  tube,  the  pressure 
on  the  surface  external  to  the  tube,  drives  the  water  up 
after  the  piston.  That  the  water  which  thus  rises  ma^y  not 
fall  again,  there  is  a  valve  or  flap  at  the  lower  part  of  the 
pump-barrel  b,  which  opens  only  to  water  passing  upwards ; 
and*  that  the  piston  may  be  allowed  to  pass  downwards 
through  the  water  in  the  barrel,  to  repeat  its  stroke,  there 
is  in  it  a  similar  valve.  The  piston,  in  rising  during  a 
second  or  succeeding  stroke,  causes  all  the  water  above  it  to 
run  over  at  the  spout  d. — Formerly  a  lifting-pump  was 
said  to  act  by  sucking  the  water  up  from  the  well  beneath 
it ;  the  true  meaning  of  which  phrase  we  now  perceive  to 
be,  that  the  piston  merely  lifts  or  holds  off  the  air  which 
was  pressing  on  the  water  within  the  barrel,  and  allows  the 
water  to  rise  in  their  obedience  to  the  external  pressure  of  the 
air  around.  The  reason  is  apparent,  then,  why,  in  the 
lifting  pump,  the  water  will  only  follow  the  piston  to  a 
certain  elevation,  viz.,  until  its  weight  balances  the  external 
pressure  of  the  atmosphere. 


172 


PNEUMATICS. 


Fig.  96. 


When  the  piston  of  a  pump  is  solid,  or  without  a 
valve,  as  at  c,  the  machine  is  called  a  forcing-pump. 
The  water  rises  beneath  the  piston,  as  already  ex- 
plained for  the  lifting-pump,  but  then  as  it  cantiot 
pass  through  the  descending  piston,  as  in  the  lifting- 
pump,  it  is  forced  into  any  other  desired  direction, 
as  to  d.  A  forcing-pump  can  bring  water  from  only 
thirty-four  feet  below  the  piston,  but  can  send  it  to 
any  elevation.  In  forcing-pumps,  it  is  usual  to  make 
the  water  enter  an  air-vessel  d  a  (already  explained 
at  page  161,)  from  which  it  is  again  urged  by  the 
elastic  air,  through  the  pipe  &,  in  a,  nearly  uniform 
stream. 

The  animal  action  of  sucking  is  an  approximation 
to  what  we  have  described  in  the  lifting-pump.  The 
difference  is  that  the  chest  or  mouth  can  make  only 
a  partial  vacuum,  and  therefore  cannot  raise  a  liquid 
very  far. 

A  syphon  remains  full  of  liquid,  although  partially 

raised  above  the  general  surface  of  the  liquid,  as  explained  above.  For  com- 
mon purposes,  a  syphon  is  made  of  the  form  here  represented,  viz.,  a  bent 
tube  c  b  a,  with  one  end  longer  than  the  other.  To  use  it,  the  end  c  is  first 
immersed  in  liquid,  and  the  end  a  being  then  stopped  for  the  time  by  the 
finger  or  a  cock,  the  air  is  extracted  by  the  mouth  or  otherwise,  through  the 
small  tube  a  d,  and  the  atmosphere  immediately  fills  the  whole  tube  with 
liquid  from  c.  If  the  instrument  be  then  left  to  act,  the  liquid  will  run  from 
the  longer  leg,  because  a  long  column  of  liquid  overbalances  a  short  one, 
until  the  shorter  has  drunk  up  all  within  its  reach.  Whether  the  external 

extremity  be  in  the  air  only,  or  immersed 
in  liquid,  makes  no  difference,  except  that 
the  immersion  shortens  so  much  the  de- 
scending column.  If  both  extremities  be 
immersed  in  liquid,  and  in  different  vessels, 
by  alternately  lifting  one  vessel  or  the 
other,  the  liquid  will  be  made  to  pass  and 
repass,  and  will  come  to  rest  in  the  syphon 
only  when  the  surfaces  in  the  two  vessels 
are  at  the  same  level.  Thus  the  same  leg 
becomes  alternately  the  long  and  the  short 
leg,  according  to  the  height  of  the  liquid 
in  which  it  is  immersed.  A  syphon  is 
sometimes  made  with  both  legs  equal  and 
turned  up,  as  here  represented,  so  that  it 
remains  full  of  liquid  although  lifted  away 
from  the  vessel,  and  therefore  is  always 
ready  for  action.  As  it  is  the  same  cause 
which  lifts  the  water  in  a  pump  and  in  a  syphon,  the  top  of  a  syphon  must 
evidently  be  within  thirty-two  feet  of  the  water-surface  below.  In  the  syphon 
as  the  cases  of  balancing  liquids,  described  at  page  131  (which  see,)  the 
comparative  diameters  of  the  legs  are  of  no  importance,  nor  their  oblique 
length,  provided  the  perpendicular  heights  of  the  two  columns  have  the  neces- 
sary relation  : — even  an  inverted  tea-pot  may  be  used  as  a  syphon.  This 
truth  is  well  exemplified  in  what  may  be  called  the  syphon-paradox  an  exact 


PRESSURE    OF    LIQUIDS.  —  SYPHON. 


173 


Fig.  98. 


counterpart  of  the  paradox  of  the  "  Hydrostatic  Bellows," 
already  explained.  If  the  apparatus  of  the  bellows  be  filled 
with  water  in  the  ordinary  way  (see  page  130,)  and  be  then 
reversed  or  turned  so  that  the  tube  becomes  like  the  long  leg  of 
a  syphon,  the  little  stream  of  water  issuing  from  it  at  a  will  lift 
as  great  a  weight  suspended  from  the  board  d,  as  the  same 
slender  column  in  the  standing  position  can  lift  upon  the  board. 
As  farther  illustrative  of  the  atmospherical  pressure  exerted  in 
producing  this  effect,  and  in  rendering  a  syphon  active,  we  m^r 
advert  to  the  striking  fact,  that  a  long  small  tube  of  water 
screwed  into  the  side  or  bottom  of  a  close  cask  of  water  so 
as  to  communicate  with  it,  and  then  allowed  to  discharge  like 
the  long  leg  of  a  syphon,  will  cause  the  cask  to  be  crushed 
inwards,  just  as  the  same  tube  screwed  into  the  top  of  the  cask,  as  repre- 
sented at  page  131,  causes  the  cask  to  burst  outwards. 

The  syphon  is  very  useful  for  drawing  off  liquids,  where  there  is  a  sedi- 
ment that  should  not  be  disturbed,  or  where  it  is  desirable  not  to  make  an 
opening  in  the  lower  part  of  the  vessel.  A  large  syphon  would  empty  a 
lake  or  mill-pond  over  its  bank  without  injuring  the  bank.  To  fill  a  large 
syphon  that  it- may  act  the  most  convenient  way  is,  instead  of  pumping  out 
the  air  from  it,  to  close  the  two  ends  for  the  time,  and  to  pour  in  water 
through  a  cock  at  the  top. 

There  is  a  pretty  syphon-toy,"called  a  Tantalus-cup,  having  in  it  a  stand- 
ing human  figure  which  conceals  a  syphon.  The  short  branch  of  the  syphhon 
rises  in  one  leg  of  the  figure  to  reach  the  level  of  the  chin,  and  the  long 
branch  descends  in  the  other  leg  to  pierce  the  bottom  of  the  cup  towards  a 
reservoir  below.  On  pouring  water  into  the  cup,  the  syphon  begins  to  act 
as  soon  as  the  water  reaches 'the  chin  of  the  figure,  and  the  cup  is  then 
emptied  as  if  by  magic. 

Among  the  infinitely  varied  water-drains  or  courses  in  the  bowels  of  the 
earth,  some  are  syphons,  and  produce  what  are  called  intermitting  wells  or 
fountains.  These  may  alternately  run  and  cease  for  longer  or  shorter 
periods,  according  to  the  comparative  magnitudes  of  the  collecting  reservoir 
and  the'  drain.  The  reservoir  may  be  an  internal  cave  of  a  mountain, 
receiving  a  regular  supply  of  water  by  a  slow  filtering  of  moisture  from 
above,  and  the  drain  is  a  syphon-formed  channel,  which^  like  that  of  the 
Tantalus-cup,  begins  to  act  only  when  the  water  in  the  reservoir  has  reached 
the  level  of  the  top  of  the  syphon,  and  then  carries  off  the  water  faster 
than  it  is  supplied.  There  are  some  fountains  that  flow  constantly,  but  at 
regular  intervals  have  a  remarkable  increase.  In  them  a  common  spring  is 
joined  with  a  syphon-spring. 

The  author  has  suggested  an  application  of  the  syphon,  which  obviates  a 
strong  objection  to  the  high  operation  for  stone,  as  explained  in  the  next 
medical  section. 

The  following  facts  have  close  relation  to  those  now  explained,  as  farther 
illustrative  of  atmospheric  pressure  on  liquids. 

A  long  glass  of  jelly,  if  inverted  and  placed  with  its  mouth  just  under 
the  surface  of  warm  water,  will  soon  be  found  to  have  lost  the  jelly,  but  to 
be  full  of  water  in  its  stead.  The  jelly  is  heavier  than  water,  and  when 
melted  by  the  heat  sinks  down,  and  is  replaced  by  water  from  below,  sent 
up  by  the  atmospheric  pressure. 


174  PNEUMATICS. 

The  slaves  in  the  West  Indies  steal  rum,  by  inserting  the  long  neck  of  a 
bottle  full  of  water  through  the  top  aperture  of  the  rum-cask.     The  water 
falls  out  of  the  bottle  into  the  cask,  while  the  lighter  rum  ascends  in  its  stead. 
The  common  water-glass  for  bird-cages  has  its  only  opening  near  the  bot- 
tom through  the  neck  b ;  yet  no  water. can  escape 
from  it  but  when  the  level  of  the  water  at  c,  in  the 
open  part,  becomes  low  enough  for  some  air  to  pass 
into  the  body  of  the  glass  by  the  channel  b.  When 
a  bubble  of  air  does  pass  in,  an  equal  bulk  of  water 
comes  out,  and  by  raising  the  water  level  in  c,  pre- 
vents the  passage  of  more. — An  ink-glass  made  on 
this  principle  preserves  the  ink  well,  because  there 
is  so  small  a  surface  exposed  to  the  air;  if  made  too 
,         .        large,  however,  the  accidental  expansion  of  the  air 
O       ]C       in  it  by  heat  may  cause  it  to  overflow. 

In  the  common  Argand  or  fountain-lamp,  the 

provision  of  oil  is  in  a  vessel  like  an  inverted  bottle,  higher  than  the  flame, 
and  with  its  mouth  immersed  in  a  small  reservoir  of  oil,  nearly  on  a  level 
with  the  flame,  then  no  oil  can  escape  from  above,  but  as  the  flame  consumes 
the  free  oil  from  the'  small  reservoir,  which  supply  is  thus  maintained 
always  at  the  same  elevation. — In  the  Hiero's  fountain  lamp,  mentioned  at 
page  164,  that  the  two  balancing  columns  of  oil  may  be  always  of  the  same 
height,  the  oil  is  suppplied  to  them  from  high  reservoirs,  with  the  mouths 
dipping  in  them  as  above  described,  and  keeping  their  tops,  therefore, 
always  at  the  same  level;  and  that  the  descending  column  may  not  be 
shortened  by  the  rising  of  the  oil  in  the  low  reservoir  c.  the  tube  containing 
it  is  turned  up  at  the  bottom  like  an  end  of  the  "  ever  ready  syphon/7  and 
discharges  near  the  top  of  c. 

We  have  hitherto  been  contemplating  only  the  direct  weight  or  downward 
pressure  of  the  atmosphere  on  liquids  :  in  the  following  instances  we  have 
proof  of  the  same  pressure  acting  upon  them  in  all  directions. 

If  a  bottle  or  cask  be  filled  with  liquid,  and  closely  corked,  and  if  a  small 
hole  be  then  drilled  in  the  bottom  or  side,  the  liquid  will  not  escape  by  it, 
because  of  the  existing  pressure  of  the  atmosphere,  and  of  there  not  being 
room  in  the  opening  for  a  current  of  air  to  enter  while  the  current  of  water 
escapes :  but  if  a  second  hole  be  drilled  in  the  top,  a  jet  from  the  lower 
opening  will  follow  immediately,  because  then  the  air  will  press  on  the  upper 
surface  of  the  liquid  as  well  as  on  the  lower,  and  the  weight  of  the  liquid 
will  be  free  to  act : — thus,  a  cask,  of  beer  or  wine  cannot  be  emptied  by  a 
cock  near  the  bottom,  unless  what  is  called  a  vent-hole  be  made  at  the  top. 
If  the  lower  opening,  however,  in  any  case  be  so  large,  that  the  air  may 
enter  by  one  side  of  it,  as  is  seen  in  decanting  a  bottle  of  wine.  In  such  a 
case,  it  is  the  interrupted  entrance  of  the  air  which  causes  that  gurgling 
sound  so  delightful  to  the  ear  of  the  drunkard,  instead  of  allowing  the 
smooth  stream  which  fall  from  a  funnel. 

Even  a  large  opening  at  the  bottom  of  a  vessel  which  is  close  above,  may 
be  prevented  by  the  pressure  of  the  air,  from  discharging  liquid  if  any 
mutual  passing  of  the  two  currents  of  air  and  liquid  be  rendered  difficult. 
An  inverted  bottle  of  water  will  not  discharge,  if  a  piece  of  paper  simply  be 
applied  against  its  mouth.  Even  a  wineglass  filled  with  water  may  be 


ATMOSPHERIC    PRESSURE.  —  BAROMETER.       175 

inverted,  and  yet  will  spill  none,  if  the  piece  of  paper,  laid  loosely  upon  its 
mouth,  be  held  to  it  during  the  turning, — the  pressure  of  the  atmosphere 
against  the  paper  keeping  it  in  its  place,  and  supporting  the  water  above  it. 
Any  vessel  or  tube  of  water  of  less  height  than  thirty-four  feet  may  be  kept 
closed  at  the  bottom  in  the  same  way. 

The  animal  body  is  made  up  of  solids  and  fluids,  and  is  affected  by  the 
atmospheric  pressure  accordingly. 

There  is  a  difficulty  at  first  in  believing  that  a  man's  body  should  be 
bearing  a  pressure  of  fifteen  pounds  on  every  square  inch  of  its  surface,  while 
he  remains  altogether  insensible  of  it :  but  such  is  the  fact,  and  the  reason  of 
his  not  feeling  the  fluid  pressure  is  its  being  perfectly  uniform  all  around. 
If  a  pressure  of  the  same  kind  be  even  many  times  greater,  such,  for  instance, 
as  fishes  bear  in  deep  water,  or  as  a  man  supports  in  the  diving-bell,  it  equally 
passes  unnoticed.  Fishes  are  at  their  ease  in  a  depth  of  water  where  the 
pressure  around  will  instantly  break  or  burst  inwards  almost  the  strongest 
empty  vessel  that  can  be  sent  down;  and  men  walk  on  earth  without  dis- 
covering a  heavy  atmosphere  about  them,  which,  however,  instantly  crushes 
together  the  sides  of  a  square  glass  bottle  emptied  by  the  air-pump,  or  even 
of  a  thick  iron  boiler,  left  for  a  moment  by  any  accident,  without  the  coun- 
teracting internal  support  of  steam  or  air. 

The  fluid  pressure  on  animal  bodies,  thus  unperceived  under  ordinary  cir- 
cumstances, may  be  rendered  instantly  sensible  by  a  little  artificial  arrange- 
ment. In  water,  an  open  tube  partialy  immersed  becomes  full  to  the  level 
of  the  water  around  it,  and  the  water  contained  in  it  is  supported,  as  already 
explained,  by  that  which  is  immediately  below  its  mouth  :  now  a  flat  fish 
resting  closely  against  the  mouth  of  the  tube,  would  evidently  be  bearing  on 
its  back  the  whole  of  this  weight,  perhaps  one  hundred  pounds;  but  the  fish 
would  not  thereby  be  pushed  away,  nor  would  it  even  feel  its  burden,  because 
the  upward  pressure  of  the  water  immediately  under  it  would  just  counter- 
balance the  weight,  while  the  lateral  pressure  around  would  prevent  any 
crushing  effect  of  the  upward  and  downward  forces.  But  if,  while  the  fish 
continued  in  the  situation  supposed,  the  hundred  pounds  of  water  were  sud- 
denly lifted  from  off  its  back  by  a  piston  in  the  tube,  the  opposite  upward 
pressure  of  one  hundred  pounds  would  at  once  crush  its  body  into  the  tube. 
At  a  less  depth,  or  with  a  smaller  tube,  the  effect  might  not  be  fatal,  but 
there  would  be  a  bulging  or  swelling  of  the  substance  of  the  fish  into  the 
mouth  of  the  tube. — In  air  and  on  the  human  body  a  perfectly  analogous 
case  is  exhibited.  A  man  without  pain  or  any  peculiar  sensation,  applies 
his  hand  closely  to  the  mouth  or  opening  of  a  tube,  or  of  any  vessel  contain- 
ing air,  but  the  instant  that  the  air  is  withdrawn  from  within  the  tube  or 
vessel,  the  then  unresisted  pressure  of  the  external  air  fixes  the  hand  upon 
the  opening,  causes  the  flesh  to  swell  or  bulge  into  it,  and  makes  the  blood 
ooze  from  any  crack  or  puncture  in  the  skin. 

These  last  lines  describe  closely  the  surgical  operation  of  cupping  ;  the 
essential  circumstances  of  which  are,  the  application  of  a  cup  or  glass,  with  a 
smooth  blunt  lip,  to  the  skin  of  any  part  of  the  body,  and  the  extraction  by  a 
syringe,  or  other  means,  of  a  portion  of  the  air  from  within  the  cup.  To 
some  minds  the  exact  comprehension  of  this  phenomenon  may  be  facilitated, 
by  considering  the  case  of  a  small  bladder  or  bag  of  India-rubber  full  of  any 
fluid  and  pressed  between  the  hands  on  every  part  of  its  surface  except  one : — 
at  that  one  part  it  would  swell,  and  even  burst  if  the  pressure  were  strong 
enough.  So  in  cupping,  the  whole  body,  except  the  surface  under  the  cup, 


176  PNEUMATICS. 

is  squeezed  by  the  atmosphere,  with  a  force  of  fifteen  pounds  to  the  square 
inch,  while  in  that  one  situation  the  pressure  is  diminished  according  to  the 
degree  of  exhaustion  in  the  cup,  and  the  blood  consequently  accumulates 
there.  The  application  of  a  cup  with  exhaustion  only,  constitutes  the  opera- 
tion called  dry-cupping.  To  obtain  blood,  the  cup  is  removed  and  the  tumid 
part  is  cut  into  by  the  simultaneous  stroke  of  a  number  of  united  lancets : 
and  the  cup  is  then  applied  again  as  before  and  exhausted,  so  that  the  blood 
may  rush  forth  under  the  diminished  pressure. 

The  partial  vacuum  in  the  cup  may  be  produced  either  by  the  action  of  a 
syringe,  or  by  burning  a  little  spirit  in  the  cup  and  applying  it  while  the 
momentary  dilation  effected  by  the  heat  has  driven  out  the  greater  part  of 
the  air.  The  human  mouth  applied  upon  any  part  becomes  a  small  cupping 
apparatus,  and  formerly,  in  cases  of  poisoned  wounds,  was  used  as  such. 
Our  present  perfect  cupping  glasses,  of  stronger  and  more  permanent  opera- 
tion, are  not  yet  always  used,  as  they  might  be,  to  assist  in  removing  the 
poison  after  the  bites  of  rabid  or  venomous  animals. 

The  author  has  suggested  an  extension  and  modification  of  the  operation 
of  the  dry  cupping,  which  he  believes  will  prove  an  important  remedy  in  the 
hands  of  the  medical  practitioner.  It  is  intended  as  a  substitute  for  bleeding 
in  certain  cases  where  blood  can  ill  be  spared,  and  as  a  more  sudden  and 
effectual  check  than  even  bleeding  itself,  in  certain  cases  of  inflammatory 
disease.  It  is  explained  in  the  next  medical  section  of  this  work. 

The  atmospheric  pressure  on  living  bodies  produces  an  effect  which  is 
rarely  thought  of,  although  of  much  importance,  viz.,  keeping  all  the  parts 
about  the  joints,  firmly  together,  by  an  action  similar  to  that  exerted  on  the 
Magdeburgh  hemispheres.  The  broad  surfaces  of  bone  forming  the  knee- 
joint,  for  instance,  even  if  not  held  together  by  ligaments,  could  not,  while 
the  capsule  surrounding  the  joint  remained  air-tight,  be  separated  by  a  force 
of  less  than  about  a  hundred  pounds ;  but  on  air  being  admitted  to  the 
articular  cavity,  the  bones  at  once  fall  to  a  certain  distance  apart.  In  the 
loose  joint  of  the  shoulder,  this  support  is  of  great  consequence.  When  the 
shoulder  or  other  joint  is  dislocated,  there  is  no  empty  space  left,  as  might 
be  supposed,  but  the  soft  parts  around  are  pressed  in  to  fill  up  the  natural 
place  of  the  bone.  When  a  thigh  bone  is  dislocated,  the  deep  socket  called 
thee  actabulum  instantly  becomes  like  a  cupping- glass,*  and  is  filled  partly  with 
fluid  and  partly  with  the  soft  solids.  In  all  joints,  it  is  the  atmospheric 
pressure  which  keeps  the  bones  in  such  steady  contact,  that  they  work 
smoothly  and  without  noise. 

The  barometer  we  have  seen  at  page  170,  is  a  column  of  fluid  supported 
in  a  tube  by  the  pressure  of  the  atmosphere,  and  therefore  indicating  most 
exactly  the  degree  of  that  pressure.  It  is  an  instrument  now  of  such  import- 
ance, both  in  a  scientific  point  of  view  and  in  the  business  of  common  life, 
that  for  the  sake  of  minds  which  conceive  such  subjects  with  difficulty  we 
shall  add  here  the  two  following  farther  illustrations  of  its  nature. 

If  mercury  be  poured  into  a  bent  tube  open  at  both  ends,  it  will  stand  at 
the  same  level  in  the  two  legs,  as  at  a  and  6,  and  the  air  will  be  pressing  on 
the  two  surfaces  at  a  and  b  with  equal  force  of  15  Ibs.  per  square  inch.  If  the 
air  be  then  removed  from  one  leg  a,  by  a  piston  or  otherwise,  while  it  continues 
to  press  in  the  other  leg  b}  the  mercury  will  be  pushed  down  in  b,  until  the 
growing  height  of  a  column  in  a  produces  a  weight  so  much  greater  than 
that  in  6,  as  just  to  counteract  the  pressure  :  now  this  balance  takes  place,  in 
fact,  when  the  mercury  in  a  stands  about  thirty  inches  higher  than  in  b;  that 
being  the  height  of  a  column  of  mercury  weighing  15  Ibs  on  the  square  inch. 


ATMOSPHERIC    PRESSURE.  —  BAROMETERS. 


177 


Fig.  100. 


cl 


If  the  top  of  the  tube  a  were  then  closed  permanently,  the  mer- 
cury would  for  ever  remain  elevated  in  it,  marking  most  perfectly 
the  atmospheric  pressure;  now  this  construction,  only  with  the 
empty  and  useless  part  of  the  tube  above  d  cut  off  or  wanting, 
forms  a  common  barometer.  The  exact  altitude  of  the  mercury 
in  it  is  known  by  observing  how  much  the  surface  near  c  is 
higher  than  that  near  d.  Often,  in  such  a  barometer,  a  little  mass 
of  metal  is  placed  to  float  on  the  mercurial  surface  at  d,  and  as 
it  rises  and  falls,  is  caused,  by  a  thread  passing  from  it  over  a 
wheel  or  pulley,  to  move  an  index  like  the  hand  of  a  clock  con- 
nected with  the  wheel,  and  this  index  tells  the  degree  of  eleva- 
tion. This  modification  is  called  the  wheel  barometer. 

Again,  as  water  at  a,  in  the  bottom  of  a  closed  pump-barrel,  if 
pressed  upon  by  the  piston  b  c,  of  which  the  rod  d  were  hollow 
or  tubular,  would  rise  in  the  rod  to  a  height  proportioned  to  the  pressure  made 
by  the  piston,  so,  in  a  straight  exhausted  barometer-tube,  which  is  as  this 
hollow  piston-rod,  the  mercury  or  water  rises,  because  the  atmospheric  pres- 
sure around  it  is  as  the  piston  forcing  the  fluid  up.      To  make  a  barometer 
of  this  kind  it  is  only  necessary  to  procure  a  glass  tube  more  than  thirty  inches 
long,  and  close  at  one  end,  and  then  having  filled  it  with 
mercury,  to  plunge  its  mouth   (stopped  by  the  finger 
while  turning)  into  a  small  cup  or  basin  of  mercury; — • 
the  fluid  falls  away  a  little  from  the  top  of  t]ie  tube, 
leaving  a  vacuum   there,  and  stands  at  the  elevation 
which   the  atmospheric   pressure  is  fitted  to  maintain. 
We  know  from  the  law  of  hydrostatics  already  explained 
that  it  is  of  no  importance,  in  such  a  case,  what  the  shape, 
or  inclination,  or  size  of  the  tube  may  be,  as  only  the 
perpendicular  height  can  measure  or  be  measured  by  the 
pressure.    This  fact  enables  us  to  construct  barometers 
with  the  upper  part  of  the  tube  bent  obliquely,  PO  that 
for  one  inch  rise  of  mercury  in  a  perpendicular  tube, 
there   shall  be  an  advance  of  several  inches  in  the  oblique  top,  rendering 
any  change  of  elevation  so  much  more  apparent. 

G-alileo  had  found  that  water  would  rise  under  the  piston  of  a  pump  to  a 
height  only  of  about  thirty-four  feet.  His  pupil  Torricelli,  conceiving  the 
happy  thought,  that  the  weight  of  the  atmosphere  might  be  the  cause  of  the 
ascent,  concluded  that  mercury,  which  is  about  thirteen  times  heavier  than 
water,  should  only  rise  under  the  same  influence  to  a  thirteenth  of  the  eleva- 
tion ; — he  tried  and  found  that  this  was  so,  and  the  mercurial  barometer  was 
invented.  Pascal  then,  to  afford  farther  evidence  that  the  weight  of  the 
atmosphere  was  the  cause  of  the  phenomena,  carried  the  tube  of  mercury  to 
the  tops  of  buildings  and  of  mountains,  and  found  that  it  fell  always  in 
exact  proportion  to  the  portion  of  the  atmosphere  left  below  it; — and  he 
found  that  water-pumps  in  different  situations  varied  as  to  sucking  power, 
according  to  the  same  law. 

It  was  soon  afterwards  discovered,  by  careful  observation  of  the  mercurial 
barometer,  that  even  when  remaining  in  -the  same  place,  it  did  not  always 
stand  at  the  same  elevation  ;  in  other  words,  that  the  weight  of  the  atmosphere 
over  any  particular  part  of  the  earth  was  constantly  fluctuating ;  a  truth  which 
without  the  barometer  could  never  have  been  suspected.  The  observation 
of  the  instrument  being  carried  still  farther,  it  was  found,  that  in  serene  dry 
weather  the  mercury  generally  stood  high,  and  that  before  and  during  storms 


Fig. 

101. 
<l 

L 

c 

178  PNEUMATICS. 

and  rain  it  fell :-— The  instrument,  therefore,  might  serve  as  a  prophet  of  the 
weather,  becoming  a  precious  monitor  to  the  husbandman  or  the  sailor. 

The  reasons  why  the  barometer  falls  before  wind  and  rain  will  be  better 
understood  a  few  pages  hence  ;  but  we  may  remark  here,  that  when  water 
which  has  been  suspended  in  the  atmosphere,  and  has  formed  a  part  of  it, 
separates  as  rain,  the  weight  and  bulk  of  the  mass  are  diminished:  and  that 
wind  must  occur  when  a  sudden  condensation  of  aeriform  matter,  in  any 
situation,  disturbs  the  equilibrium  of  the  air;  for  the  air  around  will  rush 
towards  the  situation  of  diminished  pressure. 

To  the  husbandman  the  barometer  is  of  considerable  use,  by  aiding  and  cor- 
recting the  prognostics  of  the  weather  which  he  draws  from  local  signs  familiar 
to  him;  but  its  great  use  as  a  weather-glass  seems  to  be  to  the  mariner  who 
roams  over  the  whole  ocean,  and  is  often  under  skies  and  climates  altogether 
new  to  him.  The  watchful  captain  of  the  present  day,  trusting  to  this  extra- 
ordinary monitor,  is  frequently  enabled  to  take  in  sail  and  to  make  ready  for 
the  storm,  where,  in  former  times,  the  dreadful  visitation  would  have  fallen 
upon  him  unprepared.  The  marine  barometer  has  not  yet  been  in  general 
use  for  many  years,  and  the  author  of  this  work  was  one  of  a  numerous  crew 
who  probably  owed  their  preservation  to  its  almost  miraculous  warning.  It 
was  in  a  southern  latitude ;  the  sun  had  just  set  with  placid  appearance,  closing 
a  beautiful  afternoon,  and  the  usual  mirth  of  the  evening  watch  was  proceed- 
ing, when  the  captain's  order  came  to  prepare  with  all  haste  for  a  storm.  The 
barometer  had  began  to  fall  with  appalling  rapidity.  As  yet,  the  oldest  sailors 
had  not  perceived  even  a  threatening  in  the  sky,  and  were  surprised  at  the 
extent  and  hurry  of  the  preparations  :  but  the  required  measures  were  not 
completed,  when  a  more  awful  hurricane  burst  upon  them  than  the  most 
experienced  had  ever  braved.  Nothing  could  withstand  it;  the  sails  already 
furled  and  closely  bound  to  the  yards,  were  riven  away  in  tatters  :  even  the 
yards  and  masts  themselves  were  in  great  part  disabled  ;  and  at  one  time  the 
whole  had  nearly  fallen  by  the  board.  Such,  for  a  few  hours,  was  the  rnin 
gled  roar  of  the  hurricane  among  the  rigging,  of  the  waves  around,  and  of 
the  incessant  peals  of  thunder,  that  no  human  voice  could  be  heard,  and 
amidst  the  general  consternation,  even  the  trumpet  sounded  in  vain.  In 
that  awful  night,  but  for  the  little  tube  of  mercury  which  had  given  the 
warning,  neither  the  strength  of  the  noble  ship,  nor  the  skill  and  energies 
of  the  commander,  could  have  availed  anything,  and  not  a  man  would  have 
escaped  to  tell  the  tale.  On  the  following  morning  the  wind  was  again  at 
rest,  but  the  ship  lay  upon  the  yet  heaving  waves,  an  unsightly  wreck. 

The  marine  barometer  differs  from  that  used  on  shore,  in  having  its  tube 
contracted  in  one  place  to  a  very  narrow  bore,  so  as  to  prevent  that  sudden 
rising  and  falling  of  the  mercury  which  every  motion  of  the  ship  would  else 
occasion. 

Civilized  Europe  is  now  familiar  with  the  barometer  and  its  uses,  and 
therefore,  that  Europeans  may  conceive  the  first  feelings  connected  with  it, 
they  almost  reouire  to  witness  the  astonishment  or  incredulity  with  which 
people  of  other  countries  still  regard  it.  A  Chinese  once  conversing  on  the 
subject  with  the  author,  could  only  imagine  of  the  barometer,  that  it  was  a 
gift  of  miraculous  nature,  which  fche  God  of  Christians  gave  them  in  pity, 
to  direct  them  in  the  long  and  perilous  voyages  which  they  undertook  to 
unknown  seas. 

A  barometer  is  of  great  use  to  persons  employed  about  those  mines  in 
which  hydrogen  gas,  or  fire-damp,  is  generated  and  exists  in  the  crevices. 
When  the  atmosphere  becomes  unusually  light,  the  hydrogen  being  relieved 


ATMOSPHERIC    PRESSURE. — BAROMETERS.        179 

from  a  part  of  the  pressure  which  ordinarily  confines  it  to  its  holes  and 
lurking-places,  expands  or  issues  forth  to  where  it  may  meet  the  lamp  of 
the  miner,  and  explode  to  his  destruction.  In  heavy  states  of  the  atmos- 
phere, on  the  contrary,  it  is  pressed  back  to  its  hiding-places,  and  the  miner 
advances  with  safety. 

We  see  from  this,  that  any  reservoir  or  vessel  containing  air  would  itself 
answer  as  a  barometer  if  the  only  opening  to  it  were  through  a  long  tubular 
neck,  containing  a  close-sliding  plug ;  for  then,  according  to  the  weight  and 
pressure  of  the  external  air,  the  density  of  that  in  the  vessel  would  vary,  and 
all  changes  would  be  marked  by  the  position  of  the  movable  plug.  A  baro- 
meter has  really  been  made  on  this  principle,  by  using  a  vessel  of  glass,  with 
a  long  slender  neck,  in  which  a  globule  of  mercury  is  the  movable  plug. 

The  state  of  the  atmosphere,  as  to  weight,  differs  at  different  times  in  the 
same  situation,  so  as  to  produce  a  range  of  about  three  inches  in  the  height 
of  the  mercurial  barometer;  that  is  to  say,  from  twenty-eight  to  thirty-one 
inches.  On  the  occasion  of  the  great  Lisbon  earthquake,  however,  the  mer- 
cury fell  so  far  in  the  barometers,  even  in  Britain,  as  to  disappear  from  that 
portion  at  the  top  usually  left  uncovered  for  observation.  The  uncovered 
part  of  a  barometer  is  commonly  of  five  or  six  inches  in  length,  with  a 
divided  scale  attached  to  it,  on  which  the  figures  28,  29,  &c.,  indicate  the 
number  of  inches  from  the  surface  of  the  mercury  at  the  bottom  to  the 
respective  divisions : — on  the  lower  part  of  the  scale  the  words  wind  and 
rain  are  generally  written,  meaning  that  when  the  mercury  sinks  to  them, 
wind  and  rain  are  to  be  expected ;  and  on  the  upper  part,  dry  and  fine 
appear,  for  a  corresponding  reason  :  but  we  have  to  recollect,  that  it  is  not 
the  absolute  height  of  the  mercury  which  indicates  the  existing  or  coming 
weather,  but  the  recent  change  in  its  height ;  a  falling  barometer  usually 
telling  of  wind  and  rain ;  a  rising  one  of  serene  and  dry  weather. 

The  barometer  answers  another  important  purpose,  besides  that  of  a  weather- 
glass— in  enabling  us  to  ascertain  readily  the  height  of  mountains,  or  of 
any  situation  to  which  it  can  be  carried. 

As  the  mercurial  column  in  the  barometer  is  always  an  exact  indication  of 
the  weight  or  pressure  of  air  above  its  level,  being,  indeed,  as  explained  in 
the  foregoing  paragraphs,  of  the  same  weight  as  a  column  of  the  air  of  equal 
base  with  itself,  and  reaching  from  it  to  the  top  of  the  atmosphere-— the 
niercury  must  fall  when  the  instrument  is  carried  from  any  lower  to  any 
higher  situation,  and  the  degree  of  falling  must  always  tell  exactly  how  much 
air  has  been  left  below.  For  instance,  if  thirty  inches  barometrical  height 
mark  the  whole  atmospheric  pressure  at  the  surface  of  the  ocean,  and  if  the 
instrument  be  found,  when  carried  to  some  other  situation,  to  stand  at  only 
twenty  inches,  it  proves  that  one-third  of  the  atmosphere  exists  below  the 
level  of  the  new  situation.  If  our  atmospheric  ocean  were  of  as  uniform 
density  all  the  way  up  as  our  watery  oceans,  a  certain  weight  of  air  thus  left 
behind  in  ascending  would  mark  everywhere  a  change  of  level  nearly  equal, 
and  the  ascertaining  any  height  by  the  barometer  would  become  one  of  the 
most  simple  of  calculations :  the  air  at  the  surface  of  the  earth,  being  between 
eleven  and  twelve  thousand  times  lighter  than  its  bulk  of  mercury,  an  inch 
rise  or  fall  of  the  barometer  would  mark  everywhere  a  rise  or  fall  in  the 
atmosphere  of  nearly  twelve  thousand  inches  or  one  thousand  feet.  But 
owing  to  the  elasticity  of  air,  which  causes  it  to  increase  in  volume  as  it 
escapes  from  pressure,  the  atmosphere  is  rarer  in  proportion  as  we  ascend, 


180  PNEUMATICS. 

so  that  to  leave  a  given  weight  of  it  behind,  the  ascent  must  be  greater,  the 
higher  the  situation  where  the  experiment  is  made :  the  rule,  therefore,  of 
one  inch  of  mercury  for  a  thousand  feet,  holds  only  for  rough  estimates  near 
the  surface  of  the  earth.  The  precise  calculation,  however,  for  any  case,  is 
still  very  easy ;  and  a  good  barometer,  with  a  thermometer  attached,  and 
with  tables,  or  an  algebraical  formula,  expressing  all  the  influencing  circum- 
stances, enables  us  to  ascertain  elevations  much  more  easily,  and  in  many 
cases  more  correctly,  than  by  trigonometrical  survey. 

The  weight  of  the  whole  atmospherical  ocean  surrounding  the  earth  being 
equal  to  that  of  a  watery  ocean  of  thirty-four  feet  deep,  or  of  a  covering  of 
mercury  of  thirty  inches,  and  the  air  found  at  the  surface  of  the  earth  being 
828  times  lighter  than  water,  if  the  same  density  existed  all  the  way  up,  the 
atmosphere  would  be  34  times  828  feet  high,  equal  to  about  five  miles  and 
a  half.  On  account  of  the  greater  rarity,  however,  in  the  superior  regions, 
it  really  extends  to  a  height  of  nearly  fifty  miles.  From  the  known  laws  of 
aerial  elasticity,  explained  at  page  158,  we  can  deduce  what  is  found  to  hold 
in  fact,  that  one-half  of  all  the  air  constituting  our  atmosphere  exists  within 
three  miles  and  a  half  from  the  earth's  surface ;  that  is  to  say,  under  the 
level  of  the  summit  of  Mont  Blanc.  A  person  unaccustomed  to  calculation, 
would  suppose  the  air  to  be  more  equally  distributed  through  the  fifty  miles 
than  this  rule  indicates,  as  he  might  at  first  also  suppose  a  tube  of  two  feet 
diameter  to  be  only  twice  as  capacious  as  a  tube  of  one  foot,  although  in 
reality  it  is  four  times  as  capacious. 

In  carrying  a  barometer  from  the  level  of  the  Thames  to  the  top  of  St. 
Paul's  Church,  in  London,  or  of  Hampstead  Hill,  the  mercury  falls  about 
half  an  inch,  making  an  ascent  of  about  five  hundred  feet.  On  Mont 
Blanc  it  falls  to  half  of  the  entire  barometric  height,  making  an  elevation 
of  fifteen  thousand  feet ;  and  in  Du  Lucts'  famous  balloon  ascent  it  fell  to 
below  twelve  inches,  indicating  an  elevation  of  twenty-one  thousand  feet, 
the  greatest  to  which  man  has  ever  ascended  from  the  surface  of  his  earthly 
habitation. 

The  extreme  rarity  of  the  air  on  high  mountains  must,  of  course,  affect 
animals.  A  person  breathing  on  the  summit  of  Mont  Blanc,  although 
expanding  his  chest  as  much  as  usual,  really  takes  in  at  each  inspiration 
only  half  as  much  air  as  he  does  below — exhibiting  a  contrast  to  a  man  in 
a  diving-bell,  who,  at  thirty-four  feet  under  water,  is  breathing  air  of  double 
density,  at  sixty-eight  feet  of  triple,  and  so  on.  It  is  known  that  travellers, 
and  even  their  practised  guides,  often  fall  down  suddenly  as  if  struck  by 
lightning,  when  approaching  lofty  summits,  on  account,  chiefly,  of  the 
thinness  of  the  air  which  they  are  breathing,  and  some  minutes  elapse  before 
they  recover.  In  the  elevated  plains  of  South  America,  the  inhabitants 
have  larger  chests  than  the  inhabitants  of  lower  regions — furnishing  another 
admirable  instance  of  the  animal  frame  adapting  itself  to  the  circumstances 
in  which  it  is  placed.  It  appears  from  all  this,  that  although  our  atmos- 
phere be  fifty  miles  high,  it  is  so  thin  beyond  three  miles  and  a  half,  that 
mountain  ridges  of  greater  elevation  are  nearly  as  effectual  barriers  between 
nations  of  men,  as  islands  or  rocky  ridges  in  a  sea  are  between  the  finny 
tribes  inhabiting  the  opposite  coasts.  The  intense  cold  which  appertains  to 
high  situations,  and  forms  another  obstacle  to  human  approach,  remains  to 
be  considered  in  our  next  division. 

A  barometer  connected  with  an  air-pump,  indicates  exactly  the  progress 
and  degree  of  exhaustion  in  the  receiver.  When  the  mercury  falls  to  half  its 
height,  it  shows  that  half  of  the  air  is  extracted  ;  and  so  for  all  other  propor- 


ATMOSPHERIC    PRESSURE. — BAROMETERS.        181 


Fig.  102. 
it 


tions.  A  barometer  then  is  a  necessary  appendage  to  a  complete  air-pump  ; 
but  as  its  chief  purpose  is  to  mark  when  the  exhaustion  is  carried  nearly  to 
completion,  a  very  short  tube,  corresponding  to  the  bottom  of  a  common 
barometer,  is  all  that  is  generally  provided,  and  it  is  usually  made  of  syphon 
form. 

The  ingenious  method,  mentioned  at  page  145  of  ascertaining  the  specific 
gravity  of  the  solid  material  forming  any  porous  mass  or  powder,  includes 
the  agency  of  a  barometer.  It  proceeds  upon  this  reasoning.  The  inter- 
stices of  a  porous  or  pulverized  mass  are  filled  with  air  of  the  density  of  the 
surrounding  atmosphere,  and  if  the  atmospheric  pressure  on  which  that  den- 
sity depends  be  diminished  upon  the  mass  in  any  given  degree,  an  exactly 
corresponding  proportion  of  the  air  will  issue  from  the  pores,  and  if  measured 
will  declare  the  whole  quantity,  and,  therefore,  the  amount  of  interstices 
or  pores  in  the  solid  mass.  Now  if  the  substance  were  inclosed  at  the  end 
or  bottom  of  a  syringe,  the  pressure  of  the  atmosphere  might  be  held  off 
from  it  in  any  degree  by  drawing  at  the  piston,  and  the  air  would  issue 
from  the  pores  as  described,  and  would  follow  the  piston ;  but  as,  owing  to 
the  friction  or  a  solid  piston,  it  would  be  difficult  to  measure  the  precise 
action,  the  liquid  piston  of  a  mercurial  column  has  been  substituted,  of  which 
the  force  is  always  proportioned  to  the  length.  The  operator 
takes  an  open  glass  tube,  a  e,  of  known  dimensions,  aud  pre- 
pare a  part  of  its  top,  a  b}  as  a  receptacle  for  the  substance 
under  trial,  by  affixing  a  partition  at  b,  which  shall  support  the 
substance,  but  allow  passage  to  air.  Having  then  filled  a  b 
with  the  substance,  he  gradually  immerses  the  tube  in  a  vessel 
of  mercury  df,  until  the  mercury  stand  both  inside  arid  outside 
of  the  tube  at  the  level  of  6,  the  air  from  the  tube  having 
passed  out  through  the  substance  in  a  b.  It  is  evident  that 
on  then  closing  the  tube  at  a  in  the  air-tight  manner,  and 
lifting  the  tube,  a  column  of  mercury  will  remain  standing  in 
it,  above  the  lev^l  of  the  external  mercury  at  d,  and  will  be 
acting  as  a  piston  pulling  down  from  b  with  force  proportioned 
to  its  height.  If  the  tube  be  lifted  until  such  mercurial 
column  c  d  be  just  of  half  the  length  in  a  column  in  a  common 
barometer,  the  air  in  the  pores  of  the  substance  will  be  relieved 
from  half  of  the  atmospheric  pressure,  and  will  dilate  to  double 
bulk  :  so  that  while  half  of  the  air  will  remain  in  the  pores, 
the  other  half  will  have  issued  forth  to  occupy  a  space,  as  b  c, 
between  the  surface  of  the  mercury  at  the  partition  at  b.  This 
space  b  c,  therefore  will  be  exactly  equal  to  the  amount  of 
the  pores  or  interstices  ',  and  as  it  may  be  measured  and  com- 
pared with  the  whole  space  a  b,  its  ascertained  magnitude  will 
solve  the  problem.  It  has  been  found  in  this  way  that  char- 
coal, which  is  usually  said  to  be  only  half  as  heavy  as  its  bulk 
of  water,  is  really  formed  of  matter  nearly  four  times  as  heavy  ; 
proving,  in  a  new  way,  the  identity  of  charcoal  and  diamond, 
and  that  light  pumice-stone  consists  of  matter  heavier  than  granite  or 
marble.  This  very  ingenious  application  of  the  barometer  may  lead 
ultimately  to  many  useful  results:  and  the  contrivance  merits  consideration 
here,  as  exhibiting,  under  a  new  and  interesting  aspect,  the  rationale  of 
barometric  action  aad  the  elasticity  of  air. 


ci 


182  PNEUMATICS. 

Atmospheric  pressure  determining  the  liquid  or  aeriform  state  of  certain 
substances.     (See  the  Analysis,  page  156.) 

It  has  already  been  stated  that  the  permanent  gases — or  substances  usually 
in  the  aeriform  state — may  be  reduced  to  the  liquid,  or  even  solid  form,  by 
simple  pressure,  and  abstraction  of  the  heat  which  is  combined  with  them 
in  the  aeriform  state.  Carbonic  acid,  the  common  coal  gas,  &c.,  have  been 
treated  in  this  way.  Now  it  becomes  an  interesting  question  whether  many 
of  the  substances  known  as  liquids  on  the  face  of  the  earth,  where  they  are 
bearing  the  pressure  of  the  atmosphere,  would  not  appear  as  airs  if  that 
pressure  did  not  exist. 

On  investigating  this  subject  by  experiment,  we  accordingly  find,  that 
sether,  alcohol  or  ardent  spirits,  volatile  oils,  &c.,  and  even  water  itself,  are 
known  to  us  here  as  liquids,  only  because  their  particles  are  kept  together 
by  the  weight  and  pressure  of  a  superincumbent  atmosphere  Any  of  these 
substances,  relieved  by  art  from  such  pressure,  quickly  becomes  an  air  or 
gas,  just  as  a  common  gas,  which  has  been  kept  in  the  state  of  liquid  by 
any  great  pressure,  becomes  air  again  on  being  relieved. 

In  our  first  chapter  we  explained  the  dependence  of  the  three  forms  which 
any  body  may  assume,  viz.,  of  solid,  liquid,  or  air,  on  the  quantity  of  heat 
diffused  among  the  particles ;  we  now  see,  however,  that  to  understand  the 
subject  completely,  we  must  consider  also  the  effect  of  accidental  pressure  ; 
for,  while  heat  is  the  power  separating  the  atoms  in  the  changes  mentioned, 
it  has  to  overcome  both  the  mutual  attraction  of  the  atoms  and  the  additional 
force  of  the  atmosphere  pressing  them  together.  The  combined  influence 
of  these  forces  is  fully  displayed  in  the  two  phenomena  called  boiling  and 
evaporation,  which  exhibit  the  progress  or  the  change  of  a  liquid  into  an 
aeriform  fluid.  We  now  proceed  to  examine  these  phenomena. 

Boiling. — If  water  be  placed  in  a  suitable  vessel  over  a  common  fire,  or 
over  the  flame  of  a  lamp,  it  is  gradually  heated  to  a  certain  degree ;  and 
then  small  bubbles  of  aeriform  matter,  viz.,  water,  in  the  slate  called  steam, 
are  seen  forming  at  the  bottom  of  the  vessel ;  and  successively  rising  to  the 
surface,  where  they  disappear  by  mixing  with  the  atmosphere;  and  the 
operation  being  continued,  the  quantity  of  water  diminishes  with  every 
bubble,  until  the  whole  vanishes  under  the  new  form  of  air. 

This  change  takes  place  in  water,  under  common  circumstances  at  the 
degree  of  heat  mark  212°  on  Fahrenheit's  thermometer,  and  called,  on  that 
account,  the  boiling  point  of  water;  at  which  therefore,  the  repulsive  power 
among  the  particles  is  just  sufficient  to  overcome  both  their  natural  attrac- 
tion, and  the  compressing  force  of  the  atmosphere  of  fifteen  pounds  on  the  inch. 
But  a  less  degree  of  heat  suffices  if  the  pressure  of  the  atmosphere  be  lessened 
or  removed  :  and  a  greater  degree  is  required  if  pressure  be  increased.  Water 
on  the  top  of  Mount  Blanc  boils  at  180°,  because  relieved  from  the  pressure 
of  the  air  that  is  below  the  level  of  the  mountain's  summit;  and  at  all  inter- 
mediate heights  in  descending  to  the  level  of  the  sea,  and  beyond  that  into 
mines,  there  is  a  corresponding  increase  of  the  boiling  temperature.  So 
exactly  is  this  the  case,  that  we  now  find  it  to  be  a  good  method  of  ascertain- 
ing the  heights  of  places,  merely  to  observe  the  heat  of  boiling  water  at 
them.  To  many  persons  the  information  here  given  that  boiling  water  is  not 
equally  hot  in  all  places,  will  appear  extraordinary;  and  they  will  not  under- 
stand that  even  in  the  same  place,  at  different  times,  when  the  barometer  is 
high  or  low,  there  will  be  corresponding  differences.  Again,  near  the  bottom 
of  a  boiler,  the  water  is  hotter  than  above,  because  it  is  bearing  an  additional 


ATMOSPHERIC    PRESSURE.  —  BOILING.  183 

pressure  proportioned  to  the  depth  and  does  not,  therefore,  give  out  the 
steam  which  it  would  part  with  if  a  little  higher  up.  In  very  large  and  deep 
boilers,  therefore,  such  as  are  used  in  great  porter  breweries,  the  liquor  is 
much  more  heated  that  it  can  be  in  smaller  vessels ;  a  circumstance  which 
probably  has  an  influence  on  its  ultimate  quality. 

While  water,  under  common  atmospheric  pressure,  or  when  the  barometer 
stands  at  thirty  inches,  boils  at  212°,  other  substances,  with  other  relations 
to  heat,  have  their  boiling  points  higher  or  lower :  aether,  for  instance,  at 
98° ;  spirit  or  alcohol  at  174° ;  fish-oil  and  tallow  at  about  600° ;  mercury 
at  650°. 

It  is  in  consequence  of  the  different  temperatures  at  which  the  paticles  of 
different  substances  requires  repulsion  enough  to  rise  against  the  atmospheric 
resistance  that  we  are  enabled  to  perform  the  operation  called  distilling.  If 
a  mixture  of  spirits  and  water,  for  instance,  -be  heated  up  to  180  deg.,  the 
spirit  will  pass  off  in  the  aeriform  state,  leaving  the  water  behind,  and  may 
be  caught  apart  and  cooled  to  condensation  in  any  fit  receiver.  Distillation 
is  the  best  means  we  possess  of  separating  many  substances  from  each  other, 
as  spirit  from  wine  and  other  fermental  liquor ;  various  acids  from  water ; 
water  itself  from  its  common  impurities; — and  even  the  separation  of 
mercury  from  silver  or  gold  which  it  has  been  used  to  dissolve  from  among 
the  rubbish  of  a  mine  or  river-bottom,  is  merely  a  distillation. 

We  must  call  to  mind  here  what  was  mentioned  in  a  former  part  of  the 
work,  that  a  large  quantity  of  heat  combines  with  every  substance  during  the 
change  of  form  from  solid  to  liquid,  or  from  liquid  to  air;  a  quantity  which, 
from  not  remaining  sensible  to  the  thermometer,  has  received  the  name  of 
latent  or  concealed  heat.  The  whole  of  this  is  given  out  again  in  the  contrary 
change.  In  the  conversion  of  water  into  steam,  the  heat  which  thus  disap- 
pears is  about  1,000  degrees,  or  six  times  as  much  as  is  required  to  raise  the 
cold  water  to  the  boiling  point ;  this  is  proved  by  the  time  and  fuel  expended 
in  boiling  any  quantity  to  dryness,  and  by  the  fact  that  a  pint  of  water  in 
the  form  of  steam  will  combine  instantly  with  six  pints  of  cold  water,  rais- 
ing the  whole  to  boiling  heat.  f 

But  for  the  fact  of  latent  heat,  the  conversion  of  a  liquid  into  air  would 
not  be  the  gradual  process  of  boiling  which  we  now  see,  but  a  sudden  and 
terrible  explosion  :  for  when  any  quantity  of  water  were  raised  to  the  boiling 
heat  one  degree  more  would  be  sufficient  to  convert  the  whole  into  steam. 
And  but  for  the  same  reason,  the  thawing  of  winter  snow  would  always  be 
a  sudden  and  frightful  inundation ;  the  whole  load  of  a  mountain  or  plain 
becoming  -at  once  as  a  lake  bursting  from  its  enclosing  barriers.  On  the 
other  hand,  if  water  in  freezing  had  not  to  give  out  again  its  latent  heat,  after 
any  quantity  were  at  once  cooled  down  to  the  freezing  point,  the  abstraction 
of  one  degree  more  would  instantly  convert  the'whole  into  a  solid  mass. 
Thus,  then,  by  an  arrangement  effecting  most  important  purposes  in  nature 
and  art,  all  changes  from  solid  to  liquid  and  from  liquid  to  air,  and  the 
converse  changes,  are  very  gradual. 

If  a  little  heat  be  abstracted  from  steam,  a  part  of  the  steam  proportioned 
to  the  abstraction  is  immediately  condensed  into  water.  What  is  called  steam, 
in  common  language — as  the  vapour  which  becomes  visible  at  a  little  distance 
from  the  spout  of  a  boiling  kettle  or  the  chimney  of  a  tea-urn — is  not  truly 
steam,  but  small  globules  of  water  already  condensed  by  the  cold  air  and 
mixed  with  it.  Steam  is  as  dry  and  invisible  as  air  itself;  but  the  instant 
that  it  comes  in  contact  with  air  or  other  bodies  colder  than  itself,  it  becomes 
water. 


184  PNEUMATICS. 

By  means  of  the  exhausting  air-pump  on  one  hand,  and  of  the  condensing 
syringe  on  the  other,  all  the  above-mentioned  phenomena,  depending  on 
the  atmospheric  pressure,  and  its  increase  or  diminution,  may  be  strikingly 
shown. 

Thus,  to  exhibit  the  effect  of  diminished  pressure,  water  not  heated  by 
several  degrees  to  the  boiling  point  of  ordinary  low  situations,  but  which 
would  be  boiling  at  the  top  of  Mont  Blance,  is  caused  to  boil  instantly  by 
placing  it  under  the  receiver  of  an  air-pump,  and  making  a  few  strokes  of  the 
piston  ;  if  the  exhaustion  be  rendered  nearly  complete,  the  water  will  boil, 
even  when  colder  by  20  degrees  than  the  blood  of  animals ;  and  at  degrees  of 
temperature  still  much  lower,  it  will  rapidly  assume  the  form  of  air,  although 
not  with  force  sufficient  to  produce  the  violent  agitation  of  boiling.  Other 
liquids,  as  spirits,  aether,  &c.,  from  requiring  inferior  degrees  of  heat  to 
separate  their  particles  to  aerfcform  distances,  boil  under  the  receiver  of  an 
air-pump  at  very  low  temperatures  j  aether,  for  instance,  when  as  cold  as 
freezing  water. 

On  the  other  hand,  to  exhibit  the  effect  of  increasing  pressure,  if  we  con- 
fine the  particles  of  a  liquid  still  more  than  by  a  common  atmospheric  or 
equivalent  pressure,  degrees  of  heat  higher  than  the  common  boiling  point 
will  be  required  to  separate  them.  In  a  diving-bell,  the  boiling  point  of  water 
is  higher  than  212  deg.  in  proportion  to  the  depth  which  the  bell  has  reached  : 
and  if  at  the  surface  of  the  earth,  we  heat  water  in  a  close  vessel  into  which 
air  is  forced  so  as  to  press  thirty  pounds  on  the  inch  instead  of  fifteen,  as  the 
atmosphere  does }  or  from  which  we  prevent  the  steam's  escaping  until  it 
has  acquired  the  force  of  a  double  atmosphere, — before  making  the  liquid 
boil,  we  shall  have  to  raise  the  heat,  in  a  corresponding  proportion  beyond 
212  deg.  Under  a  very  strong  pressure,  water  may  be  rendered  almost  red- 
hot,  but  the  force  with  which  its  particles  are  then  tending  to  separate  is 
almost  that  of  inflamed  gunpowder.  Even  then,  however,  if  a  gradual  issue 
were  allowed,  only  a  certain  quantity  of  the  water  would  absorb  and  render 
latent  the  existing  excess  of  heat  above  212  deg.,  and  would  become  common 
steam,  leaving  behind  a  considerable  portion  as  boiling  water  of  the  ordinary 
temperature. 

The  fact  that  liquids  are  driven  off,  or  made  to  boil  at  lower  degrees  of  heat 
when  the  atmospheric  pressure  is  lessened  or  removed,  has  recently  been 
applied  to  some  very  useful  purposes. 

The  process  of  refining  sugar  is  to  dissolve  impure  sugar  in,  water,  and 
after  clarifying  the  solution,  to  boil  off  or  evaporate  the  water  again,  that 
the  dry  crystallized  mass  may  remain. 

Formerly  this  evaporation  was  performed  under  the  atmospheric  pressure, 
and  a  heat  of  21.8°  or  220°  was  required  to  make  the  syrup  boil;  by  which 
degree  of  heat,  however,  a  portion  of  the  sugar  was  discoloured  and  spoiled, 
and^  the  whole  product  was  deteriorated.  The  valuable  thought  occurred  to  J 
Mr.  Howard,  that  the  water  might  be  dissipated  in  boiling  the  syrup  in  a 
vacuum,  or  at  least  a  place  from  which  air  was  nearly  excluded,  and  therefore, 
at  a  low  temperature.  This  was  done  accordingly ;  and  the  saving  of  sugar 
and  the  improvement  of  quality  were  such,  as  to  make  the  patent-right, 
which  secured  the  emoluments  of  the  process  to  him  and  other  parties,  worth 
many  thousand  pounds  a  year.  The  syrup,  during  this  process,  is  not  more 
heated  than  if  in  a  vessel  merely  exposed  to  a  summer  sun. 

In  the  preparation  of  many  medicinal  substances,  the  process  of  boiling  in 


ATMOSPHERIC    PRESSURE.- —  BOILING. 


185 


Fig-  103. 


var.no  is  equally  important.  Many  extracts  from  vegetables,  have  their 
virtues  impaired,  or  even  destroyed,  by  a  heat  of  212°;  but  when  the  water 
used  in  making  the  extract  is  driven  off  in  vacuo,  the  temperature  need 
never  be  higher  than  blood-heat,  and  all  the  activity  of  the  fresh  plant 
remains  in  the  extract. 

In  the  same  manner,  in  the  process  of  distillation,  —  which  is  merely  the 
receiving  and  condensing  again  in'  appropriate  vessels  the  aeriform  matter 
raised  by  heat  from  any  mass,  —  substances  which  are  changed  and  injured 
by  an  elevated  temperature  may  be  obtained  of  admirable  quality  by  carry- 
ing on  the  operation  in  a  vacuum.  The  essential  oils  of  lavender,  pepper- 
mint, &c.,  never  had  the  natural  flavour  and  virtues  of  the  plants  until 
within  the  last  few  years,  since  this  plan  has  been  adopted. 

The  influence  on  the  human  system  of  vegetable  medicines  obtained  in 
the  old  or  in  the  new  way,  is  so  different,  that  the  prescriber  should  carefully 
advert  to  the  circumstance. 

The  apparatus  for  evaporating  and  distilling  in  vacuo  consists  of  vessels 
strong  enough  to  bear,  when  quite  empty,  the  external  atmospheric  pressure 
and  which  are  therefore  generally  of  arched  form.  The  vacuum  is  produced 
and  maintained  by  air-pumps  driven  by  a  steam  engine  or  otherwise;  or  by 
first  admitting  steam  to  expel  the  air,  and  then  condensing  the  steam  into 
water. 

The  author  has  suggested  a  very  simple  contrivance  to  answer,  in  certain 
cases,  the  purpose  of  such  air-pun\,ps  and  steam-engines  or 
apparatus.  It  is  merely  to  establish  a  communication 
between  a  close  boiler,  as  a,  and  the  vacuum  at  the  top  of 
a  water  barometer,  as  b,  To  produce  that  vacuum,  the 
strong  vessel  b  forming  the  top  of  the  barometer,  and 
thirty-six  feet  of  tube  below,  reaching  to  d  are  first  filled 
with  water  through  a  cock  c  at  the  top  ;  this  cock  being 
then  shut,  and  another  cock  d  at  the  bottom,  which  was 
shut,  being  opened,  the  water  will  sink  down  out  of  the 
vessel  6,  until  the  column  in  the  tube  be  only  thirty-four 
feet  high  as  at/,  that  being  the  height  which  the  atmos- 
phere will  support.  On  then  opening  a  communication 
between  the  boiler  a  and  the  vacuum  in  b,  the  operation 
will  go  on  as  desired,  and  the  steam  rising  from  a  may  be 
condensed  in  b  by  a  little  stream  of  cold  water  allowed  con- 
stantly to  run  through  from  above.  This  water,  it  is  evi- 
dent, would  always  pass  downwards  to  form  part  of  the 
column  below,  without  filling  up  or  impairing  the  vacuum. 
If  air  should  find  admittance  in  any  way.  the  original  degree 
of  vacuum  could  easily  be  reproduced  as  at  first  ;  and  to 
prevent  interruptions,  it  might  be  convenient  to  have  two 
vessels  like  6,  of  which  one  could  always  be  in  action  while 
the  other  was  being  emptied  of  air.  The  author  planned  this  as  a  simple 
apparatus  for  the  preparation  of  medical  extracts  ;  and  it  appears  well  suited 
also  for  the  manufacture  of  sugar  in  the  colonies,  where  air-pumps  and  nice 
machinery  can  with  difficulty  be  either  obtained  or  managed.  On  many 
sugar  estates  there  is  a  fall  of  water,  which  would  supply  the  barometer  with- 
out the  trouble  of  pumping.  The  tube  d  c  need  not  be  perpendicular,  pro- 
vided it  be  longer  in  proportion  to  its  obliquity  ;  and  it  may  be  very  small  ; 
some  yards  of  common  lead-pipe  would  answer. 

When  it  was  understood  that,  at  common  temperatures,  water  and  many 


186  PNEUMATICS. 

fe 

other  liquids  would  be  existing  in  the  form  of  air,  but  for  an  atmospheric 
pressure  opposing  the  separation  of  the  particles,  it  became  of  great  import- 
ance in  many  of  the  arts,  and  for  comprehending  certain  phenomena  of 
nature,  to  ascertain,  very  exactly,  with  respect  to  some  of  these  liquids,  the 
degrees  of  expansive  force  belonging  to  them  at  different  degrees  of  tempera- 
ture. The  subject,  as  far  as  water  is  concerned,  has  been  investigated  with  great 
care,  and  the  following  table  shows  part  of  the  results.  The  left-hand  column 
marks  temperature  from  32  deg.  of  Fahrenheit's  thermometer,  or  the  freez- 
ing point  of  water,  to  290  deg. ;  and  the  right-hand  column  marks  the  cor- 
responding degrees  of  force  with  which  the  water  tends  to  expand  into  the 
state  of  steam,  and  therefore  also  the  force  and  density  of  the  steam  existing 
in  any  vessel  above  the  water  which  it  contains.  One  ounce  and  a  half 
per  square  inch,  is  the  force  exerted  on  the  sides  of  any  containing  vessel  by 
steam  rising  from  freezing  water,  that  is  to  say,  the  force  with  which  freezing 
water  seeks  to  dilate  into  steam  or  air  ;  and  sixty  pounds  per  inch  is  the  force 
of  water  heated  to  290  deg.  9  To  many  readers  the  idea  will  be  quite  new 
and  surprising,  that  if  some  freezing  water,  or  even  ice,  be  placed  in  a  bladder 
containing  nothing  else,  and  the  bladder  be  then  placed  in  the  exhausted 
receiver  of  an  air-pump  or  other  vacuum,  the  bladder  will  quickly  be  dis- 
tended with  steam  strong  enough  to  support  one  ounce  and  a  half  on  every 
square  inch  of  its  upper  surface. 


At  32°  force  of  steam  is 
50 

1|  oz.  per  inch. 

O  3.        f\rj 

100 

—  4      U/«  . 

IS 

J.O 

4-     Ibs 

180     
212     

7£  Ibs'. 
15     Ibs. 

250 30     Ibs. 

290 60    Ibs! 

"*  In  this  table  we  have  to  remark  how  much  more  rapidly  the  tendency 
to  become  steam  increases  than  the  temperature  of  the  water :  for  a  rise  of 
eighteen  degrees,  viz.,  from  32°  to  50°,  at  the  beginning  of  the  scale,  only 
increases  the  dilating  force  one  ounce  and  a  quarter  on  the  inch,  while  an 
equal  rise  at  the  top  of  the  scale,  viz.,  from  Ii72  deg.  to  290  deg.,  increases 
it  fifteen  pounds.  It  is  most  important  to  distinguish,  however,  between  the 
tendency  to  form  steam  at  any  temperature,  and  the  bulk  or  quantity  of 
steam  formed  by  a  given  quantity  of  heat ;  for  the  matter  imperfectly  under- 
stood has  led  to  many  vain  schemes  for  improving  the  steam-engine.  The 
truth  is,  that  high-pressure  steam  is  merely  condensed  steam,  as  high-pres- 
sure air  is  condensed  air;  in  other  words,  the  density  of  steam  is  greater, 
or  there  must  be  more  of  it,  exactly  as  its  force  is  greater  according  to  the 
rule  explained  at  page  160 :  and  the  heat  absorbed  in  its  formation  being 
proportioned  to  the  quantity  of  steam  in  a  given  space,  or  the  density,  the 
force  and  the  cost  in  fuel  have  always  nearly  the  same  relation  to  each  other. 
In  one  pint  of  steam,  at  29t)  deg.,  having  an  elastic  force  of  sixty  pounds  on 
the  inch,  there  are  very  nearly  four  times  as  much  water  and  four  times  as 
much  latent  heat  as  in  one  pint  of  steam  at  212  deg.,  which  has  a  force  of 
fifteen  pounds  on  the  inch ;  indeed,  the  one  pint,  at  290  deg.,  may  be 
changed  into  four  pints  at  212  deg.,  or  the  contrary,  by  merely  changing 
the  degrees  of  pressure.  It  does  not  accord  with  the  plan  of  the  present 


STEAM    ENGINE.  187 

work  to  enter  farther  into  the  details  of  this  subject,  but  they  may  be  found 
in  various  modern  treatises. 

Seeing  the  rapid  increase  of  the  expansive  force  in  the  preceding  table, 
we  have  the  explanation  of  the  terrible  effects  occasionally  produced  by  con- 
fined water  when  overheated.  A  boiler  of  any  kind  completely  closed,  and 
having  no  safety  valve,  if  heated  to  a  certain  degree,  will  explode  as  if 
charged  with  gunpowder.  Unhappily  the  instances  are  too  numerous  where 
the  incautious  or  ignorant  use  of  steam  has  produced  explosions,  which  have 
shattered  buildings  and  destroyed  whole  neighborhoods. 

The  prodigious  force  generated  by  heating  water  would  at  first  only  sur- 
prise and  terrify  men,  but  in  the  course  of  time  would  lead  inventive 
minds  to  enquire  whether  it  might  not  be  turned  to  use ;  in  other  words, 
whether  some*  mechanism,  to  be  called  a  steam-engine,  might  not  be  con- 
trived to  enable  men  to  make  it  aid  them  in  their  various  labours.  To  this 
inquiry,  after  numerous  less  successful  attempts,  a  glorious  answer  has  been 
given  in  our  own  day  by  the  illustrious  WATT  ; — and  to  this  part  of  our 
work  it  belongs  to  consider  what  he  has  accomplished,  viz.,  to  describe 

The  Steam- Engine, 

which,  in  the  few  years  since  the  genius  of  WATT  carried  it  to  its  present 
state  of  perfection,  has  changed  the  direction, of  human  industry,  and  may 
almost  be  said  to  have  elevated  man  in  the  scale  of  existence. 

The  name  of  steam  enyine,  to  most  persons,  brings  the 
idea  of  a  machine  of  the  most  complex  nature,  and  hence  to  Fig.  104. 
be  understood  only  by  those  who  will  devote  much  time  to 
the  study  of  it ;  but  he  who  can  understand  a  common  pump, 
may  understand  a  steam-engine.  It  is  in  fact  only  a  pump 
in  which  the  fluid  passing  through  it  is  made  to  impel  the  [~r  ~~t 

piston  instead  of  being  impelled  by  it,  that  is  to  say,  in  which 
the  fluid  acts  as  the  power  instead  of  being  the  resistance. 
It  may  be  described  simply  as  a  strong  barrel  or  cylinder  c  d, 
with  a  closely  fitting  piston  in  it,  here  shown  at  b,  which  is 
driven  up  and  down  by  steam  admitted  alternately  above  and 
below  it  from  a  suitable  boiler ;  while  to  the  end  of  the  pis- 
ton  rod  a,  at  which  the  whole  force  may  be  considered  as  . 
concentrated,  there  is  attached  in  any  convenient  way  the 
work  which  is  to  be  performed.  The  power  of  the  engine  is  of  course  pro- 
portioned to  the  size  or  area  of  the  piston,  on  which  the  steam  acts  with  a 
force,  according  to  its  density,  of  from  15  to  100  or  more  pounds  to  each 
square  inch.  In  some  of  the  Cornish  mines  there  are  cylinders  and  pistons 
of  more  than  ninety  inches  in  diameter,  on  which  the  pressure  of  the  steam 
equals  the  efforts  of  six  hundred  horses. 

In  one  place  this  wonderful  piston-rod  may  be  seen  acting  upon  the  end 
of  a  great  vibrating  beam,  to  the  other  end  of  which  capacious  water-pumps 
are  attached,  whose  motion  causes  almost  a  river  to  gush  up  from  the  bowels 
of  the  earth.  In  another  place,  it  is  seen  working  a  crank,  and  urging  com- 
plicated machinery.  One  steam-engine  four  miles  from  London  is  at  the 
same  instant  filling  all  the  water  reservoirs,  and  baths,  and  fountains  of  the 
finest  quarter  of  the  town.  One  engine  stretching  long  arms  over  a  great 
barrack,  or  manufactory,  keeps  in  one  quarter,  thousands  of  spinning-wheels 
in  motion,  while  in  another  it  is  carding  the  material  of  the  thread,  and  in 
another  weaving  the  cloth.  In  like  manner,  one  steam  engine,  in  a  great 


188 


PNEUMATICS. 


metropolitan  brewery,  may  be  seen  at  the  same  time  grinding  the  malt,  pull- 
ing up  supplies  of  all  kinds  from  wagons  around  the  building,  pumping  cold 
water  into  some  of  the  coppers,  sending  the  boiling  wort  from  others  up  to 
lofty  cooling-pans,  over  which  it  is  turning  the  fans,  perhaps  also  working 
the  mash-tub,  drawing  water  from  the  deep  wells  under  ground,  and  loading 
the  drays — in  a  word,  performing  the  offices  of  a  hundred  hands.  Again, 
there  are  manufactories  where  this  resistless  power  is  seen  with  its  mechanic 
claws  seizing  masses  of  iron,  and  in  a  few  minutes  delivering  them  out  again 
pressed  into  thin  sheets,  or  cut  into  bars  and  ribbons,  as  if  the  iron  had  be- 
come to  it  like  soft  clay  in  the  hands  of  the  potter.  And  now  for  some 
years,  over  nearly  the  whole  world,  has  this  wonderful  piston-rod,  working  at 
its  crank,  been  turning  the  paddle-wheels  of  innumerable  steam-boats,  there- 
by sitting  at  defiance  the  violence  of  the  winds  and  waves,  aifd  the  currents 
of  the  fleetest  rivers,  while  it  carries  men  and  civilization  into  the  remote 
recesses  of  all  the  great  continents.  To  wherever  a  river  leads,  the  region, 
although  concealed  perhaps  since  the  beginning  of  the  world,  is  now  by  the 
steam-engine  called  as  it  were  from  its  solitude,  to  form  a  part  of  the  great 
garden  which  civilized  man  is  beautifying. — Such  are  a  few  of  the  prodigies 
which  this  machine  is  already  performing,  and  every  day  is  witnessing  new 
applications  of  its  utility. 

The  following  account  of  the  parts  of  the  steam-engine  is  intended,  with- 
out entering  into  minute  practical  details,  still  fully  to  explain  the  principle 
or  general  nature  of  the  machine.  It  should  serve  to  render  very  interesting 
to  an  attentive  reader,  a  visit  to  any  place'  where  a  steam-engine  is  in  use : 
and  it  should  make  evident  the  folly  of  many  of  the  modern  schemes  for 
improving  the  engine.  To  avoid  complexity  in  the  figure,  the  parts  which 
the  reader  can  easily  conceive  are  not  here  sketched. 

Fig.  105. 


1st.  The  part  which  first  claims  attention  is  the  great  barrel  c  d,  already 
spoken  of  as  the  centre  or  main  portion  of  the  machine,  in  which  the  piston 
P  is  moved  up  and  down  by  the  action  of  steam  entering  alternately  above 
and  below  it,  through  the  pipes  e  c  and  e  d.  The  barrel  or  cylinder  is 
bored  with  extreme  accuracy,  and  the  piston  is  padded  round  its  edge  with 
hemp  or  other  soft  material  so  as  to  be  perfectly  air  or  steam-tight.  Lately 


STEAM    ENGINE.  189 

pistons  have  been  made  altogether  of  metal,  and,  in  some  cases,  from  work- 
ing with  less  friction,  these  answer  even  better  than  the  others. — 2d.  The 
next  part  to  be  mentioned  is  the  boiler  B,  which  is  made  of  suitable  size  and 
strength. — 3d.  The  steam  passes  from  the  boiler  along  the  pipe  to  e,  and  there 
by  any  suitable  cock  or  valves,  worked  by  the'  engine  itself,  is  directed  alter- 
nately to  the  upper  and  under  part  of  the  barrel;  and  while  it  is  entering  to 
press  on  one  side  of  the  piston,  the  waste  steam  is  allowed  to  escape  from 
the  other  side,  either  to  the  atmosphere,  for  high-pressunj  engines,  or  into — 
4th,  the  condenser  at  C,  for  those  of  low-pressure ;  the  condenser  being 
always  kept  at  a  low  temperature  by  cold  water  running  into  it  arid  pumped 
out  again  by  the  piston  k, — 5th.  The  supply  of  steam  from  the  boiler  to  the 
cylinder  is  regulated  by  a  valve  placed  somewhere  in  the  pipe  B  6,  and  made 
obedient  to  what  is  called — 6th,  the  governor,  a  contrivance  not  represented 
here,  but  already  described  at  page  52,  to  illustrate  centrifugal  force.  We 
may  recall  it  by  saying,  that  it  consists  of  two  balls  hanging  by  jointed  rods 
like  the  legs  of  a  tongs,  from  opposite  sides  of  an  upright  spindle,  which  is 
made  to  revolve  by  connection  with  some  turning  part  of  the  machinery : — 
when  the  spindle  turns  at  all  faster  than  with  the  desired  speed,  the  balls  fly 
more  apart  and  are  made  to  affect  the  steam  valve  so  as  to  narrow  the  passage ; 
and,  on  the  contrary,  when  it  turns  more  slowly  than  is  desired,  they  collapse, 
and  by  so  doing  open  the  valve  wider. — 7th.  The  supply  of  water  to  the 
boiler  is  regulated  by  afloat  on  the  surface  of  the  water  in  the  boiler;  which 
float,  on  descending  to  a  certain  point,  by  reason  of  the  consumption  of 
water,  opens  the  valve  to  admit  more. — 8th.  There  is  a  safety  valve  in  the 
boiler,  viz.,  a  well-fitted  flap  or  stopper,  held  against  an  opening  by  a  weight, 
but  loaded  so  as  to  open  before  danger  can  arise  from  the  over-heating  of  the 
water. — 9th.  The  rapidity  of  the  combustion,  or  force  of  the  fire,  is  exactly 
regulated  by  the  state  of  the  boiler  and  the  wants  of  the  machine,  thus : — 
there  is  a  large  open  tube  (not  represented  here)  rising  from  near  the  bottom 
of  the  boiler,  through  its  top,  to  the  height  of  several  feet,  and  when  the 
water  in  the  boiler  is  too  hot,  and  the  steam  therefore  too  strong,  part  of  the 
water  is  pressed  up  into  this  tube,  and  by  the  agency  of  a  float  which  rests 
on  its  surface,  it  shuts  the  chimney  valve  or  damper :  the  draught  is  then 
diminished  and  the  fuel  saved," until  a  brisker  fire  is  again  required. — 10th. 
In  this  figure  a  i  g  marks  the  place  of  the  great  beam,  turning  on  an  axis 
at  i,  and  transmitting  the  force  of  the  piston  to  the  remote  machinery. 
When  the  object  is  to  raise  water,  the  pump  rods  are  simply  connected  with 
the  end  g  of  the  beam  ;  but  when  any  rotatory  motion  is  wanted,  the  end  g 
is  made  to  turn — llth.  A  crank  I  n  by  the  rod  g  I;  and  uniformity  of 
motion  is  obtained  by  the  influence  of — 12th,  the  great  fly  wheel  m,  fixed 
to  the  axis  of  the  crank. 

The  smallest  and  simplest  steam  engine,  and  therefore  the  cheapest,  is 
that  called  the  high-pressure  engine.  In  it  steam  is  used  of  great  density, 
and  consequently  of  great  force,  as  of  50  Ibs.  or  more  to  the  inch;  and  while 
the  fresh  steam  is  admitted  to  press  on  one  side  of  the  piston,  the  steam 
which  has  already  worked,  is  allowed  to  escape,  or  is  driven  out  to  the  air, 
from  the  other  side.  The  atmospheric  resistance  to  the  issue  of  the  steam 
diminishes  the  working  force  of  the  piston  just  15  Ibs.  per  inch.  The  sim- 
plicity of  this  form  of  engine  recommends  it,  but  the  danger  of  a  large 
boiler  of  overheated  water,  always,  like  inflamed  gunpowder,  seeking  to 
escape,  bas?  by  numberless  fatal  accidents,  been  proved  to  be  so  great,  that 
the  use  of  such  an  engine  is  limited  to  certain  situations.  Notwithstanding 
all  the  ingenious  securities  recently  contrived  against  the  danger,  and  which 


190  PNEUMATICS. 

wijl  suffice  for  small  engines,  such  as  are  required  for  steam  carriages,  the 
high-pressure  engine  is  not  employed  in  a  single  English  passage-vessel.* 

In  the  low-pressure  engine,  the  steam  is  used  generally  of  force  not 
exceeding  20  Ibs.  on  the  inch)  which  force  is  only  5  Ibs.  more  than  the  atmos- 
pheric pressure,  and  is  insufficient  to  burst  a  common  boiler,  or  to  do  serious 
mischief  :f  but  as  the  interior  of  the  low-pressure  engine  is  kept  in  a  state  of 

*  In  this  country,  a^o,  what  is  called  the  low  pressure  engine,  is  at  present  employed  in 
all  the  steamboats  on  our  eastern  waters,  but  we  have  reason  to  believe  that  few,  if  any,  of 
them  are  worked  with  a  less  pressure  than  25  Ibs.  on  the  square  inch,  and  we  have  seen 
many  of  them  worked  with  a  much  higher  pressure.  It  is  stated,  that  the  steam  guage  on 
board  the  Pulaski,  just  before  the  fatal  explosion,  indicated  a  pressure  of  26  Ibs.,  which  was 
considered  by  the  engineer  a  safe  working  force.  The  British  Ocean  Steamers  work  with  a 
pressure  of  only  3£  to  4i  Ibs.  to  the  square  inch.  On  our  western  waters,  the  low  pressure 
engine  has  been  discarded  in  favour  of  the  high  pressure.  AM.  ED. 

f  Our  author  must  be  understood  as  saying,  only,  that  explosions  do  not  take  place  in  low 
pressure  engines  whilst  working  under  a  pressure  of  only  20  Ibs.  to  the  square  inch,  But 
from  various  circumstances,  the  elastic  force  of  the  steam  in  a  low  pressure  engine  may  be 

freatly  increased,  and  instead  of  its  ordinary  power  of  20  Ibs.,  it  may  acquire  one  of  100  or 
00  Ibs.,  and  if  the  boiler,  as  is  often  the  case,  is  unable  to  bear  this  pressure,  it  will  burst. 
In  this  way  explosions  have  repeatedly  taken  place  in  low  pressure  engines.  Several  of 
these»will  be  found  related  in  an  interesting  memoir  by  M.  Arago,  originally  published  in 
ihe"  Annuaire  du  Bureau  des  Longitudes,"  and  which  has  been  translated  and  published  in 
that  useful  periodical,  "  The  Journal  of  the  Franklin  Institute  of  Pennsylvania."  M.  Arago, 
in  this  memoir,  remarks,  "  I  ought  not  to  conclude  so  long  a  paper  on  the  subject  of  the 
explosion  of  steam  boilers,  without  explaining  why  I  have  not  separated  the  examples  of 
the  explosion  of  high  pressure  boilers  from  those  of  the  low  pressure  ;  it  is  because  I  think 
there  is  no  reason  to  make  such  distinction.  Every  one  must,  in  fact,  admit,  that  at  the 
time  of  an  explosion,  all  boilers  contain  high  pressure  steam." 

The  belief  that  low  pressure  boilers  are  not  liable  to  burst,  or  do  mischief,  has  led,  as 
has  been  already  observed,  to  their  exclusive  use  in  passage  vessels ;  this  belief,  however, 
is  founded  in  error.  M.  Arago,  in  the  memoir  already  quoted,  observes,  •'  it  does  not 
appear  established  by  any  means,  that  explosions  take  place  more  frequently  in  hi<rh  than 
in  low  pressure  boilers;  the  contrary  has  been  maintained  by  different  engineers,  among 
whom  may  be  classed  Messrs.  Perkins,  Oliver  Evans,  Ac."  Indeed,  a  little  reflection  will 
show  that  high  pressure  boilers  ought  not  to  be  more  liable  to  explosion  than  the  low. 
Boilers  may  be  made  of  either  iron  or  copper  of  sufficient  strength  to  resist  a  much  greater 
force  than  that  of  the  steam  ever  employed  in  high  pressure  engines.  Now  boilers  are 
always  constructed  of  a  strength  proportionate  to  the  pressure  they  are  to  sustain.  Thus, 
in  a  low  pressure  engine  working  with  a  force  of  20  Ibs.,  the  boiler  is  made  of  strength 
calculated  to  support  from  3  to  5  times  that  pressure,"  in  a  high  pressure  engine  destined 
to  work  with  a  pressure  of  150  Ibs.,  the  boiler  is  constructed  so  as  to  resist  from  3  to  5  times 
that  pressure.  It  will  immediately  be  asked,  why  cannot  the  boiler  of  a  low  pressure 
engine  be  made  of  the  same  strength  as  if  they  were  for  a  high  pressure  engine  ?  In  answer 
to  this  it  may  be  remarked,  that  independent  of  many  difficulties  to  be  overcome  before  we 
can  exceed  certain  limits  in  the  thickness  of  boilers,  the  weight  and  cost  of  the  low  presure 
engine  are  already  so  great  that  it  would  be  impossible  to  persuade  owners  of  steamboats 
to  incur  any  addition  to  these  particulars — and  even  would  they  do  so,  perfect  safety  would 
still  not  thus  be  obtained.  We  have  already  observed,  that  at  the  time  of  an  explosion,  all 
boilers  contain  high  pressure  steam,  and  as  we  know  no  limits  to  the  force  of  this  steam, 
however  strong  the  boiler  may  be,  it  may  burst,  unless  this  be  prevented  by  other  means. 

It  was  long  ago  known,  that  if  a  vessel,  however  strong  it  might  be,  containing  water, 
be  placed  over  a  fire,  it  will  burst,  unless  an  opening  is  provided  for  the  escape  of  the 
steam  as  fast  as  produced.  The  temperature  which  will  cause  the  rending  of  a  vessel, 
must  depend  upon  its  form  and  dimensions,  and  upon  the  tenacity  and  thickness  of  the 
material  of  which  it  is  made.  If  we  could  keep  the  heat  of  our  furnace  below  a  certain 
limit,  no  other  precaution  would  be  required  to  prevent  explosions.  But  it  is  evident  that 
this  cannot  be  done,  we  must,  therefore,  resort  to  some  other  expedient,  and  the  safety 
valve,  invented  by  Papin,  would  seem  to  answer  this  purpose.  We  must  be  allowed  to 
anticipate  the  subject  a  little,  in  order  to  explain  the  nature  of  this  valve. 

The  safety  valve  consists  of  a  hole,  say  of  an  inch  square,  made  in  the  upper  part  of  the 
boiler,  upon  which  is  placed  a  metal  plate  loaded  with  a  certain  weight.  It  is  evident 
that  the  hole  will  remain  closed  as  long  as  the  pressure  of  the  steam  within  the  boiler  is 
less  that  the  weight  of  the  valve,  together  with  that  of  the  atmosphere,  upon  the  square 
inch,  and  that  as  soon  as  the  pressure  within  shall  exceed  this,  the  valve  will  be  raised 
and  give  a  free  vent  to  the  steam. 

It  would  lead  us  too  far  to  explain  how  it  has  happened  that  so  simple  and  apparently 


STEAM    ENGINE.  191 

vacuum,  except  where  the  steam  is  acting,  the  whole  pressure  of  20  Ibs.  is 
made  available,  and  the  engine  has  the  same  power,  if  of  equal  size,  as  a 
high  pressure  engine  working  with  steam  of  35  Ibs.  on  the  inch  The  re- 
quired vacuum  is  preserved  by  means  of  a  separate  vessel  or  box,  represented 
at  C.  called  the  condenser,  into  which  cold  water  is  constantly  running  to 
condense  the  steam,  and  is  afterwards  pumped  out  with  the  condensed 
steam,  and  with  any  little  air  that  may  have  entered  :  the  pump  is  repre- 
sented at  k  in  the  figure.  Steam,  on  coming  into  contact  with  a  cold  body, 
is  condensed  almost  with  the  rapidity  of  an  explosion ;  and  therefore  the 
instant  that  the  opened  valves  make  a  communication  between  the  cold  con- 
denser and  any  part  of  the  engine  containing  steam,  this  rushes  to  the  con- 
denser, and  becomes  water,  leaving  a  vacuum  behind.  The  great  merit  of 
Mr.  Watt  was  in  the  contrivance  of  this  separate  condenser,  for,  until  his 
time,  cold  water  had  always  been  thrown  directly  into  the  working  cylinder, 
cooling  it  so  much,  that  twice  or  thrice  its  fill  of  steam  was  destroyed  at 
each  stroke  to  warm  it  again  before  it  could  work.  This  single  change 
saved  three-fourths  of  the  quantity  of  fuel  formerly  expended. 

Before  Watt's  day,  the  only  steam  engine  in  use  was  a  rude  single- stroke 
engine  as  it  was  called,  in  which  steam,  admitted  under  the  piston,  allowed 
the  weight  of  the  pump-rods  at  the  far  end  of  the  beam  to  lift  the  piston, 
and  the  steam  being  then  condensed  so  as  to  leave  a  vacuum  in  the  cylinder, 
the  pressure  of  the  atmosphere  pushed  the  piston  down  to  do  its  work  :  on 
this  last  account  the  engine  was  also  called  au'atmospheric  engine.  It  was 
used  almost  solely  for  pumping  water ;  but  it  wasted  so  much  fuel,  from 
causes  of  which  the  chief  is  mentioned  in  the  last  paragraph,  that  the 
expense  was  not  much  less  than  that  of  employing  horses. 

In  the  atmospheric  engine,  the  steam  which  lifted  the  piston  against  the 
atmospheric  pressure,  required  to  be  at  least  as  strong  as  that  pressure,  to  the 
very  end  of  the  stroke.  Another  of  Watt's  great  improvements  was,  his 


efficient  means  has  not  always  proved  efficacious;  since  these  causes  are  various  and  many 
of  them  as  yet  not  perfectly  understood. 

Those  who  wish  to  investigate  the  subject,  we  refer  to  the  memoir  of  M.  Arago,  and 
to  the  report  of  a  committee  of  the  Franklin  Institute,  who  have  collected  an  account  of 
all  the  explosions  in  this  country,  and  who  have  instituted  a  very  interesting  series  of  ex- 
periments, in  order  to  examine  into  the  causes  of  the  explosion  of  steam  boilers,  and  devise 
means  for  its  prevention.  This  report  has  been  published  in  the  Journal  of  the  Institute. 

We  must  not  omit,  however,  to  mention  that  when  a  low  pressure  boiler  does  explode, 
it  hns  ifeen  found  to  r roduce  greater  destruction  than  a  high  pressure  one,  in  consequence 
of  the  great  size,  and,  therefore,  larger  quantity  of  water  contained  in  the  former.  It 
may,  perhaps,  be  supposed  that  the  steam  from  a  high  pressure  engine  would  scald  more 
severely  than  that  from  a  low  pressure  one.  This,  however,  is  not  the  fact :  on  the  contrary, 
whilst  the  steam  issuing  from  a  low  pressure  engine  scalds  at  all  moderate  distances  from 
the  boiler,  that  from  a  high  pressure  one  scalds  only  at  certain  distances.  Thus  the  hand 
may  be  placed  an  inch  from  an  aperture  in  a  high  pressure  engine  without  any  inconve- 
nience being  felt:  at  a  greater  distance,  however,  it  will  scald  most  severely.  A  friend  has 
informed  us  that  he  has  placed  his  hand  within  an  inch  of  the  aperture  in  a  boiler  from 
which  the  steam  was  issuing  at  a  time  when  the  force  of  the  steam  within  the  boiler  was 
equal  to  300  Ibs.  on  the  square  inch,  without  feeling  any  inconvenience.  Some  interesting 
experiments  on  this  subject  have  been  instituted  by  Peter  Ewort,  Esq.,  and  an  account  of 
which  will  be  found  in  the  fifth  volume  of  the  Journal  of  the  Franklin  Institute. 

It  must  not  be  supposed  from  any  thing  that  we  have  said  in  this  note,  that  explosions 
of  steam  boilers  cannot  be  prevented.  But  we  may  be  allowed  to  quote  on  this  subject 
the  following  remarks  of  M.  Arago.  '•  No  cause  of  explosion  exists  which  cannot  be  avoided, 
by  means  at  once  simple  and  within  the  reach  of  every  one.  As  we  should  not  trust  fire- 
arms in  the  hands  of  children,  so,  I  think,  we  should  not  trust  the  direction  of  a  steam 
engine  to  a  man  either  unskilful,  without  experience,  or  wanting  in  intelligence.  It  is  a 
mistaken  idea,  that  because  steam  engines  usually  move*  without  attention  to  them,  such 
attention  is  not  required;  Watt  contended  strongly *against  this  error."  AM.  ED. 


192  PNEUMATICS. 

excluding  altogether  the  air  from  his  machine,  by  doing  which  he  not  only 
avoided  the  cooling  effect  of  the  air,  but  was  at  liberty  to  shut  off  the  steam, 
as  it  is  expressed,  or  to  stop  the  supply  for  each  stroke  before  the  cylinder 
was  full,  and  then  to  make  the  farther  expansion  of  the  quantity  admitted 
impel  the  piston  to  the  end  of  the  stroke.  This  principle  of  causing  the  mere 
expansion  of  steam  to  do  work  was  afterwards  carried  to  a  great  extent  by 
Messrs.  Hornblower,  Woolfe,  and  others,  who  constructed  engines  with  two 
barrels,  in  the  first  and  smaller  of  which,  the  steam  was  made  to  act  in  its 
dense  or  strong  state,  as  it  issued  from  the  boiler,  and  when  it  had  finished 
a  stroke  there,  instead  of  being  at  once  sent  useless  to  the  condense,]",  it  was 
admitted  to  a  larger  piston,  which  it  moved  by  its  continued  expansion  alone  : 
— the  same  steam  thus  doing  double  work  or  more.  All  the  advantages  of 
the  two  cylinders,  however,  are  obtainable  from  the  single  cylinder  with  its 
condenser,  as  now  used  in  most  of  the  Cornish  mines.  Steam  of  about  60  Ibs. 
pressure  on  the  inch  is  admitted  to  the  cylinder,  until  the  piston  is  driven 
nearly  one-third  of  its  way,  and  the  valve  being  then  shut,  the  same  steam  is 
left  to  finish  the  stroke  by  its  expansion.  The  pressure  of  the  expanding 
steam  gradually  diminishes,  it  is  true,  in  proportion  as  the  volume  increases  : 
but  in  pumping  water  there  is  a  great  saving  of  time,  from  having  the  power 
more  intense  at  the  beginning  of  the  stroke,  when  the  vast  mass  of  water  and 
machinery  has  first  to  be  put  into  motion.  Steam,  while  doubling  its  volume 
by  mere  expansion,  will  do  about  two-thirds  as  much  work  as  while  origi- 
nally rising  from  the  boiler,  and  by  every  subsequent  doubling  it  might  do 
as  much  as  by  the  first :  the  increasing  size  of  the  cylinder,  however,  and 
increased  friction,  confine  this  mode  of  using  it  to  narrow  limits. 

It  might  be  supposed  that  high  pressure  engines  without  condensers  would 
be  comparatively  wasteful,  because  in  them  the  steam  which  has  acted  must 
be  driven  out  of  the  cylinder  against  the  powerful  resistance  of  the  atmos- 
phere, while  in  the  low  pressure  engine  it  has  instant  access  to  the  con- 
denser, and  leaves  effective  the  whole  pressure  of  the  fresh  steam  on  the 
opposite  side  of  the  piston.  But  as  in  the  low  pressure  engine,  nearly  half 
the  power  of  the  steam  is  .expended  in  overcoming  the  friction  and  other 
impediments  of  the  numerous  parts,  while  in  that  of  high  pressure,  the  parts 
are  so  much  fewer,  and  the  piston  is  so  much  smaller  in  proportion  to  the 
force  acting  upon  it,  that  the  loss  from  friction  is  often  less  than  a  fourth  or 
even  a  sixth  of  the  steam  power,  although  the  resistance  of  the  air  is  to  be 
overcome  by  the  high  pressure  engine,  still  there  is  often  a  saving  on  the 
whole.  The  saving  becomes  very  considerable  if  the  steam  be  allowed  to 
act  by  its  expansion  also,  as  described  in  the  last  paragraph. 

From  misapprehension  of  the  law  of  increase  of  force  by  increase  of  heat 
in  water,  explained  by  the  table  at  page  186,  some  exceedingly  false  conclu- 
sions have  been  drawn  and  acted  upon  at  great  expense  (as  lately  by  Mr. 
Perkins)  in  attempts  to  make  engines  work  with  an  excessively  high  pres- 
sure. Besides  making  the  error  now  alluded  to,  and  others,  Mr.  Perkins 
overlooked  the  fact,  that  we  possess  no  material  for  cylinders  and  pistons 
strong  enough  to  bear  the  contemplated  pressure  and  friction  even  for  a 
moderate  time.  Perhaps  more  striking  examples  could  not  be  adduced  of 
the  absurdities  into  which  even  highly  ingenious  men  may  fall,  when  not 
sufficiently  acquainted  with  the  general  truths  of  nature  on  which  the  arts 
which  occupy  them  are  founded,  than  in  the  history  of  supposed  inventions 
and  improvements  connected  with  the  steam-engine. 

The  fertile  genius  of  Jame«  Watt  did  not  stop  at  the  accomplishment  of  the 
two  or  three  important  particular's  described  above,  but^throughout  the  whole 


EXPLOSION.  193 

detail  of  the  component  parts,  and  of  the  various  applications  of  the  engine, 
he  contrived  miracles  of  simplicity  and  usefulness.  We  should  exceed  the 
prescribed  boun'ds  of  this  work  by  entering  more  minutely  into  the  subject ; 
but  we  may  remark  that,  in  the  present  perfect  state  of  the  engine,  it  appears 
a  thins  almost  endowed  with  intelligence.  It  regulates  with  perfect  accuracy 
and  uniformity  the  number  of  its  strokes  in  a  given  time,  counting  or  record- 
ing them,  moreover,  to  tell  how  much  work  it  has  done,  as  a  clock  records 
the  beats  of  its  pendulum  ; — it  regulates  the  quantity  of  steam  admitted  to 
work  ; — the  briskness  of  the  fire  ; — the  supply  of  water  to  the  boiler  ; — the 
supply  of  coals  to  the  fire  ; — it  opens  and  shuts  its  valves  with  absolute  pre- 
cision as  to  time  and  manner ; — it  oils  its  joints ; — it  takes  out  any  air  which 
may  accidentally  enter  into  parts  which  should  be  vacuous ; — and  when  any 
thing  goes  wrong  which  it  cannot  of  itself  rectify,  it  warns  its  attendants  by 
ringing  a  bell : — yet  with  all  these  talents  and  qualities,  and  even  when  exert- 
ing the  force  of  hundreds  of  horses,  it  is  obedient  to  the  hand  of  a  child  ; — 
its  aliment  is  coal,  wood,  charcoal,  or  other  combustible  ; — it  consumes  none 
while  idle  ; — it  never  tires,  and  wants  no  sleep ; — it  is  not  subject  to  malady 
when  originally  well  made ;  and  only  refuses  to  work  when  worn  out  with 
age; — it  is  equally  active  in  all  climates,  and  will  do  work  of  any  kind; — it 
is  a  water-pumper,  a  miner,  a  sailor,  a  cotton-spinner,  a  weaver,  a  blacksmith, 
a  miller,  &c.,  &c. ;  and  a  small  engine  in  the  character  of  a  steam  pony,  may 
be  seen  draging  after  it  on  a  rail-road  a  hundred  tons  of  merchandize,  or  a 
regiment  of  soldiers,  with  thrice  the  speed  of  our  fleetest  horse  coaches.  It 
is  the  king  of  machines,  and  a  permanent  realization  of  the  Genii  of  Eastern 
fable,  submitting  supernatural  powers  to  the  command  of  man. 

We  need  not  wonder  that  the  inventor  of  an  engine  having  such  qualities, 
should  be  deemed  deserving  of  the  highest  honours  from  his  fellow-men.  In 
November,  1825,  a  public  meeting  was  called,  to  vote  a  monument  to  WATT, 
then  not  long  deceased;  and  the  most  distinguished  men  of  the  empire,  of 
all  parties,  philosophers  and  statesmen,  met  to  vie  with  each  other  in  speak- 
ing his  praise.  Perhaps  a  series  of  such  eloquent  discourses  has  rarely  been 
pronounced  at  one  time  ;  but  perhaps  in  the  progress  of  the  arts  of  civiliza- 
tion there  can  rarely  be  offered  such  motive  and  occasion.  The  common 
voice  of  that  assembly  scarcely  exaggerated,  when  attributing  to  WATT'S 
genius  and  perseverance  that  increase  of  our  national  commerce  and  riches, 
which  had  enabled  free  Britian,  single-handed,  at  an  extraordinary  crisis  of 
human  affairs,  to  contend  with  Europe  combined  against  her,  and  at  last  to 
triumph,  so  as  to  secure  her  own  happy  destinies,  and  probably  much  to 
influence  those  of  the  human  race. 

As  science  and  the  twin  sister  art  are  making  constant  advances,  who 
shall  say  that  even  the  steam-engine,  perfect  as  we  have  described  it,  forms 
the  limit  to  human  discovery  of  mighty  yet  obedient  force  ?  It  is  true  that 
the  nature  of  steam,  and  the  laws  of  its  formation  and  action,  are  now  so 
well  understood,  that  the  intelligent  engineer  no  more  hopes  for  great 
improvement  in  steam-engines,  than  he  hopes  for  it  in  the  mode  of  using 
a  waterfall  to  turn  a  mill ;  but  still  there  are  kindred  regions  of  nature  left 
almost  unexplored.  We  shall  have  occasion  to  make  a  remark  on  this 
subject  in  our  chapter  on  the  nature  of  heat. 

The  explosion  of  gunpowder  and.  of  all  fulminating  mixtures  bears  so  strong 
an  analogy  to  the  phenomenon  of  the  formation  of  steam,  that  the  mind 
may  advantageously  contemplate  the  subject  in  this  place. 

The  ingredients  of  which  gunpowder  is  formed  are  chiefly  substances 

13 


194  PNEUMATICS. 

which,  when  separate,  exist,  at  any  common  temperature,  in  the  form  of 
air;  and  the  combustion  sets  them  loose,  with  a  production  of  intense  heat, 
causing  an  increase  of  volume  which  is  instantaneous,  and  almost  irresistible. 
By  experiment  and  mathematical  deduction,  it  appears  that  the  exploding 
particles  begin  to  separate  from  each  other  with  a  velocity  as  if  ten  thou- 
sand volumes  of  air  had  been  condensed  into  one  :  and  this  explains  the 
corresponding  force  and  swiftness  with  which  a  bullet  is  propelled. 

All  the  fulminating  metals  are  chiefly  combinations  of  the  like  sub- 
stances with  the  metals  ;  and  the  ingredients  are  held  together  by  so  slight 
a  tie,  that  a  little  friction  or  elevation  of  temperature  disunites  them  so  as 
to  produce  the  explosion. 

The  escape  of  condensed  air  from  the  chamber  of  an  air-gun,  is  a  species 
of  explosion ;  but  is  very  gentle  compared  with  the  shock  of  discharged 
gunpowder. 

It  has  lately  been  shown  that  a  gun-barrel  may  be  connected  with  a  high- 
pressure  steam-boiler,  in  the  same  manner  as  with  a  chamber  of  condensed 
air;  and  as  the  steam  may  be  supplied  as  long  as  water  remains  in  the 
boiler,  if  bullets  be  allowed  to  fall  into  the  barrel  fast  enough,  a  hundred  or 
more  may  be  thrown  out  every  minute,  with  the  same  force  and  precision 
as  if  each  issued  from  a  common  fire-arm.  The  rapid  succession  resembles 
the  issue  of  water  from  a  jet  pipe ;  and  if  such  an  engine  were  used  in  a 
field  of  battle,  its  barrel  of  death,  made  to  point  gradually  along  a  line  of 
men,  would  mow  them  down  like  corn-stalks  before  the  scythe — none  could 
escape.  The  horrible  idea  and  proposal  have  been  excused  by  saying,  that 
to  prove  the  possibility  of  such  carnage  must  have  the  effect  of  putting  an 
end  to  war  altogether. 

The  invention  of  gunpowder,  with  the  consequent  change  of  military 
tactics,  because  it  gave  to  a  handful  of  men  possessing  it  the  mastery  over 
thousands  who  bad  it  not,  was  hailed  by  the  philosophers  of  the  day  as  a 
certain  security  against  the  relapse  of  civilized  mankind  into  such  a  state  of 
barbarism  as  followed  the  irruption  into  Europe  of  the  Goths  and  Vandals : — 
none  but  well-instructed  and  disciplined  armies  could  then  enter  a  European 
kingdom.  This  consideration,  however,  has  lost  its  interest,  since  the  in- 
vention of  printing,  and  other  changes  in  society,  have  afforded  still  better 
and  more  humane  securities. 

Besides  the  interesting  instances  above  cited  of  the  pressure  of  the  atmos- 
phere determining  whether  certain  substances  shall  or  shall  not  have 
the  form  of  air,  there  are  others  that  deserve  mention,  where  the  effect  is 
modified  by  the  mutual  attraction  of  substances. 

The  pressure  of  the  atmosphere  at  the  surface  of  the  earth  keeps  a  cer- 
tain quantity  of  air  in  combination  with  water,  so  as  to  form  part  of  the 
liquid  mass.  This  air  re-appears  at  once  on  taking  off  the  pressure.  If  we 
place  a  glass  of  water  under  the  receiver  of  an  air-pump  and  then  exhaust 
this,  the  water  is  soon  crowded  with  bubbles  of  air,  seen  adhering  to  the 
glass  all  round,  or  rising  through  the  water.  This  admixture  of  air  in 
water  is  necessary  to  the  life  of  fishes.  It  is  driven  off  by  boiling,  and  hence 
the  vapid  taste  of  water  that  has  recently  been  boiled. 

In  the  making  of  beer,  wine,  and  other  fermented  liquors,  there  is  formed, 
during  the  fermentation,  a  large  quantity  of  the  substance  called  carbonic 
acid.  Much  of  it  flies  off  in  its  usual  form  of  gas,  but,  because  of  the  pres- 
sure of  the  atmosphere,  much  still  remains  in  union  with  the  liquid.  On 


PRESSURE    AFFECTING    TEMPERATURE.  195 

removing  this  pressure  suddenly,  the  liquid  appears  almost  to  boil,  as  when 
a  glass  of  warm  beer  is  placed  in  the  air-pump  vacuum, 

A  degree  of  pressure  still  greater  than  that  of  the  atmosphere  keeps  a  pro- 
portionably  larger  quantity  of  this  carbonic  acid  in  liquid  combination;  as  in 
bottled  porter  or  sparkling  champagne  before  the  cork  is  drawn  ;  but  as  soon 
as  the  compression  maintained  by  the  cork  is  removed,  the  gas  escapes,  caus- 
ing the  thin  champagne  to  sparkle,  and  the  more  viscid  beer,  which  retains 
the  little  bubbles  as  they  rise,  to  be  covered  with  froth.  After  the  spark- 
ling or  frothing  has  ceased  under  the  atmospheric  pressure,  the  phenomenon 
may  be  renewed  by  placing  the  glass  in  the  air-pump  receiver. 

Carbonic  acid  so  rtadily  becomes  liquid  when  its  attraction  for  water  assists 
the  compression,  that  enough  of  it  may  be  united  with  water  to  make  a  pint 
become  a  pint  and  a  half.  The  soda  water,  or  aerated  water,  now  so  gene- 
rally used  as  a  drink  in  warm  weather,  is  water  with  several  times  its  bulk 
of  carbonic  acid  forced  into  it  by  pressure  ;  and  a  part  of  this  is  seen  escaping 
always  at  the  instant  of  the  confining  cork  being  drawn. 

Carbonic  acid  forms  nearly  half  of  the  substance  of  marble  or  lime-stone. 
"When  an  acid  with  stronger  attraction,  as  vinegar  or  sulphuric  acid,  is 
poured  upon  marble,  it  dispossesses  the  carbonic  acid,  and  unites  itself  with 
the  pure  lime.  The  carbonic  acid  in  rising,  constitutes  the  effervescence  which 
then  appears.  Carbonic  acid,  for  the  manufacture  of  the  common  soda  water 
and  other  aerated  drinks,  is  obtained  in  this  way. 

Many  mineral  waters  contain  carbonic  acid,  which  remains  in  tranquil 
combination,  while  the  water  is  bearing  a  certain  pressure  under  ground,  but 
which  in  part  escapes  as  soon  as  the  water  issues  to  the  air  and  only  the 
atmospheric  pressure  remains :  such  waters  are  called  sparkling  waters. 

The  reason  that  champagne  and  aerated  waters  are  so  cool  when  first 
decanted  is,  that  the  carbonic  acid,  in  assuming  its  gaseous  form,  absorbs 
as  latent  heat,  a  large  proportion  of  the  heat  which  was  previously  existing 
in  the  liquid. 

The  atmospheric  pressure,  by  making  the  density  of  the  air  in  any  place 
dependant  upon  the  height  of  the  place  above  the  level  of  the  sea,  causes 
corresponding  differences  of  temperature. 

The  explanation  of  this  is  simple.  If  a  gallon  of  air,  at  the  surface  of  the 
earth,  contain  a  certain  quantity  of  heat,  this  must  be  diffused  equally  through 
the  space  of  the  gallon;  but  if  the  air  be  then  compressed  into  one-tenth  of 
the  bulk,  there  will  be  ten  times  as  much  heat  in  that  tenth  as  there  was  be- 
fore ;  the  increase  affecting  the  thermometer  to  an  extent  modified  by  circum- 
stances explained  in  a  future  part  of  this  work.  In  like  manner,  if  by  taking 
off  pressure,  the  gallon  be  made  to  dilate  to  ten  gallons,  the  heat  will  be  in 
the  same  degree  diffused,  and  any  one  part  will  be  colder  than  before.  It  is 
known  that  air  may  be  so  much  compressed  under  the  piston  of  a  syringe, 
that  the  heat  in  it,  similarly  concentrated,  becomes  intense  enough  to  inflame 
tinder  attached  to  the  bottom  of  the  piston ; — this  means,  under  the  name  of 
the  match-syringe,  being  in  common  use  for  obtaining  an  instantaneous  light. 

Now,  for  the  reason  here  explained,  the  air  near  the  surface  of  the  earth, 
forming  the  bottom  of  the  atmosphere,  because  condensed  by  the  weight  of 
the  air  above  it,  is  much  warmer  than  if  it  were  suddenly  carried  higher  up, 
to  where,  from  the  pressure  being  less,  it  would  be  more  expanded  or  thin. 
In  many  cases  the  height  of  mountains  may  be  estimated  by  the  difference  of 
temperature  observed  at  the  bottom  and  at  the  top.  While  a  thermometer 


196  PNEUMATICS. 

stands  at  60°  at  the  bottom  of  St.  Paul's  Cathedral,  in  London,  another 
marks  only  58°  at  the  top  of  the  dome  ;  and  in  the  lofty  ascent  of  a  balloon, 
the  thermometer  soon  falls  to  the  freezing  point  and  below  it,  the  cold  to 
the  aeronaut  becoming  almost  insupportable. 

In  every  part  of  the  earth,  at  a  certain  elevation  in  the  atmosphere,  differ- 
ent according  to  the  latitude  or  proximity  to  the  equator,  the  thermometer 
never  rises  above  the  freezing  point, — and  this  limit  in  the  atmosphere  is 
called  the  line  or  level  of  perpetual  congelation.  In  Norway  it  is  at  five 
thousand  feet  above  the  level  of  the  sea ;  in  Switzerland  at  six  thousand  five 
hundred;  in  Spain  and  Italy  at  seven  thousand;  farther  south,  at  Teneriffe, 
at  nine  thousand ;  directly  under  the  sun,  as  in  central  Africa,  and  among  the 
Andes  in  America,  it  is  about  fourteen  thousand.  We  see  therefore  why  the 
snow-capt  mountains  are  not  the  tenants  only  of  high  northern  and  southern 
latitudes.  In  this  effect  of  elevation  which  renders  many  of  the  tropical 
regions  of  the  earth  not  only  tolerable  abodes  for  man,  but  as  suitable  as  any 
others,  contrary  to  the  opinion  of  the  ancient  philosophers  of  Europe,  who 
accounted  them  by  reason  of  the  great  heat,  an  everlasting  barrier,  as  regard- 
ed man,  between  the  northern  and  southern  hemispheres.  Much  of  the 
tropical  land  of  America  is  so  raised,  that  it  rivals,  as  to  agreeable  tempera- 
ture, even  a  European  climate ;  while  the  lightness  and  purity  of  the  air,  and 
the  brightness  of  the  sun,  add  delightfully  to  its  charms.  The  vast  expanse 
of  table  land  forming  the  empire  of  Mexico  is  of  this  kind,  enjoying  the 
immediate  proximity  of  the  sun,  and  yet,  by  its  elevation  of  seven  thousand 
feet  above  the  level  of  the  ocean,  possessing  the  most  healthful  freshness. 

The  land  in  many  parts  has  the  fertility  of  a  cultivated  garden,  and  can 
produce  naturally  most  of  the  riches  which  vegetation  offers  over  the  diver- 
sified face  of  the  globe.  The  plains  of  Columbia  in  South  America,  and 
indeed  all  along  the  ridge  of  the  Andes,  are  similarly  circumstanced.  The 
contrast  is  very  striking,  after  sailing  a  thousand  miles  up  the  level  river 
Magdelena,  in  a  heat  scarcely  equalled  on  the  plains  of  India,  at  once  to 
climb  to  the  table-land  above,  where  Saute  Fe  de  Bogota,  the  capital  of  the 
republic,  is  seen  smiling  over  interminable  plains,  that  bear  the  livery  of  the 
fairest  fields  of  Europe. 

Persons  not  understanding  the  law  which  we  are  now  illustrating,  will 
express  surprise  that  wind  or  air  blowing  down  upon  them  from  a  snow- 
clad  mountain,  should  still  be  warm  and  temperate.  The  truth  is,  that  there 
is  just  as  much  heat  combined  with  an  ounce  of  the  air  on  the  mountain  top 
ae  in  the  valley;  but  above,  the  heat  is  diffused  through  a  space  perhaps 
twice  as  great  as  when  below,  and,  therefore,  is  less  sensible.  It  may  be  the 
same  air  which  sweeps  along  as  a  warm  gale  on  a  plain  at  the  foot  of  a  moun- 
tain,— which  then  rises  and  freezes  water  on  the  summit — and  which  in  an 
hour  after,  or  less,  is  playing  among  the  flowers  of  another  valley,  as  warm 
and  genial  as  before. 

As  the  temperature  in  different  parts  of  the  atmosphere  depends  thus 
upon  the  rarity  of  the  air,  and  therefore  upon  the  height,  the  vegetable  pro- 
ductions of  each  distinct  region  or  elevation  are  of  a  distinct  character  ;  and 
many  other  peculiarities  of  place  and  climate  acknowledge  the  same  cause. 

Because  the  atmospheric  pressure  determines  the  temperature  of  the  air  in 
different  situations,  as  BOW  explained,  it  has  also  a  corresponding  influence 
upon  the  state  of  aerial  humidity,  which  is  modified  by  the  temperature. 

It  was  explained  at  page  184.  that  water  and  other  liquids  under  a  vacuum, 


PRESSURE    AFFECTING    TEMPERATURE.  197 

rise  in  the  form  of  air  or  vapour  with  force,  and  in  quantity  having  a  strict 
relation  to  the  temperature — heat  being  in  fact  the  cause  of  their  rising;  and 
the  table  at  page  186  exhibits  the  force,  and  therefore  the  density  of  watery 
vapour  corresponding  to  some  certain  temperatures.  Now  it  is  a  remarkable 
circumstance,  that  vapour  in  the  same  quantity  and  of  equal  tension  rises 
from  any  liquid,  whether  placed  under  the  pressure  of  air,  or  under  a  vacuum ; 
only  through  a  space  containing  air  it  diffuses  itself  more  slowly  than  if  the 
air  were  not  present.  As  regards  the  former  case,  it  was  for  a  long  time 
supposed  that  the  air  dissolved  a  liquid  as  a  liquid  dissolves  a  salt:  but  it 
now  appears  that  there  is  merely  a  mechanical  mixture  of  the  two.  If  the 
vapour,  while  rising  from  a  liquid,  has  not  a  tension  or  elastic  force  equal 
to  the  pressure  of  the  atmosphere  the  process  is  tranquil,  and  is  called 
evaporation,  and  it  goes  on  only  as  the  vapour  can  diffuse  itself  among  the 
particles  of  the  air,  and  therefore  slowly  in  air  perfectly  quiescent,  but  quicker 
as  the  air  is  moving  more,  or  as  the  density  of  the  air  is  less.  But  when 
the  vapour,  owing  to  a  greater  heat,  is  strong  enough  to  overcome  the  atmos- 
pheric pressure  of  fifteen  pounds  per  inch,  and  the  weight  of  a  certain  quantity 
of  liquid  over  it,  the  phenomena  of  boiling  arises  as  already  described. 

For  the  reason  now  explained,  the  air  of  our  atmosphere  contains  diffused 
through  it  a  large  quantity  of  invisible  aeriform  water;  and  if  there  were  no 
intestine  motion,  and  no  changes  of  temperature  in  the  atmosphere,  the 
quantity  of  water  would  soon  everywhere  reach  a  maximum,  or  would  be 
the  greatest  that  the  temperature  of  the  place  could  support ;  instead  of  this, 
however,  from  a  variety  of  causes  to  be  explained  below,  the  air  is  moving 
about  constantly  as  winds,  and  the  local  temperatures  are  ever  fluctuating, 
and  when  the  temperature  is  lowered,  in  situations  where  a  maximum  of 
watery  vapour  is  present,  part  of  this  is  instantly  reduced  to  the  state  of 
water  again,  and  appears,  according  to  circumstance,  in  the  form  of  mist, 
rain,  snow  or  hail;  while  to  supply  material  for  these  phenomena,  evapo- 
ration is  going  on  wherever,  over  water,  there  is  not  a  maximum  of  vapour 
in  the  air.  These  opposing  operations  of  evaporation  and  condensation  keep 
up  that  constant  circulation  of  moisture  which  is  the  life  of  nature. 

When  a  given  quantity  of  water  assumes  the  aeriform  state,  it  contains  the 
same  quantity  of  latent  heat  in  all  cases,  whether  rising,  for  instance,  from 
a  boiling  caldron,  or  from  the  surface  of  the  lake.  Hence  we  see  why  evapo- 
ration is  so  cooling  a  process  to  any  liquid  or  moistened  solid  from  which  it 
is  arising  :  and  as  we  have  already  shown  that  a  rapid  passing  of  dry  air,  or 
the  substance  being  placed  in  a  vacuum,  quickens  evaporation,  we  now  see 
why  both  of  these  conditions  accelerate  the  cooling.  Wet  linen  placed  in  a 
strong  wind,  which  does  not  contain  a  maximum  of  moisture,  becomes  dry 
almost  immediately ;  a  bottle  of  wine  ccTvered  with  a  wet  cloth  and  suspended 
in  a  current  of  air,  as  is  practised  in  warm  climates  to  prepare  wine  for  the 
table  is  quickly  cooled;  mats  hung  around  the  walls  of  houses  in  India,  and 
frequently  wetted  through  the  day,  preserve  a  delightful  freshness  in  the 
apartments.  Sprinkling  water  or  vinegar  over  a  hot  sick  room  cools  and 
refreshes  it ;  and  watering  the  streets  of  a  city  moderates  in  them  the  inten- 
sity of  summer  heat.  In  warm  climates  water  is  cooled  for  drinking  by 
being  put  into  vessels  so  porous  that  the  external  surface  is  always  moist, 
the  vessel  being  then  suspended  in  a  current  of  air,  or  during  a  calm  being 
made  to  vibrate  in  the  manner  of  a  pendulum.  Again,  the  rapidity  of  evapo- 
ration from  water  under  the  exhausted  receiver  of  an  air-pump,  and  particu- 
larly when  some  other  substance  which  powerfully  absorbs  watery  vapour  is 
included  in  the  receiver,  is  so  rapid,  and  carries  off  the  heat  so  quickly,  that 


198  PNEUMATICS. 

the  mass  of  water  freezes  before  much  of  it  has  been  carried  away.  This 
process  is  used  for  making  ice  in  India. 

It  is  partly  because  air  saturated  with  moisture,  that  is  to  say,  having 
as  much  water  diffused  in  it  as  can  be  supported  in  the  invisible  or  aeriform 
state  of  the  existing  temperature, — lets  fall  a  part  on  any  reduction  of  the 
temperature,  that  air  which  as  a  portion  of  the  atmosphere,  has  been  heated 
by  the  sun  during  the  day,  and  has  received  such  moisture,  lets  it  fall  again 
during  the  night,  and  exhibits  the  night  fogs  of  certain  seasons,  which  float 
upon  the  surface  of  the  earth,  until  again  acted  upon  by  the  beams  of  the 
next  morning's  sun.  Fog,  when  farther  condensed,  by  groups  of  the  minute 
particles  uniting,  forms  rain;  and  rain  when  cooled  becomes  snow  or  hail. 

The  quantity  of  de\7  which  falls  at  night  is  influenced  by  the  quantity  of 
moisture  taken  up  by  the  atmosphere  during  the  heat  of  the  day;  and  the 
immediate  cause  of  the  dew  is,  as  was  ingeniously  proved  by  Dr.  Wells, 
some  years  ago,  that  the  temperature  of  the  objects  on  which  it  settles  has 
become  lower  during  the  night  than  that  of  the  air  around,  and  than  is 
required  to  maintain  in  the  invisible  state,  the  moisture  in  the  surrounding 
atmosphere.  There  is  a  tendency  in  heat  to  diffuse  itself  uniformly  among 
bodies,  by  a  constant  radiation  from  one  to  another,  rapid  in  proportion  to 
the  differences  of  temperature,  and  which,  if  continued,  would  reduce  all  to 
the  same  degree.  The  earth,  therefore,  during  the  day,  receives  radiated 
heat  from  the  sun,  and  becomes  comparatively  hot,  and  during  the  night  it 
gives  out  heat  again  by  radiation  towards  the  sky,  from  which  there  is  little 
or  no  return.  When  there  are  clouds  in  the  atmosphere  at  night,  they 
receive  the  heat  darted  upwards  from  the  bodies  on  the  earth's  surface,  and 
they  radiate  heat  back,  becoming  thus,  as  it  were,  a  clothing  to  maintain  the 
warmth  of  the  earth  beneath  them, — and  on  cloudy  nights  there  is  no  dew, — 
but  with  a  clear  sky,  the  heat  radiated  upwards,  darts  into  boundless  space, 
and  is  lost  altogether  to  the  objects  which  emitted  it.  These  objects,  there- 
fore, which  during  the  day  had  the  same,  or  even  a  higher  temperature  than 
the  atmosphere  around,  now  become  colder,  and  the  aeriform  water  which 
comes  in  contact  with  them  is  condensed,  and  forms  what  we  call  dew.  This 
beautiful  provision  of  nature  supplies  the  necessary  moisture  to  vegetables 
during  seasons  when  rain  is  deficient.  Dew  on  very  cold  objects  freezes  as 
it  settles,  and  is  then  called  hoar  frost.  A  phenomenon  which  may  be 
classed  with  dew,  is  the  perspiration,  as  it  is  vulgarly  called,  of  massive 
walls  and  furniture,  occurring  on  the  sudden  setting  in  of  warm  weather,  or 
on  the  occasion  of  a  warm  moist  air  of  higher  temperature  than  the  walls 
being  suddenly  introduced,  as  when  a  crowd  assembles  in  a  cold  church  : — 
the  wall  or  other  object  then,  from  not  having  yet  acquired  the  temperature 
of  the  surrounding  air,  condenses  upon  itself  a  copious  deposition  of  the 
atmospheric  moisture.  For  a  similar  reason  a  bottle  of  wine  brought  from  a 
cold  cellar  or  from  an  ice-pail,  into  a  room  with  company,  is  soon  covered 
with  thick  moisture  or  dew;  as  are  the  glasses  also  into  which  the  wine  is 
poured.  It  is  another  phenomenon  of  the  same  kind  when  we  see  the 
moisture  of  warm  breath  condensed  on  any  cold  polished  surface,  as  on  a 
mirror's  face,  or  on  the  glasses  of  a  carriage  shut  up,  or  on  the  windows  of 
a  room  in  winter,  when  the  surface  is  very  cold,  the  moisture  being  frozen 
with  the  appearance  of  beautiful  aboresence. 

Many  instruments  have  been  contrived,  with  the  name  of  hygrometers, 
for  indicating  the  quantity  of  water  in  the  atmosphere.  A  prepared  human 
hair  is  the  essential  part  of  one  of  the  best  of  those  formerly  used;  the 
lengthening  or  shortening  of  the  hair,  according  to  the  quantity  of  moisture 


PRESSURE    AFFECTING    MOISTURE.  199 

around  it,  being  caused  to  move  an  index  like  that  of  a  wheel-barometer,  to 
mark  the  degrees.  This,  however,  and  other  common  hygrometers,  are  only 
philosophical  toys;  but  Mr.  Daniel  (see  his  excellent  work,  entitled  Meteoro- 
logical Essays)  has  lately  given  to  the  philosophical  world  a  correct  and 
simple  instrument  for  .the  purpose,  depending- on  the  principle  explained 
above, — that  whenever  the  temperature  of  a  body  in  the  atmosphere  is 
reduced  below  that  at  which  the  quantity  of  watery  vapour  in  the  air  around 
it  can  be  maintained  in  the  aeriform  or  invisible  state,  dew  forms  on  the 
body.  His  apparatus  consists  of  a  bulb  of  glass,  which  can  be  cooled  to  any 
desired  degree  from  being  connected  with  another  bulb  enveloped  in  an 
evaporating  liquid;  and  when  moisture  begins  visibly  to  settle  upon  the  first, 
its  temperature  is  exhibited  on  a  thermometer  enclosed  within  it;  and  the 
proportion  of  water  mixed  with  the  air  around  is  then^,  as  indicated  by  the 
table,  partially  copied  here,  at  page  186. 

A  great  fall  of  the  barometer  marks  a  diminished  pressure  in  the  atmos- 
phere around,  with  a  consequent  dilatation  of  the  air  and  fall  of  temperature, 
as  explained  a  few  pages  back ;  and  if  the  air  at  such  a  time  hold  a  maxi- 
mum of  moisture,  a  part  of  this  must  become  visible  as  fog  or  rain.  Thua^ 
a  fall  of  the  barometer,  a  fall  of  temperature,  and  a  fall  of  rain,  often  occur 
as  associated  phenomena. 

Illustrating  this  by  experiment,  we  find,  that  on  the  extraction  of  air  from 
the  receiver  of  an  air-pump,  a  cloud  of  mist  generally  appears  in  it  with  the 
first  strokes  of  the  piston  : — the  reason  being  that  the  still  remaining  air, 
because  cooled  by  the  refraction,  absorbs  heat  from  the  vapor  in  combina- 
tion with  it,  and  renders  the  water  visible.  The  mist  is  then  removed  by 
the  subsequent  action  of  the  machine,  or  is  re-dissolved  when  the  usual 
quantity  of  air  is  re-admitted. 

We  understand  from  this  why  rain  happens  so  much  more  frequently 
among  mountains  than  on  extended  plains.  When  air  saturated  with  mois- 
ture approaches  the  mountain  ridge  to  rise  over  it.  for  every  foot  that  it  rises 
it  escapes  from  a  degree  of  the  pressure  which  it  bore  while  lower  down, 
and  in  then  dilating,  it  becomes  colder,  and  lets  fall  part  of  its  moisture. 
It  is  the  rain  copiously  thus  produced  in  mountainous  regions  which  consti- 
tutes the  chief  supply  of  their  many  rivers,  and  which,  with  periodical 
changes  of  wind  bringing  more  moisture,  causes  the  extraordinary  annual 
overflowing  of  such  rivers  as  the  Nile,  the  Ganges,  &c. 

Those  who  have  visited  the  Cape  of  Grood  Hope,  will  recollect  a  striking 
phenomenon  illustrative  of  our  present  subject,  observed  there  when  the  wind  .. 
blows  from  the  south-east.  Beyond  the  city,  as  viewed  from  the  bay,  there 
is  a  mountain  of  great  elevation,  called  from  its  extended  flat  summit,  the 
Table  Mountain.  In  general  its  rugged  steeps  are  seen  rising  in  a  clear  sky ; 
but  when  the  south-east  wind  blows,  the  whole  summit  becomes  enveloped 
in  a  cloud  of  singular  density  and  beauty.  The  inhabitants  call  the  pheno- 
menon the  spreading  of  the  table-cloth.  The  cloud  does  not  appear  to  be  at 
rest  on  the  hill,  but  to  be  constantly  rolling  onward ;  yet  to  the  surprise  of 
the  beholder,  it  never  descends,  for  the  snowy  wreaths  seem  falling  over  the 
precipice  towards  the  town  below,  vanish  completely  before  they  reach  it, 
while  others  are  formed  on  the  other  side  to  replace  them.  The  reason  of 
the  phenomena  is  this  :  The  air  constituting  the  wind  from  the  south-east 
having  passed  over  the  vast  southern  ocean,  comes  charged  with  as  much 
invisible  moisture  as  its  temperature  can  sustain.  In  rising  up  the  side  of 
the  mountain  it  is  rising  in  the  atmosphere,  and  is  therefore  gradually  escap- 
ing from  a  part  of  the  pressure  lately  borne ;  and  on  attaining  the  summit  it 


200  PNEUMATICS. 

has  dilated  so  much,  and  has  consequently  become  so  much  colder,  that  it 
lets  go  part  of  its  moisture.  This  then  appears  as  the  cloud  just  described; 
but  it  no  sooner  falls  over  the  edge  of  the  mountain,  and  again  descends  in 
the  atmosphere  to  where  it  is  pressed,  and  condensed,  and  heated  as  before, 
than  it  is  re-dissolved  and  disappears  : — the  magnificent  apparition  dwelling 
only  on  the  mountain  top. 

When  the  elevation  to  which  moisture  is  suddenly  carried  is  very  great, 
the  fall  of  temperature  is  proportioned,  and  the  separating  water  becomes 
snow  instead  of  rain.  This  phenomenon  is  remarkably  illustrated  by  a  great 
Hiero's  fountain,  used  in  one  of  the  mines  of  Hungary ;  during  the  play  of 
which,  the  air  in  one  place  is  so  compressed,  that  on  being  suddenly  released 
it  expands  and  cools  enough  to  cause  the  moisture  driven  out  with  it  to 
appear,  even  in  summer,  as  a  shower  of  snow.  . 

The  foregoing  reasoning  explains  why,  along  the  sides  o/  mountain  ridges, 
clouds  are  generally  seen  floating  at  a  certain  height  only,  and  therefore  in 
horizontal  strata.  The  water  is  separated  from  the  air  at  a  certain  tempera- 
ture, which  is  dependent  on  the  height,  and  above  that  height  the  air  is  at 
the  time  too  dry  and  rare  to  have  clouds.  Very  lofty  summits  are  always 
seen  much  above  the  clouds,  and  the  admirer  of  nature  who  climbs  towards 
them,  may  often  contemplate  the  grand  phenomena  of  the  thunder-storm 
far  beneath  his  feet.  Teneriffe  soars  so  sublimely,  that  the  distant  sailor 
not  unfrequently  mistakes  the  line  of  clouds  hanging  around  its  sides  for 
the  white  streak  which  elsewhere  indicates  the  cliffs  and  waves  of  the 
sea-shore. 

fluid  support  or  floating  in  air.     (Read  the  Analysis,  page  156.) 

When  it  was  explained  under  "  Hydrostatics,"  that  any  body  immersed  in 
a  fluid  has  its  downward  tendency  or  weight  resisted  with  exactly  the  force 
which  supported  the  quantity  of  the  fluid  previously  occupying  the  same 
space,  and  therefore  that  the  body  will  sink  or  swim,  according  as  it  is 
heavier  or  lighter  than  its  bulk  of  the  fluid,  the  reasoning  was  as  applicable 
to  the  case  of  a  body  immersed  in  an  air  or  gas  as  in  a  liquid. 

We  hence  see  why  a  body  weighed  in  an  air  appears  lighter,  by  the  exact 
weight  of  its  bulk  of  the  air,  than  when  weighed  in  an  empty  space  or 
vacuum ; — and  why,  for  the  same  reason,  the  jocular  question,  whether  a 
pound  of  lead  or  a  pound  of  cork  be  the  heavier,  it  is  not  truly  answered  by 
saying  that  they  are  of  equal  weight;  the  cork  being  really  the  heavier,  for 
when  balanced  in  air,  bulky  cork  is  more  supported  than  dense  lead.  A 
small  weighing  beam  having  attached  to  its  opposite  ends  pieces  of  cork  and 
lead  which  equipoise  in  the  air,  if  placed  under  the  exhausted  receiver  of  an 
air-pump,  quickly  exhibits  the  cork  preponderating. 

As  any  liquid  lighter  than  water,  such  as  oil  or  spirits,  on  being  set  at 
liberty  under  the  surface  of  water,  will  rise,  while  any  heavier  liquid,  such 
as  brine,  syrup,  or  sulphuric  acid,  will  sink ;  and  in  both  cases  with  force 
proportioned  to  the  difference  of  specific  gravities :  so  we  find  that  in  com- 
mon air,  a  mass  of  hydrogen,  or  hotter  air,  ascends,  because  specifically 
lighter ;  while  oxygen,  carbonic  acid  gas,  or  colder  air,  descends,  because 
specifically  heavier.  This  truth  is  well  exemplified  in 

The   Balloon,  - 

which  is  a  thin  light  bag  of  varnished  silk,  generally  shaped  like  a  globe  or 
egg,  and  filled  with  a  fluid  lighter  than  common  air.    It  is  made  sufficiently 


FLOATING.  —  BALLOONS.  201 

large  that  the  difference  between  its  weight  when  filled  and  that  of  an  equal 
bulk  of  common  air,  may  enable  it  to  carry  aloft  the  material  of  which  it  is 
constructed,  with  the  aeronauts,  and  their  apparatus.  It  is  in  principle  like 
a  bladder  of  oil  immersed  in  water.  A  globe  of  thirty -five  feet  diameter 
has  a  capacity  of  nearly  twenty-two  thousand  cubic  feet.  This  quantity  of 
common  air  weighs  cft>out  sixteen  hundred  pounds^  and  the  same  quantity 
of  hydrogen  gas,  of  easily  obtained  purity,  weighs  only  one-eighth  as  much 
as  two  hundred  pounds.  Such  a  globe,  therefore,  being  buoyed  up,  or 
supported  in  common  air,  with  a  force  of  sixteen  hundred  pounds,  while,  if 
filled  with  hydrogen,  it  only  weighs  two  hundred,  will  carry  up  into  the  sky 
founteen  hundred  pounds  of  material  and  load. 

The  first  balloon  was  exhibited  by  a  man  ignorant  of  what  he  was  really 
effecting.  Seeing  the  clouds  float  high  in  the  atmosphere,  he  thought  thac 
if  he  could  make  a  cloud  and  enclose  it  in  a  bag,  it  might  rise  and  carry 
him  with  it.  Then,  erroneously  deeming  smoke  and  a  cloud  the  same,  he 
made  a  fire  of  green  wood,  wool,  &c.,  and  placed  a  great  bag  over  it  with  the 
mouth  downwards  to  receive  the  smoke.  He  soon  had  the  joy  to  see  the 
bag  full,  and,  when  set  free,  ascending ;  but  he  understood  not  that  the 
cause  was  the  hot  and  dilated  air  within,  which,  being  lighter  than  the  sur- 
rounding air  was  buoyed  up;  while  the  visible  parts  of  the  smoke,  which 
chiefly  engaged  his  attention,  was  really  heavier  than  the  air,  and  was  an 
impediment  to  his  wishes. 

This  modification  called  the  hot  air  or  fire  balloon,  was  afterwards  better 
understood,  and  was  used  by  aeronauts,  until  the  more  commodious  and  less 
dangerous  modification,  called  the  inflammable  air  balloon,  or  balloon  of 
hydrogen  gas,  was  substituted. 

Since  the  modern  introduction  of  gas  lights,  the  carburretted  hydrogen 
prepared  for  them  is  generally  employed  for  filling  balloons.  It  is  con- 
siderably heavier  than  pure  hydrogen,  but  is  so  much  more  readily  obtained, 
that  aeronauts  like  better  to  make  a  larger  balloon  to  suit  it,  than  a  smaller 
one  which  obliges  them  to  prepare  the  other.— A  thin  paper  bag,  filled  with 
the  hot  air  rising  from  a  large  lamp,  is  a  miniature  hot  air  or  fire  balloon; 
and  a  common  soap  bubble,  filled  with  hydrogen  is  a  little  inflammable  air 
balloon,  which  mounts  with  great  rapidity. 

There  are,  perhaps,  few  occasions  on  which  a  youth  is  more  surprised  and 
delighted  than  when  he  first  beholds  a  balloon  sailing  high  in  the  bosom  of 
the  air  and  bearing  a  human  being  to  regions  far  beyond  what  the  soaring 
eagle  has  ever  reached;  while  to  the  intrepid  aeronaut  himself,  the  scene  of 
a  world  displayed  beneath  him  is  unquestionably  the  grandest,  except  that 
of  the  starry  heavens,  which  mortal  eye  has  ever  compassed.  To  him  even 
wide  spread  London,  the  queen  of  the  cities  of  the  earth,  and  a  little  world 
within  itself,  when  viewed  from  a  great  elevation  in  the  sky,  appears  but 
as  a  dusky  patch  upon  a  map,  with  the  far-famed  Thames  winding  there 
as  a  silvery  line,  and  the  magnificent  temples  and  palaces  scattered  around 
appearing  but  as  darker  points  rising  out  of  the  general  mist  of  buildings, 
in  which  a  million  and  a  half  of  human  beings  reside. 

The  first  aeronautic  expeditions  astonished  the  world,  and  endless  reveries 
passed  through  mens'  minds  of  important  uses  to  which  the  new  discovery 
might  be  applied  ;  but  more  mature  reflection,  and  now  frequent  trials  have 
shown  that  the  balloon,  while  furnishing  philosophers  with  the  opportunity 
of  making  some  observations  in  elevated  regions  of  the  atmosphere,  is  still 
interesting  chiefly  as  a  philosophical  toy.  The  French,  under  the  Directory 


202  PNEUMATICS. 

in  1796,  attempted  to  use  it  as  a  military  station,  from  which  the  position 
and  motions  of  an  enemy  might  be  descried :  but  the  plan  was  eventually 
abandoned.  It  has  since  been  thought  of  as  a  means  by  which  travellers 
might  obtain  information  while  penetrating  into  unknown  countries,  like  the 
almost  interlineable  plain  of  Australasia.  Although  aeronauts,  while  aloft, 
have  the  power  of  making  the  balloon  rise  farther  by*tb  rowing  out  part  of 
the  sand-ballast  which  they  carry  with  them,  or  of  making  it  descend  by 
opening  a  valve  at  the  top,  through  which  the  hydrogen  may  escape,  still 
they  have  no  power  of  producing  a  lateral  motion.  The  idea  which  yet 
strongly  excites  the  minds  of  some  projectors,  that  by  wings  or  other  means, 
a  balloon  may  be  directed  in  the  sky  nearly  as  a  ship  is  directed  on  the«sea, 
is  not  much  more  reasonable  than  to  suppose  that  an  insect,  suspended  to  a 
huge  block  of  wood,  driven  along  at  the  rate  of  eight  or  ten  miles  an  hour 
by  river  torrent,  should  have  power  to  stop  or  sail  against  the  stream.  A 
man  in  a  balloom  would  generally  have  to  resist  or  change  a  motion  exceed- 
ing fifty  miles  in  an  hour. 

A  balloon  which  is  only  half  full  at  the  surface  of  the  earth,  becomes 
quite  full  when  it  has  risen  three  miles  and  a  half,  because,  at  that  altitude, 
air  from  below  doubles  its  volume  on  account  of  the  diminished  pressure. 
A  balloon,  therefore,  if  quite  distended  on  first  rising,  must  let  air  escape 
as  it  ascends,  or  it  will  burst :  this  is  true  also  of  the  drum  of  the  human  ear 
under  the  same  circumstances,  and  in  a  contrary  way  under  the  opposite 
circumstances  of  descending  in  a  diving-bell. 

The  downy  seeds  of  plants  seen  floating  about  upon  the  winds  of  autumn 
are  not  lighter  than  air,  but  have  so  much  bulk  and  surface  in  proportion  to 
their  weight,  that  the  friction  upon  them  of  the  moving  air,  is  greater  than 
their  weight,  and  carries  them  along. 

A  sheet  of  paper  made  in  some  degree  to  resemble  a  balloon,  by  its  having 
a  little  weight,  representing  the  hanging  car,  attached  by  threads  from  its 
angles,  is  often  seen  rising  at  a  street  corner,  to  the  delight  of  the  boy  who 
watches  it.  Its  rise  depends  upon  eddy  winds  or  currents  which  the  corner 
produces. 

The  ascent  ofjlame  and  smoke 

in  the  atmosphere,  affords  other  examples  of  a  lighter  fluid  rising  in  a 
heavier ;  for  both  these  are  merely  hotter  air  rising  in  the  midst  of  colder. 

The  phenomenon  of  flame  is  produced  when  a  burning  substance  contains 
some  ingredient  capable,  on  being  heated,  of  assuming  the  form  of  air  or  gas, 
which  ingredient,  or  ascending,  burns  or  combines  with  the  oxygen  of.  the 
atmosphere,  with  intensity  of  action  sufficient  to  produce  a  white  heat.  It 
is  because  charcoal  and  coke  have  nothing  in  them  thus  volatile,  that  they 
burn  without  flame,  appearing  like  red-hot  stones.  The  flame  of  a  lamp  or 
candle  is  merely  the  oil,  wax,  or  tallow  converted  into  gas,  and  allowed  to 
burn  as  it  is  disengaged  and  rises.  The  same  gas  obtained  by  heating  the 
oil,  &c.,  in  vessels  which  exclude  the  atmosphere,  so  as  to  prevent  immediate 
combustion,  and  from  which  tubes  lead  to  suitable  receptacles,  is  the  common 
oil-gas  used  for  illumination. 

Smoke  consists  of  all  the  dust  and  visible  particles  which  are  separated 
from  the  fuel  without  being  burned,  and  are,  moreover,  light  or  minute 
enough  to  be  carried  aloft  by  the  rising  current  of  heated  air ;  but  all  that  is 
visible  of  smoke  is  really  heavier  than  air,  and  soon  falls  again  as  powdered 
chalk  falls  in  water.  In  the  receiver  of  an  air-pump,  where  a  candle  has 
been  extinguished  by  exhausting  the  air,  the  steam  of  smoke  that  continues 


FLOATING.  —  FLAME    AND    SMOKE.  203 

to  pour  from  the  wick  after  the  exhaustion,  is  seen  to  fallen  the  pump-plate, 
because  there  is  no  air  to  support  it. 

Chimneys  quicken  the  ascent  of  hot  air  by  keeping  a  long  column  of  it 
together.  A  column  of  two  feet  high  rises,  or  is  pressed  up  with  twice  as 
much  force  as  a  column  of  one  foot,  and  so  in'proportion  for  all  other  lengths  j 
just  as  two  or  more  corks  strung  together  and  immersed  in  water,  tend  up- 
wards with  proportionally  more  force  than  a  single  cork  ;  or  as  a  long  spear  of 
light  wood,  allowed  to  ascend  perpendicularly  from  a  great  depth  in  water, 
acquires  a  velocity  which  makes  it  dart  above  the  surface,  while  a  short  piece 
under  the  same  circumstances  rises  very  slowly.  In  a  chimney  where  one 
foot  in  height  of  the  column  of  hot  air  is  one  ounce  lighter  than  the  same 
bulk  of  the  external  cold  air,  if  the  chimney  be  one  hundred  feet  high,  the 
air  or  smoke  in  it  is  propelled  upwards  with  a  force  of  one  hundred  ounces. 
In  all  cases,  therefore,  the  draught,  as  it  is  called  of  a  chimney,  is  pro- 
portioned to  its  length.  The  following  facts  are  consequences  of  this  truth. 

In.  low  cottages,  and  in  the  upper  floors  of  houses,  the  annoyance  of 
smoky  rooms  is  much  more  frequent  than  were  chimneys  are  longer. 

If  there  are  two  fires  in  the  same  room,  or  in  any  rooms  open  to  each 
other,  which  have  chimneys  of  different  lengths,  and  of  which  the  doors  and 
windows  are  very  close,  so  that  the  air  to  supply  the  draught  cannot  enter 
by  them,  the  taller  chimney  will  overpower  the  shorter,  and  cause  it  to 
smoke  into  the  room  ;  just  as  the  long  leg  of  a  syphon  overcomes  the  short 
one,  or  as  a  long  log  of  wood,  held  down  in  water  by  a  cord  passing  from  it 
round  a  pulley  at  the  bottom  to  a  shorter  log  also  floating,  will  rise,  and 
pull  down  the  shorter  log.  | 

A  long  chimney,  for  the  reasons  above  explained,  causes  a  current  of 
air  to  pass  through  the  fire  very  rapidly,  and  it  has  the  advantage  also  of 
acting  more  uniformly  than  any  bellows  or  blowing  machine.  On  these 
accounts,  of  fires  of  steam  engines,  and  many  others,  it  is  the  means  of 
blowing  generally  preferred.  The  importance  of  length,  in  a  chimney 
explains  the  remarkable  appearance  of  some  mining  districts  and  modern 
English  towns,  where  steam-engines  abound. 

When  we  heap  dying  embers  together,  so  that  the  hot  air  rising  among 
them  may  become  a  mass  or  column  of  considerable  altitude,  this  column 
has  the  effect  of  blowing  them  gently,  and  helps  to  light  them  up  .again, 
A  piece  of  burning  paper  thrown  upon  the  top  of  a  half-extinguished  fire, 
often  makes  it  blaze  afresh,  by  causing  a  more  rapid  current  of  air  to  pass 
through  it  from  below. 

The  action  or  draught  of  a  chimney,  influenced  as  we  have  seen,  by  its 
length,  depends  also  on  the  degree  in  which  the  air  in  it  is  heated,  because 
this  determines  the  dilitation,  or  comparative  lightness,  which  makes  the 
air  ascend. 

In  what  are  called  open  fire-places,  such  as  those  in  the  sitting-rooms  of 
Britain,  a  large  quantity  of  air  directly  from  the  apartment  enters  the  chimney 
above  the  fire,  and  mixes  with  the  hot  air  from  the  fire  itself.  This  mixture 
ascends  more  slowly  than  if  hot  air  alone  entered,  and  in  a  proportion 
dependent  on  the  degree  of  mixture.  The  effect  of  excluding  a  part  of  this 
colder  air,  is  seen  when  a  board  or  plate  of  metal  is  suspended  across  the 
opening  of  the  chimney,  so  as  to  narrow  the  entrance  : — almost  instantly  a 
quicker  action  is  produced,  and  the  fire  begins  to  roar  as  if  blown  by  a  bel- 
lows. This  means  is  often  used  to  blow  the  fire  instead  of  bellows,  or  to 
cure  a  smoky  chimney  by  increasing  the  draught.  What  is  call  a  register 


204  PNEUMATICS. 

stove  is  a  kindred  contrivance.  It  has  a  flap  placed  in  the  throat  of  the  chim- 
ney, which  serves  to  widen  or  contract  the  passage  at  pleasure.  Because  the 
flap  is  generally  opened  only  enough  to  allow  that  air  to  pass  which  rises 
directly  from  the  fire,  the  chimney  receives  only  very  hot  air,  and  therefore 
acts  well.  The  register  stove  often  cures  smoky  chimneys  :  and  by  preven- 
ing  the  too  ready  escape  of  the  moderately  warmed  air  of  the  room,  of  which 
so  much  is  wasted  by  a  common  fire-place,  it  also  saves  fuel.  In  what  are 
called  close  fire-places,  as  those  of  steam-engines,  or  brewers'  coppers,  when 
the  furnace  door  is  shut,  no  air  can  enter  the  chimney  but  directly  through 
the  fire  ;  hence  the  action  of  such  chimneys  is  very  powerful. 

In  a  room  with  two  fires,  or  in  drawing  rooms  communicating  with  each 
other,  although  the  chimneys  be  of  equal  length,  that  one  over  the  best  fire 
will  act  the  most  strongly ;  and  if  the  doors  and  windows  of  the  apartment  be 
so  close  as  to  prevent  a  sufficiency  of  air  from  entering  by  them  to  supply 
both  fires,  cold  air  will  enter  by  that  chimney  which  has  the  weakest  fire,  and 
the  smoke  from  it  will  spread  into  the  room.  How  often  is  an  assembling 
dinner  party  annoyed  by  the  smoke  of  a  second  drawing-room  fire  just  lighted 
before  their  arrival,  and  which  had  therefore  to  contend  with  the.  antagonist 
fire  already  in  powerful  action  all  the  day.  "While  only  one  fire  was  lighted,  the 
cold  chimney  was  admitting  the  air  to  feed  it,  just  as  an  open  pane  in  the 
window  would  have  done.  A  room  may  be  so  close  that  no  air  can  find  en- 
trance, and  in  such  a  case  the  smoke  of  its  fire  must  all  spread  into  the  room. 
When  all  the  windows  and  doors  of  a  house  fit  so  closely  as  not  to  admit 
air  for  the  acting  chimneys,  the  supply  comes  down  the  chimneys  that  are  not 
in  use.  Inattention  to  this  fact  causes  many  a  good  chimney  to  incur  the 
imputation  of  being  smoky,  because  on  the  attempt  being  made  to  light  a 
fire  at  it,  the  smoke  at  first  is  always  thrown  back.  The  truth  is,  that  at 
the  time  when  the  servant  begins  to  light  the  fire,  there  is  a  downward  cur- 
rent in  the  chimney,  repelling  of  course,  any  heated  air  and  smoke  that 
approaches  it,  and  spreading  them  over  the  whole  house;  but  were  the 
room  door  to  be  shut  for  a  few  minutes,  so  as  to  cutoff  communication  with 
the  other  drawing  chimneys  in  the  house,  while  at  the  same  time  the  windows 
were  opened,  the  chimney  would  act  at  once ;  and  when  sufficiently  heated, 
would  continue  to  act  in  spite  of  the  others,  as  well  as  they. 

There  are  some  cases  of  smoky  rooms  not  to  be  so  easily  corrected  as  what 
we  have  now  mentioned.  When  a  low  house  adjoins  a  lofty  house,  the  wind 
blowing  towards  the  latter,  is  obstructed  and  becomes  a  gathering  or  conden- 
sation of  air  against  the  wall ;  and  if  the  top  of  a  low  chimney  be  there,  the 
compressed  air  enters  it  and  pours  downwards.  The  same  happens  occasionally 
from  the  proximity  of  trees  or  rocks.  In  such  cases,  to  avoid  the  influence, 
the  chimneys  of  the  low  houses  are  often  made  very  lofty.  Again,  whenever, 
from  the  nature  of  buildings,  eddies  of  wind  occur,  or  unequal  pressures,  as 
at  street  corners,  &c.,  the  chimneys  around  do  not  act  regularly.  It  is  pro- 
verbial, that  corner  houses,  or  those  at  the  end  of  a  row,  are  smoky  houses ; 
and  we  see  the  uniformity  of  architecture  in  a  street  often  destroyed  by  the 
necessity  of  lengthening  the  chimneys  of  the  houses  at  the  extremities. 

When  smoke  is  found  descending  into  a  room  where  there  is  no  fire,  the 
empty  chimney  is  serving  as  an  inlet  for  air  to  the  house,  while  the  smoke 
of  a  neighboring  chimney  is  passing  closely  over  the  top  of  ijb. 

In  summer,  when  fires  are  not  in  use,  there  is  often  a  strong  smell  of  soot 
perceived  in  the  apartments  during  the  whole  of  the  day,  but  which  ceases 
at  night.  The  reason  is,  that  during  the  day  the  chimney  is  colder  than  the 
external  air,  and  by  condensing  the  air  which  enters  it,  causes  a  downward 


FLOATING.  —  FLAME    AND    SMOKE.  205 

current  through  the  soot.  During*he  night,  again,  when  the  external  air 
becomes  colder,  owing  to  the  absence  of  the  sun,  the  chimney,  by  retaining 
the  heat  absorbed  during  the  day,  is  hot  enough  to  warm  the  air  in  it,  and 
to  cause  an  upward  current.  These  currents,  in  chimneys  left  open  during 
the  days  and  nights  of  summer,  are  almost  as  regular  as  the  land  and  sea 
breezes  of  tr6pical  countries. 

All  these  remarks  prove  how  important  it  is  to  be  able  to  conceive  clearly 
of  the  motions  going  on,  according  to  the  simple  laws  of  matter,  in  the 
invisible  air  around  us.  Were  such  subjects  better  and  more  generally 
understood,  many  prevalent  errors  in  the  arts  of  life,  influencing  much  the 
comforts  and  health  of  the  community,  would  soon  be  corrected. 

If  we  are  filled  with  admiration  on  discovering  how  perfectly  the  simple 
law  of  a  lighter  fluid  rising  in  a  heavier,  provides  a  constantly  renewed  sup- 
ply of  fresh  air  to  our  fires,  which  supply  we  should  else  have  to  furnish  by 
the  unremitted  action  of  some  expensive  blowing  apparatus,  still  more  must 
we  admire  that  the  operation  of  this  law  should  effect  the  more  important 
purpose  of  furnishing  the  ever  renewed  supply  of  the  same  vital  fluid  to 
breathing  creatures.  The  air  which  a  man  has  once  respired  becomes  poison 
to  him ;  but  because  the  temperature  of  his  body  is  generally  higher  than 
that  of  the  atmosphere  around  him,  as  soon  as  he  has  discharged  any  air 
from  the  lungs,  it  ascends  completely  away  from  him  into  the  great  purify- 
ing laboratory  of  the  atmosphere,  and  new  air  takes  its  place.  No  art  or 
labor  of  his,  as  by  the  use  of  fans  or  punkas,  could  have  done  half  so  well 
what  this  simple  law  unceasingly  and  invisibly  accomplishes,  and  accom- 
plishes without  effort  or  even  attention  on  his  part,  and  in  his  sleeping  as  in 
his  waking  hours.  Truly  in  this,  may  he  be  said  to  be  watched  over  by  a 
kind  Providence. 

The  warming  and  ventilating  of  houses, 

is  an  important  art,  founded  chiefly  on  the  foregoing  considerations,  and  at 
present  too  little  understood,  not  only  by  the  public  at  large,  but  even  by 
medical  practitioners,  whose  management  of  disease,  though  judicious  in 
other  respects,  is  often  rendered  vain  by  error  or  omission  in  this. 

Excellent  fuel  is  so  cheap  in  Britain,  owing  to  the  profusion  which 
beds  of  rich  coal  are  scattered  in  it,  that  a  careless  domestic  expenditure  has 
arisen ;  which,  however,  instead  of  securing  the  comfort  and  health  that 
might  be  expected,  has  led  to  plans  of  warming  which  often  prove  destruc- 
tive to  both.  The  mischief  lies  chiefly  in  the  unsteadiness  or  fluctuations  of 
our  domestic  temperature ;  for  in  still  colder  countries,  and  where  fuel  is 
more  expensive,  as  in  the  north  of  continental  Europe,  the  necessity  for 
economy  has  led  to  contrivances  which  give  steady  temperature  and  impunity. 

In  cold  countries,  to  retain  and  preserve  the  heat  once  obtained,  the  houses 
are  made  with  thick  walls,  double  windows,  and  nice  fittings ;  and,  more- 
over, with  close  stoves  or  fire-places,  which  draw  their  supply  of  air,  not 
from  the  apartments  where  they  are  placed,  wasting  the  temperate  air  of 
these,  but  directly  from  without.  Thus  fuel  is  saved  to  a  great  extent,  and 
a  uniformity  of  temperature  is  produced,  both  as  regards  the  different  parts 
of  the  room,  so  that  the  occupiers  may  sit  with  comfort  where  they  please, 
and  as  regards  the  different  times  of  the  day,  for  the  stove  being  once  heated 
in  the  morning,  often  suffices  to  maintain  a  steady  warmth  until  night. 
The  temperature  can  be  carried  to  any  required  degree,  'and  sufficient 
ventilation  is  easily  effected. 

In  England,  again,  the  apartments,  with  their  open  chimneys,  may  be 


206  PNEUMATICS. 

compared  to  great  air  funnels,  constancy  pouring  out  their  warm  contents 
through  a  large  opening,  and  constantly  requiring  to  be  replenished.  They 
thus  waste  fuel  exceedingly,  because  the  chimney  being  large  enough  to 
allow  a  whole  room-full  of  air  to  pass  away  in  two  or  three  minutes,  the  air 
of  the  room  has  to  be  warmed,  not  once  in  the  course  of  the  day,  but  very 
many  times.  The  temperature  in  them  is  made  to  fluctuate  by  the  slightest 
causes,  as  the  opening  a  door,  the  omitting  to  stir  the  fire,  &c.  The  heat  is 
very  unequal  in  different  parts  of  the  room,  rendering  it  necessary  in  general 
for  the  company  to  sit  near  the  fire ;  where  they  must  often  submit  to  be 
almost  scorched  on  one  side,  while  they  are  chilled  on  the  other.  There  is 
generally  a  warm  stratum  of  air  above  the  level  of  the  chimney-piece,  sur- 
rounding, therefore,  the  upper  part  of  the  bodies  of  persons  in  the  room,  while 
a  cold  stratum  below  envelopes  the  sensitive  feet  and  legs.  As  a  very  rapid 
current  is  constantly  ascending  in  the  chimney,  a  corresponding  supply  must 
be  entering  somewhere ;  and  it  can  only  enter  by  the  crevices  and  defects 
in  the  doors,  windows,  floors,  &c.  : — now  there  is  nothing  more  dangerous 
to  health  than  to  sit  near  such  inlets,  as  is  proved  by  the  rheumatisms,  stiff 
necks  and  catarrhs,  not  to  mention  more  serious  diseases,  which  so  frequently 
follow  the  exposure.  There  is  an  old  Spanish  proverb,  thus  translated, 

"  If  cold  wind  reach  you  through  a  hole, 
Go  make  your  will  and  mind  your  soul," 

which  is  scarcely  an  exaggeration. 

Consumption  is  the  disease  which  carries  off  a  fifth  or  more  of  the  persons 
born  in  Britain;  owing  in  part,  no  doubt,  to  the  changeableness  of  the  exter- 
nal climate,  but  much  more  to  the  faulty  modes  of  warming  and  ventilating 
the  houses.  To  judge  of  the  influence  of  temperature  in  producing  this  disease 
we  may  consider, — that  miners  who  live  under  ground,  and  are  always, 
therefore,  in  the  same  temperature,  are  strangers  to  it,  while  their  brothers, 
and  relatives,  exposed  to  the  vicissitudes  above,  fall  victims, — that  butchers 
and  others,  who  live  almost  constantly  in  the  open  air,  so  as  to  be  hardened 
by  the  exposure,  enjoy  nearly  equal  immunity, — that  consumption  is  scarcely 
known  in  Russia,  \ihere  close  stoves  and  houses  preserve  a  uniform  tempera- 
ture within  doors,  while  fit  clothing  gives  safety  on  going  out, — and  that  in 
all  countries  and  situations,  whether  tropical,  temperate  or  polar,  the  fre- 
quency of  the  disease  bears  relation  to  the  degree  and  manner  of  change. 
We  may  here  remark,  also,  that  it  is  not  consumption  alone  which  springs 
from  changes  of  temperature,  but  a  great  proportion  of  acute  diseases,  and 
particularly  of  the  common  winter  diseases  of  England.  There  are  a  few 
cases  of  these  in  which  the  invalid  has  not  to  remark,  that  if  he  had  avoided 
cold  or  wet  on  some  certain  occasion,  he  might  yet  have  been  well. 

Wnile  temperature  is  thus  so  frequently  an  original  cause  of  disease,  it  is 
also  a  circumstance  of  the  very  highest  importance  in  the  treatment, — as  is 
proved  by  every  fact  bearing  upon  the  question.  We  may,  therefore,  at  first 
wonder  that  it  should  be  so  negligently  and  unskilfully  controlled  as  we  often 
see  it  'j  disease  and  death  being  thence  allowed  to  lurk,  as  it  were,  undis- 
turbed in  the  sanctuaries  of  our  homes :  but  when  we  reflect  on  the  subtile 
and  invisible  nature  of  air  and  heat,  and  that  the  science  which  detects  their 
agencies  has  been  hitherto  so  little  an  object  of  general  study,  and  is,  indeed, 
of  modern  discovery,  the  fact  is  accounted  for. 

In  England',  the  open  fire-place  is  so  generally  in  use  for  common  dwell- 
ings, and  the  cheerful  'blaze  is  accounted  so  essential  to  the  comforts  of  the 
winter  days  and  long  evenings,  that  it  would  be  difficult  to  persuade  persons 


•FLOATING. — WARMING    AND   VENTILATING.      207 

to  abandon  it :  let  us  hope,  then,  that  when  the  subject  which  we  are  now 
discussing  comes  to  be  better  and  more  generally  understood,  the  open  fire, 
with  close  flooring,  better  for  double  windows,  doors  that  fit  well,  register 
stoves,  and  good  general  management,  may  be  rendered  almost  as  efficient 
for  warming,  and  as  safe  to  health,  as  any  other  contrivance. 

The  following  considerations  present  themselves  in  this  place: — Small 
rooms  in  winter  are  more  dangerous  to  health  than  large  ones,  because  the 
cold  air,  entering  towards  the  fire  by  the  doors  and  windows,  reaches  the  per- 
sons in  the  room  before  it  can  be  tempered  by  mixing  with  the  warmer  air 
already  around  them.  Stoves  in  halls  and  stair-cases  are  useful,  because 
they  warm  the  air  before  it  enters  the  rooms ;  and  they  prevent  the  hurtful 
chills  often  felt  on  passing  through  a  cold  stair-case  from  one  warm  room  to 
another.  It  is  important  to  admit  no  more  cold  air  into  the  house  than  is 
just  required  for  the  fires  and  for  ventilation ;  hence  there  .is  a  great  error 
in  the  common  practice  of  leaving  all  the  chimneys  that  are  not  in  use  quite 
open,  each  admitting  air  as  much  as  a  hole  in  the  wall,  or  an  open  pane  in 
the  window  would  do.  Perhaps  the  best  mode  of  admitting  air  to  feed  the 
fires  is  through  tubes,  leading  directly  from  the  outer  air  to  the  fire-place,  and 
provided  with  what  are  called  throttle-valves,  for  the  regulation  of  the  quan- 
tity ;  the  fresh  air  admitted  by  them  being  made  to  spread  in  the  room 
either  at  once,  or  after  having  been  warmed  during  its  passage  inwards,  by 
coming  near  the  fire.  In  a  very  close  apartment,  ventilation  must  be  ex- 
pressly provided  for  by  an  opening  near  the  ceiling,  through  which  the  impure 
air,  rising  from  the  respiration  of  the  company,  may  pass  away.  With  an 
open  fire  the  purpose  is  effected,  although  less  perfectly,  by  the  frequent 
change  of  the  whole  air  of  the  room  which  that  construction  occasions. 

With  a  view  to  have,  in  rooms  intended  for  invalids,  the  most  perfect 
security  against  cold  blasts  and  fluctuations  of  temperature,  and  still  to  retain 
the  so  much  valued  appearance  of  the  open  fire,  a  glazed  frame  or  window 
may  be  placed  at  the  entrance  to  the  chimney  or  stove,  so  as  completely  to 
prevent  the  passage  of  air  from  the  room  to  the  fire.  The  room  will  then  be 
warmed  by  the  fire  through  the  glass,  nearly  as  a  green-house  is  warmed  by 
the  rays  of  the  sun.  It  is  true  that  the  heat  of  combustion  does'not  pass 
through  glass  so  readily  as  the  heat  of  the  sun ;  but  the  difference  for  the 
case  supposed  is  not  important.  The  glass  of  such  a  window  must,  of 
course,  be  divided  into  small  panes,  and  supported  by  a  metallic  frame-work 
to  resist  the  heat '}  and  there  must  be  a  flap  or  door  in  the  frame- work,  for 
the  purpose  of  admitting  the  fuel  and  stirring  the  fire.  Air  must  be  supplied 
to  the  fire,  as  described  above,  by  a  tube  leading  directly  from  the  external 
atmosphere  to  the  ash-pit.  The  ventilation  of  the  room  may  be  effected  by 
an  opening  into  the  chimney  near  the  ceiling;  and  the  temperature  may  be 
regulated  with  great  precision. by  a  valve  placed  in  this  opening,  and  made 
to  obey  the  dilatation  and  contraction  of  a  piece  of  wire  affixed  to  it,  the 
length  of  which  will  always  depend  on  the  temperature  of  the  room.  The 
author  contrived  the  arrangements  here  described,  for  the  winter  residence  of 
a  person  threatened  with  consumption,  and  the  happy  issue  of  that  particu- 
lar case,  and  of  others  treated  on  similar  principles,  has  led  him  to  doubt 
whether  many  of  the  patients  with  incipient  consumption  who  are  usually 
sent  to  warmer  climates,  and  who  die  there  after  suffering  hardships  on  the 
journey,  and  distress  from  the  banishment  sufficient  to  shake  even  strong 
health,  might  not  be  saved  by  judicious  treatment  in  properly  warmed  and 
ventilated  apartments,  under  their  own  roofs,  and  in  the  midst  of  affectionate 
kindred.  And  if  a  boy  be  almost  certainly  secured  from  consumption  by 


208  PNEUMATICS. 

being  made  a  miner  or  a  butcher,  may  we  not  hope  that,  when  all  the  influ- 
encing circumstances  come  to  be  better  understood,  something  of  the  same 
immunity  may  be  obtained  for  persons  in  all  the  professions- and  conditions 
of  civilized  society  ? 

It  must  not  be  supposed  that  the  remarks  made  in  this  section  exhaust 
even  nearly  the  very  important  subject  of  temperature  as  affecting  health. 
The  questions  of  clothing,  of  hot  and  cold  bathing,  of  exercise,  and  others, 
equally  belong  to  it,  but  the  consideration  of  them  falls  under  other  depart- 
ments of  study. 

Winds  or  currents  in  the  atmosphere 

are  also  phenomena,  in  a  great  measure  dependent  on  the  law,  that  lighter 
fluids  rise  in  heavier.  As  oil  let  loose  under  water  is  pressed  up  to  the 
surface  and  swims,  so  air  near  the  surface  of  the  earth,  when  heated  by  the 
sun,  rises  to  the  top  of  the  atmosphere,  and  spreads  there,  forced  up  by  the 
heavier  air  around;  this  heavier  air  rushing  inwards,  constitutes  the  wind 
felt  at  the  surface  of  the  earth.  The  cross  currents  in  the  atmosphere  arising 
as  now  described,  are  often  rendered  evident  by  the  motion  of  clouds  or 
balloons. 

If  our  globe  were  at  rest,  and  the  sun  were  always  beaming  over  the  same 
part,  the  earth  and  air  directly  under  the  sun  would  become  exceedingly 
heated,  and  the  air  there  would  be  constantly  rising  like  oil  in  water,  or  like 
the  smoke  from  a  great  fire ;  while  currents  or  winds  below  would  be  pour- 
ing towards  the  central  spot,  from  all  directions.  But  the  earth  is  constantly 
turning  round  under  the  sun,  so  that  the  whole  middle  region  or  equatorial 
belt  may  be  called  the  sun's  place :  and  therefore,  according  to  the  principle 
just  laid  down,  there  should  be  over  it  a  constant  rising  of  air,  and  constant 
currents  from  the  two  sides  of  it,  or  the  north  and  south,  to  supply  the 
ascent.  Now  this  phenomenon  is  really  going  on,  and  has  been  going  on 
ever  since  Jthe  beginning  of  the  world,  producing  the  steady  winds  of  the 
northern  and  southern  hemispheres,  called  trade  winds,  on  which  in  most 
places  within  thirty  degrees  of  the  equator,  mariners  reckon  almost  as  con- 
fidently as  on  the  rising  and  setting  of  the  sun  himself. 

The  trade  winds,  however,  although  thus  moving  from  the  poles  to  the 
equator,  do  not  appear  on  the  earth  to  be  directly  north  and  south,  for  the 
eastward  whirling,  or  diurnal  rotation  of  the  earth,  causes  a  wind  from  the 
north  to  appear  as  if  coming  from  the  north-east,  and  a  wind  from  the  south 
as  if  coming  from  the  south-east.  This  fact  is  illustrated  by  the  case  of  a 
man  on  a  galloping  horse,  to  whom  a  calm  appears  to  be  a  strong  wind  in  his 
face ;  and  if  he  be  riding  eastward,  while  the  wind  is  directly  north  or  south, 
such  wind  will  appear  to  him  to  come  from  the  north-east,  or  south-east : — 
or  again,  is  illustrated  by  the  case  of  a  small  globe  made  to  turn  upon  a  per- 
pendicular axis,  while  a  ball  or  some  water  is  allowed  to  run  from  the  top  of 
it  downwards;  the  ball  or  water  will  not  immediately  acquire  the  whirling 
motion  of  the  globe,  but  will  fall  almost  directly  downwards,  in  a  track 
which,  if  marked  upon  the  globe,  will  appear  not  as  a  direct  line  from  the 
axis  to  the  equator,  that  is,  from  north  to  south,  but  as  a  line  falling  obliquely. 
Thus,  then,  the  whirling  of  the  earth  is  the  cause  of  the  oblique  and  west- 
ward direction  of  the  trade  winds;  and  not,  as  has  often  been  said,  the  sun 
drawing  them  after  him. 

The  reason  why  the  trade  winds  at  their  external  confines,  which  are  about 
30  degrees  from  the  sun's  place,  appear  almost  directly  east}  and  become 


FLOATING.  —  WINDS.  209 

more  nearly  north  and  south  as  they  approach  the  central  line,  is,  that  at  the 
confine  they  are  like 'fluid  coming  from  the  axis  of  a  turning  wheel,  and  which 
has  approached  the  circumference,  but  has  not  yet  acquired  the  velocity  of 
the  circumference ;  while,  nearer  the  line,  they  are  like  the  fluid  after  it  has 
for  a  considerable  time  been  turning  on  the  circumference,  and  has  acquired 
the  rotary  motion  there,  consequently  appearing  at  rest  as  regards  that 
motion,  but  still  leaving  sensible  any  motion  in  a  cross  direction. 

While,  in  the  lower  regions  of  the  atmosphere,  air  is  thus  constantly 
flowing  towards  the  equator  and  forming  the  steady  trade  winds  between 
the  tropics  in  the  upper  regions,  there  must,  of  course,  be  a  counter-current 
distributing  the  heated  air  again  over  the  globe :  accordingly,  since  reasoning 
led  men  to  expect  this,  many  striking  proofs  have  been  detected.  At  the 
summit  of  the  Peak  of  Teneriffe,  observations  now  show  that  there  is  always 
a  strong  wind  blowing  in  a  direction  contrary  to  that  of  the  trade  wind  on 
the  face  of  the  ocean  below.  Again,  the  trade  winds  among  the  West  India 
Islands  are  constant,  yet  volcanic  dust  thrown  aloft  from  the  Island  of  St. 
Vincent,  in  the  year  1812,  was  found,  to  the  astonishment  of  the  inhabitants 
of  Barbadoes,  hovering  over  them  in  thick  clouds,  and  falling,  after  corning 
more  than  100  miles  directly  against  the  strong  trade  wind,  which  ships 
must  take  a  circuitous  course  to  avoid.  Persons  sailing  from  the  Cape  of 
Grood  Hope  to  St.  Helena,  have  often  to  remark  that  the  sun  is  hidden  for 
days  together,  by  a  stratum  of  dense  clouds  passing  southward  high  in  the 
atmosphere }  which  clouds  consist  of  the  moisture  raised  near  the  equator 
with  the  heated  air,  and  becoming  condensed  again  as  it  approaches  the 
colder  regions  of  the  south. 

Beyond  the  tropics,  where  the  heating  influence  of  the  sun  is  less,  the 
winds  occasionally  obey  other  causes  than  those  we  have  now  been  con- 
sidering, which  causes  have  not  yet  been  fully  investigated.  The  winds  of 
temperate  climates  are  in  consequence  much  less  regular,  and*  are  called 
variable  ;  but  still,  as  a  general  rule,  whenever  air  is  moving  towards  the 
equator,  from  the  north  or  south  poles  where  it  was  at  rest,  it  must  have  the 
appearance  of  an  east  wind,  or  a  wind  moving  in  the  contrary  direction  of  the 
earth  itself,  until  it  has  gradually  acquired  the  whirling  motion  of  that  part  of 
the  surface  of  the  earth  on  which  it  is  found;  and  again,  when  air  is  moving 
from  the  equator,  where  it  had  at  last  acquired  nearly  the  same  motion  as 
that  part  of  the  earth,  on  reaching  parts  nearer  the  poles,  and  which  have  less 
eastward  motion,  it  continues  to  run  faster  than  they,  and  becomes  a  westerly 
wind.  In  many  situations  beyond  the  tropics,  the  westerly  winds,  which  are 
merely  the  upper  equatorial  currents  of  air  falling  down,  are  almost  as  regular 
as  the  easterly  winds  within  the  tropics,  and  might  also  be  called  trade  winds : 
— witness  the  usual  shortness  of  the  voyages  from  New  York  to  Liverpool, 
and  the  length  of  those  made  in  the  contrary  direction.  North  of  the  equator, 
then,  on  earth,  true  north  winds  appear  to  be  north-east,  and  true  south  winds 
appear  to  be  south-west : — which  are  the  two  winds  that  blow  in  England  for 
three  hundred  days  of  every  year.  In  southern  climates  the  converse  is  true. 

While  the  sun  is  beaming  directly  over  a  tropical  island,  he  warms  very 
much  the  surface  of  the  soil,  and,  therefore,  also,  the  air  over  it  j  but  the  rays 
which  fall  upon  the  ocean  around  penetrate  deep  into  the  mass,  and  produce 
little  increase  of  superficial  temperature.  As  a  consequence  of  this,  there  is 
a  rapid  ascent  of  hot  air  over  the  island  during  the  day,  and  a  cooler  wind 
blowing  towards  its  centre  from  all  directions.  This  wind. constitutes  the 
refreshing  sea-breeze  of  tropical  islands  and  coasts.  A  person  must  have 
been  among  these,  to  conceive  the  delight  which  the  sea-breeze  brings  after 

14 


210  PNEUMATICS. 

the  sultry  stagnation  which  precedes  it.  The  welcome  ripple  shorewards  is 
first  perceived  on  the  surface  of  the  lately  smooth  or  glassy  sea ;  and  soon 
the  whole  face  of  the  sea  is  white  with  little  curling  waves,  among  which 
the  graceful  canoe,  lately  asleep  on  the  water,  now  shoots  swiftly  along. 

During  the  night  a  phenomenon  of  opposite  nature  takes  place.  The 
surface  of  the  earth  then  no  longer  receiving  the  sun's  rays,  is  soon  cooled 
by  radiation,  while  the  sea,  which  absorbed  heat  during  the  day,  not  on  the 
surface  only,  but  through  its  mass,  continues  to  give  out  heat  all  night. 
The  consequence  is,  that  the  air  over  the  earth  becoming  colder  than  that 
over  the  sea,  sinks  down,  and  spreads  out  on  all  sides,  producing  the  land- 
freeze  of  tropical  climates.  This  wind  is  often  charged  with  unhealthy 
exhalations  from  the  marshes  and  forests,  while  the  sea-breeze  is  all  purity 
and  freshness.  Many  islands  and  coasts  would  be  absolutely  uninhabitable 
but  for  the  sea-breeze. 

The  peculiar  distribution  of  land  in  the  Asiatic  part  of  the  globe,  produces 
the  curious  effect  there  of  a  sea-breeze  of  six  months,  and  a  land-breeze  of 
six  months.  The  great  continent  of  Asia  lies  chiefly  north  of  the  line,  and 
during  its  summer,  the  air  over  it  is  so  much  heated,  that  there  is  a  constant 
steady  influx  from  the  south — appearing  south-west,  for  the  reason  given  in 
a  preceding  page;  and  during  its  winter  months,  while  the  sun  is  over  the 
southern  ocean,  there  is  a  constant  land  breeze  from  the'  north — appearing, 
for  a  like  reason,  north-east.  These  winds  are  called  monsoons  ;  and  if  their 
utility  to  commerce  were  to  be  a  reason  for  a  name,  they  also  deserve  the 
name  of  trade  winds.  In  early  periods  of  navigation,  they  served  to  the 
mariner  the  purpose  of  compass,  as  well  as  of  moving  power  ;  and  one 
voyage  outward,  and  another  homeward  with  the  changing  monsoons,  filled 
up  his  year. — On  the  western  shores  of  Africa  and  America,  also,  the  trade 
winds  are  interfered  with  by  the  heating  of  the  land;  but  much  less  so  than 
in  Asia,  and  always  in  accordance  with  the  laws  now  explained. 

The  frightful  tornadoes,  or  whirlwinds,  which  occasionally  devastate 
certain  tropical  regions,  making  victims  of  every  ship  or  bark  caught  on 
the  waters,  and  the  shore  gusts  or  squalls  met  with  every  where,  are 
owing  to  some  sudden  chemical  changes  in  the  atmosphere,  not  yet  fully 
understood. 

The  Pneumatic  Trough  and  Gasometer 

of  the  chemist  are  contrivances  constantly  displaying  the  truth  now  under 
consideration,  "  that  a  lighter  fluid  is  pushed  up  and  floats  on  a  heavier." 
They  are  important  parts  of  the  apparatus  for  operating  on  substances  while 
in  the  form  of  air. 

The  trough  a  may  be  made  of  metallic  plate,  or  of  wood  lined  with  metal, 
and  of  any  convenient  size.     It  is  nearly  filled 
Fig.  106.  with  water,  and  has  at  one  end  about  an  inch 

under  the  surface  of  the  water,  a  shelf  on  which 
jars  or  vessels,  as  b  and  c,  may  rest.  Any 
particular  air  or  gas  is  preserved  separate  from 
the  atmosphere,  by  being  placed  in  one  of  these 
jars  with  the  mouth  downwards.  The  gas  is 
passed  into  the  jar  by  the  operator  first  immers- 
ing the  jar  in  the  trough,  so  as  to  fill  it  with 
water  and  to  expel  the  common  air  from  it;  and 
then  holding  its  mouth  over  the  gas  while  rising 
under  the  water  from  another  vessel  or  pipe  : — 


FLOATING.  —  GAS    APPARATUS. 


211 


Fig.  107. 


CL 


-s 


a 

ci 


d  represents  a  long-necked  vessel,  used  to  contain  the  ingredients  for  the  pro- 
duction of  gases  by  chemical  action.  The  gas  of  course  rises  to  the  top  of 
the  jar  b,  and  gradually'displaces  the  water.  During  the  operation  of  filling, 
the  jar  may  be  supported  by  the  hand  or  by  resting  on  the  shelf; — in  the 
latter  case  the  gas  is  allowed  to  rise  into  it  through  a  hole  in  the  shelf,  pro- 
vided with  a  small  funnel  gaping  downwards  to  catch  the  air  more  readily. 
The  shelf  may  have  room  on  it  for  many  jars,  and  it  may  have  more  holes 
than  one ;  and  if  the  gas  under  operation  be  such  that  water  absorbs  or 
changes  it,  some  other  liquid,  as  mercury,  may  be  used  instead  of  water. 

A  gasometer  or  gas-holder,  is  merely  a  larger  jar  or  vessel  as  or,  dipping 
into  water,  with  its  mouth  downwards,  in  a 
trough  of  its  own  shape,  6  c,  and  so  sup- 
.  ported  or  counterpoised  by  a  weight  at  d, 
over  pullies,  that  very  little  force  suffices  to 
move  it  up  or  down.  Air  forced  into  it 
through  a  pipe  /opening  under  it,  causes  it 
to  rise  or  float  higher  in  proportion  to  the 
quantity.  The  air  is  made  to  pass  from  it 
again  when  wanted,  either  through  the  same 
tube  or  through  another  as  e. 

The  huge  gasometers,  exceeding  in  size 
an  ordinary  house,  and  containing  the  supply 
of  gas  for  the  lamps  of  a  town,  are  vessels 
suspended  as  above  represented,  in  great  pits 
or  troughs,  filled  with  water.  The  gas  issues 
with  force  proportioned  to  the  downward 
pressure  of  the  containing  vessel,  which  may 
be  nicely  regulated  in  a  variety  of  ways,  and 
is  generally  made  to  equal  the  action  of  a  column  of  water  of  two  inches  in 
height;  that  is  to  say,  such,  that  a  pipe  issuing  from  the  gas  holder,  and 
dipping  into  water  at  its  other  end,  shall  allow  gas  to  escape,  if  immersed 
less  than  two  inches  perpendicularly. 

It  would  be  encroaching  on  the  province  of  the  chemist  to  treat  here  par- 
ticularly of  the  substances  which  most  generally  exist  in  the  aeriform  state ; 
but  to  give  an  increased  interest  to  the  description  of  the  gas  apparatus,  a 
few  leading  facts  may  be  mentioned. 

Of  about  fifty  distinct  substances  known  as  the  materials  of  our  globe,  five, 
when  uncombined,  and  under  common  circumstances  of  heat  and  pressure, 
exist  as  air  or  gases  The  water  used  to  fill  the  apparatus  above  described 
is  a  compound  of  two  of  the  substances,  viz.,  oxygen  and  hydrogen.  By 
directing  an  electrical  current  through  water,  it  is  gradually  decomposed, 
and  from  one  side,  a  stream  of  aeriform  oxygen  may  be  received,  and  from 
the  other  a  stream  of  hydrogen  The  two  gases  may  be  again  united  to 
form  water,  by  mixing  them  in  a  proper  vessel,  and  passing  an  electric  spark 
through  them.  They  combine  with  explosion. 

This  oxygen,  so  called  from  its  relation  to  acids,  (the  name  consisting  of 
two  Greek  words,  signifying  acid  and  to  form,)  has  been  accounted,  for 
many  reasons,  the  most  important  substance  in  nature.  It  forms  eight- 
ninths,  by  weight  of  the  ocean ;  one-fourth  of  the  atmosphere ;  and  perhaps 
one-fourth  of  the  solid  matter  of  the  globe :  possibly,  therefore,  although 
most  persons  think  of  it  only  as  an  air  or  gas,  there  is  not  a  millionth  part 
of  the  quantity  of  oxygen  in  the  world  existing  as  air.  It  unites  readily 
with  most  other  substances,  and  generally  with  such  intense  action  as  to 


212  PNEUMATICS. 

produce  the  phenomena  of  fire  or  combustion;,  the  word  combustible  chiefly 
apply  to  substances  that  quickly  combine  with  oxygen. 

Oxygen  assumes  a  singular  variety  of  character  in  its  different  combina- 
tions. Thus  with  hydrogen,  it  forms  water;  with  lead,  it  forms  the  sub- 
stance called  red-lead  ;  with  nitrogen,  in  one  proportion,  it  forms  atmospheric 
air,  in  another  proportion,  the  nitrous  oxide,  or  what  is  called  the  laughing 
gas,  in  a  third,  the  acid  called  aqua  fortis ;  with  sulphur  it  forms  the 
sulphuric  acid  or  oil  of  vitriol;  with  iron,  and  all  metals  it  forms  their  ores 
called  oxides :  and  so  forth.  But  the  most  important  character  in  which  we 
know  it,  is  as  that  ingredient  of  our  atmosphere,  without  which  animals  and 
vegetables  cannot  live,  and  fire  cannot  burn.  Oxygen,  from  this  part  of  its 
history,  was  long  named  vital  or  pure  air. 

Pure  oxygen,  in  the  state  of  air  is  a  little  heavier  than  common  air;  but, 
when  holding  a  quantity  of  charcoal  in  solution,  it  forms  aeriform  carbonic 
acid,  which  is  nearly  twice  as  heavy  as  common  air,  and  may  be  poured  out 
of  one  vessel  into  another  like  water.  Carbonic  acid  is  what  issues  from 
soda-water,  brisk  ale,  champagne,  &c.,  while  they  sparkle.  If  drawn  into 
the  lungs  in  breathing,  it  is  fatal  to  life.  A  charcoal  fire  left  in  a  close  room 
with  sleeping  persons,  has  often  been  fatal  to  them,  because  carbonic  acid 
gas  is  the  product  of  the  combustion.  So  likewise,  houseless  wretches  in 
winter  lying  down  in  a  bricktnaker's  field  to  leeward  of  a  burning-heap  of 
bricks,  often  fall  asleep  for  ever.  The  famous  Grotto  del  Cane,  in  Italy,  is 
a  cavern  always  full  of  carbonic  acid,  which  springs  into  it  from  below,  as 
water  springs  into  a  well,  and  runs  over  like  water  from  a  well : — it  received 
its  name  from  the  circumstance  of  dogs  dying  instantly  when  thrown  into  it. 
Carbonic  acid  rising  in  fermentation  has  often  proved  fatal  to  persons  leaning 
over  the  edge  of  fermenting  vats.  It  is  common  to  see  a  rat  die  instantly, 
in  the  attempt  to  run  a  plank  laid  across  the  mouth  of  a  fermenting  tub. 

Hydrogen,  the  other  ingredient  of  water,  so  called  from  its  relation  to 
water  (the  name  consists  of  the  Greek  words  for  water  and  to  form,)  when 
in  the  state  of  air,  is  sixteen  times  as  light  as  oxygen.  With  it  balloons  are 
filled.  When  it  holds  in  solution  a  certain  quantity  of  carbon  or  charcoal  it 
becomes  the  common  gas  used  for  illumination,  and  is  the  fire-damp  of 
mines,  of  which  the  burning  and  explosion  are  so  terrible.  It  forms  one- 
ninth  of  the  ocean,  and  much  of  the  animal  and  vegetable  bodies. 

Nitrogen,  so  called  from  its  relation  to  nitric  acid,  is  the  third  and  last 
substance  which  we  shall  mention.  It  is  what  remains  of  the  atmosphere 
when  the  oxygen  is  removed.  It  forms  about  four-fifths  of  the  atmosphere, 
one-fourth  of  the  animal  flesh,  and  is  found  in  small  quantities  in  the  other 
combinations.  It  will  not  support  life  by  itself,  and  therefore  formerly  was 
called  azote :  with  a  larger  portion  of  oxygen,  it  forms  nitric  acid  or  the 
aquafortis  of  old. 

The  last  few  paragraphs  may  serve  to  show  how  many  of  the  manipula- 
tions of  chemistry  are  directed  by  the  principles  of  physics  or  mechanical 
philosophy;  and  therefore,  how  essential  to  the  chemist  the  preliminary 
study  of  physics  becomes, 


DISCHARGE  FROM  APERTURES.         213 

PART   III. 

OR 

THE  PHENOMENA  OF  FLUIDS. 

• 

(  CONTINUED.  ) 


SECTION   III— HYDRAULICS— PHENOMENA   OF  FLUIDS    IN 

MOTION. 


ANALYSIS   OF   THE    SECTION. 

Whether  the  particles  of  matter  exist -in  the  form  of  solid  or  fluid,  the  cir- 
cumstance does  not  affect  their  properties  of  INERTIA  and  GRAVITY.— 
Hence  liquids  and  airs,  in  proportion  to  their  quantity,  resist,  receive, 
and  impart  motion,  and  have  weight  and  friction,  as  is  true  of  solids. 
This  is  seen  in  the  phenomena  of 

1.  Fluids  issuing  from  vessels,  or  moving  in  pipes  and  channels. 

2.  Waves. 

8.  Fluids  resisting  the  motion  of  bodies  immersed  in  them;  or  themselves 

moving  against  other  bodies. 
4.  Fluids  lifted,  or  moved  in  opposition  to  gravity. 


"Fluids  issuing  from  vessels,  or  moving  in  channels.1' 

WATER  admitted  to  a  tube  ascending  from  near  the  bottom  of  a  reservoir 
will  rise  in  it,  as  already  explained,  to  the  level  of  the  liquid-surface  in  the 
reservoir.  If  such  a  tube  be  afterwards  cut  off,  except  a  small  part  at  the 
bottom,  then  prepared  as  a  jet-pipe,  the  water  will  spout  from  this  still  to 
the  same  height,  with  a  certain  deduction  for  the  resistance  of  the  air  and 
friction.  Now  as  a  body  shot  upwards  to  any  height  has  that  velocity  in 
departing,  which  it  again  acquires  by  falling  back  to  the  same  place  or  level, 
(with  a  certain  deduction  for  the  resistance  of  the  air,)  as  explained  at  page 
60,  it  follows  that  fluid  issues  from  any  orifice  in  a  reservoir  with  velocity 
equal  to  what  a  body  acquires  in  falling  as  far  as  from  the  level  of  the  fluid 
surface  in  the  reservoir  to  the  orifice.  By  referring  then  to  the  law  of  fall- 
ing bodies,  as  explained  at  page  50,  we  may  learn  the  velocity  of  the  issue 
of  water  in  any  case,  and  therefore  tthe  quantity  delivered  by  an  opening  of 
a  given  magnitude. 


214  HYDRAULICS. 

Thus,  a  body  by  gravity  falls  sixteen  feet  in  the  first  second,  with  speed 
gradually  increasing,  and  at  the  end  of  the  second  has  a  velocity  of  thirty-two 
feet  per  second;  therefore  a  reservoir  with  an. opening  of  an  inch  square  at 
sixteen  feet  below  the  water's  surface,  will  deliver,  in  one  second  of  time,  with 
a  certain  deduction  for  resistance  of  air,  friction,  &c.,  thirty-two  feet  of  a  jet 
of  water  of  an  inch  square ;  and  according  to  the  same  rule,  an  opening  at 
four  times  the  depth  should  deliver  a  double  quantity ;  at  nine  times  the 
depth,  a  triple  quantity  ;  and  so  on,  as  really  happens.  An  inquirer  is  at 
first  surprised  that  the  quantity  should  not  be  quadruple,  where  the  height  of 
column  or  pressure  forcing  it  out  is  quadruple,  ninefold  when  the  pressure 
is  ninefold,  &c.,  but  on  reflection,  he  may  perceive  that  the  real  effects,  as 
stated  above,  are  still  exactly  proportioned  to  the  causes ;  for  when  only 
twice  as  much  water  is  forced  out  in  the  same  time,  there  is  still  an  effect, 
four  times  as  powerful,  because  each  particle  of  the  double  quantity  issues 
with  twice  the  force  or  velocity,  and  increase  of  velocity  costs  just  as  much 
force  as  increase  of  quantity.  Similar  reasoning  holds  with  respect  to  the 
triple  or  other  quantities.  Because  a  body  shot  upward  with  a  double  velo- 
city gains  a  quadruple  height,  (  see  page  60, )  the  jet  issuing  with  only  double 
velocity  from  four  times  the  depth,  still  reaches  the  level  of  the  surface  of 
the  reservoir. 

The  knowledge  of  this  rule  for  discharging  orifices  is  of  the  greatest 
importance  in  the  construction  of  water-works,  because  when  joined  with 
other  rules  assigning  the  effects  of  friction,  bending,  unequal  width,  &c.,  in 
pipes,  it  ascertains  the  quantity  of  water  which  a  conduit  of  any  magnitude, 
length  and  slope,  will  deliver. 

It  is  a  curious  fact,  that  more  water  issues  from  a  vessel  through  a  short 
pipe,  than  through  a  simple  aperture  of  the  same  diameter  as  the  pipe;  and 
still  more  if  the  pipe  be  funnel-shaped,  or  wider  towards  its  inner  extremity. 

The  explanation  is,  that  the  issuing  particles  .coming  from  all  sides  to 
escape,  cross  and  impede  each  other  in  rushing  through  a  simple  opening, 
as  is  proved  by  the  narrow  neck  which  the  jet  exhibits  a  little  beyond  the 
opening ;  but  in  a  tube,  this  narrowing  of  the  jet  cannot  happen  without 
leaving  a  vacuum  around  the  part,  and  the  pressure  of  the  atmosphere,  re- 
sisting the  vacuum,  causes  a  quicker  flow.  The  funnel-shape  again  leads 
the  water  by  a  more  gradual  inclination  to  the  point  of  exit,  and  thus  con- 
siderably prevents  the  crossing  among  the  particles ;  besides  that,  because 
its  mouth  surrounds  the  narrow  neck  of  the  jet,  it  allows  that  part  to  be 
deemed  the  commencement  of  the  jet. 

Another  remarkable  effect  of  atmospheric  pressure  on  running  liquids  is, 
that  in  a  tube  of  considerable  length,  descending  from  the  reservoir,  it  much 
quickens  the  discharge.  Water  naturally  falls  like  any  other  body  with 
accelerating  velocity,  but  if  it  so  fall  in  a  tube  which  it  fills  like  a  piston, 
either  portions  of  it  below  must  outstrip  portions  above,  leaving  vacuous 
spaces  between,  or  water  from  above  must  be  pressed  into  the  tube  by  some 
other  force  than  its  weight.  Now  the  atmospheric  pressure  becomes  this 
force,  and  it  prevents  a  vacuum,  partly  by  impelling  water  more  rapidly  into 
the  top  of  the  tube,  and  partly  by  resisting  the  discharge  from  below.  The 
forcing  in  of  the  water  at  the  top  of  the  tube  causes  that  depression  of  the 
water  surface  in  the  reservoir  over  it,  which  becomes  more  conspicuous  as 
the  depth  in  the  reservoir  diminishes,  and  at  last  is  a  deep  hole  in  the  water 
extending  far  into  the  tube,  and  sometimes  even  as  in  a  common  funnel 
extending  quite  through. 

The  friction  or  resistance  which  fluids  suffer  in  passing  along  pipes  is 


DISCHARGE    FROM    APERTURES.  215 

i 

much  higher  than  might  be  expected.  It  depends  on  the  cohesion  of  the 
particles  to  the  surface  of  the  pipe  and  among  one  another,  and  on  the  par- 
ticles near  the  outside  being  constantly  driven  from  their  straight  course  by 
the  irregularities  in  the  surface  of  the  pipe.  An  inch  tube  of  two  hundred 
feet  in  length,  placed  horizontally,  is  found  to  discharge  only  a  fourth  part 
of  the  water  which  escapes  by  aa  simple  aperture  of  the  same  diameter.  All 
passing  along  tubes  is  still  more  retarded.  A  person  who  erected  a  great 
bellows  at  a  water-fall,  to  blow  a  furnace  two  miles  off,  found  that  his 
apparatus  was  totally  useless.  When  gas  lights  were  first  proposed, 
some  engineers  feared  that  the  resistance  by  friction  to  the  passing  air 
would  be  fatal  to  the  enterprise. 

Higher  temperature  in  a  liquid  increases  remarkably  the  quantity  dis- 
charged by  an  orfice  or  pipe, — apparently  by  diminishing  that  cohesion  of 
the  particles  which  exists  in  certain  degrees  in  all  liquids  and  affects  so 
much  their  internal  movement.  The  additition  of  100  degrees  of  heat  will, 
in  certain  cases,  nearly  double  the  discharge. 

The  flux  of  water  through  orifices  under  uniform  circumstances  is  so 
steady,  that  before  the  invention  of  clocks  and  watches,  it  was  employed  as 
a  means  of  measuring  time.  The  vessels  were  called  clepsydrae,.  That  of 
Ctesibius  is  famous,  in  which  the  issuing  water  took  the  form  of  tears  from 
the  eyes  of  a  figure,  deploring  the  rapid  passing  of  precious  time ;  and  these 
tears  being  received  into  a  fit  vessel,  gradually  filled  it  up  and  raised 
another  floating  figure,  who  pointed  to  the  hours  marked  on  an  upright 
scale.  This  vessel  was  daily  emptied  by  a  syphon,  when  charged  to  a 
certain  height,  and  its  discharge  worked  machinery  which  told  the  month 
and  the  day.  The  common  hour-glass  of  running  sand  is  another  modifica- 
tion of  the  same  principle,  with  this  remarkable  difference,  however,  that 
depth  of  the  sand  does  not  quicken  the  flux. 

The  progress  of  water  in  an  open  conduit,  such  as  the  channel  of  a  river 
or  acqueduct,  is  influenced  by  friction,  &c.,  in  the  same  manner  as  in  close 
pipes.  But  for  this,  a  river  like  the  Rhone,  drawing  its  waters  from  the 
elevation  of  1,000  feet  above  the  level  of  its  mouth,  would  pour  them  out, 
with  the  velocity  of  water  issuing  from  the  bottom  of  a  reservoir,  1,000  feet 
deep;  that  is  to  say,  at  the  rate  of  about  170  miles  per  hour.  The  ordinary 
flow  of  rivers  is  about  three  miles  per  hour,  and  their  channels  slope  three 
or  four  inches  per  mile. 

The  velocity  of  a  water  current  is  easily  ascertained  by  immersing  in  it 
an  upright  tube,  of  which  the  bottom  bent  at  right  angles  becomes  an  open 
mouth  turned  towards  the  stream.     The  water  in  the  tube 
will  stand  above  the  surface  of  the  stream,  as  much  as  would         Fig.  108. 
be  necessary  in  a  reservoir,  according  to  the  explanation          j  ( 
given  above,  to  cause  a  velocity  of  jet  equal  to  the  velocity       ct 
of  the  stream.     A  modification  of  this  contrivance  may  be 
made  to  measure  the  velocity  of  the  wind.    A  common  mode 
of  telling  the  velocity  of  an  open  stream,  is  to  observe  with 
a  stop-watch  the  progress  of  a  body  floating  in  some  part  of 
it  from  which  its  medium  speed  maybe  known;  and  know- 
ing that  speed  and  the  depth  and  width  of  the  channel  the 
quantity  delivered  in  a  given  time  becomes  a  matter  of  sim- 
pie  calculation.     The  speed  of  the  wind  may  be  ascertained 
by   observing  how  long  the  shadow  of  a  cloud   takes   to 
pass  across  a  field  of  known  dimensions. 

The  friction  of  water  moving  in  water  is  such,  that  a  small  stream  directed 


216  HYDRAULICS. 

through  a  pool,  with  speed  enough  to  rise' over  the  opposite  bank,  will  soon 
empty  the  pool.  Extensive  fens  have  been  drained  on  this  principle.  The 
friction  between  air  and  water  is  also  singularly  strong,  and  is  proved  on  a 
great  scale  by  the  magnitude  of  the  ocean-waves,  which  is  a  consequence  of 
it ;  and  on  a  small  scale,  by  the  amusing  experiment  of  making  a  light 
round  body  dance  or  play  upon  the  summit  of  a  water-jet — a  chief  cause  of  its 
remaining  there  being,  that  the  current  of  air  which  rises  around  the  jet  by 
reason  of  the  friction,  presses  it  inward  again,  whenever  it  inclines  to  fall 
over.  Oil  thrown  upon  the  surface  of  water,  soon  spreads  as  a  film  over  it. 
and  defends  it  from  farther  contact  and  friction  of  the  air.  If  oil  be  thus 
spread  at  the  windward  side  of  a  pond  where  the  waves  begin,  the  whole 
surface  of  a  pond  becomes  as  smooth  as  glass ;  and  even  out  at  sea,  where 
the  commencement  of  the  waves  cannot  be  reached,  oil  thrown  upon  them 
smooths  their  surface  to  leeward  of  the  place,  and  prevents  their  curling 
over  or  breaking.  It  is  said  that  boats  having  to  reach  the  shore  through 
a  raging  surf,  have  been  preserved  by  the  crews  first  spilling  a  cask  of  oil 
in  the  offing. 

The  most  magnificent  examples  that  ever  existed,  or  probably  ever  will ' 
exist  of  artificial  water-courses,  were  the  acqueducts  of  ancient  Borne,  about 
twenty  in  number.  Several  of  them  exceeded  forty  miles  in  length,  passing 
Ihrough  hills  in  their  way,  and  resting  on  tiers  of  splendid  arches  across  the 
valleys.  They  were  constructed  of  such  durable  materials,  and  so  skilfully, 
that  the  principal  of  them  remain  perfect  to  this  day.  Considered  as  one 
object,  they  rank,  in  point  of  magnitude,  with  any  other  work  of  human 
labour,  not  excepting  the  pyramids  of  Egypt. 

While  the  acqueducts  are  cited  as  specimens  of  grandeur,  we  may  mention 
the  fountains  in  the  gardens  of  France  and  Italy  as  specimens  of  beauty. 
Those  at  Versailles  are  well  known.  In  them  the  most  magical  effects  are 
produced  by  varying  the  ways  in  which  water  is  made  to  spout  from  orifices. 
In  one  place  it  is  seen  darting  into  the  air  as  a  single  upright  pillar  :  in  others 
many  such  pillars  rise  together,  like  giant  stalks  of  corn  ;  sometimes  an 
inclination  given  to  the  jets  makes  them  bend  so  as  to  form  beautiful  arches, 
of  which  a  portion  appear  as  the  roofs  of  apartments  built  of  water  while 
others  mingle  together  with  endless  variety  :  here  and  there  water-throwing 
wheels  throw  out  spiral  streams,  and  hollow  spheres  with  a  thousand  openings 
are  the  centres  of  immense  bushes  or  trees  of  silvery  boughs.  Such  effects 
amidst  cascades,  smooth  lakes,  and  scenes  of  lovely  landscapes,  constitute  a 
whole  as  enchanting,  perhaps,  as  art  by  moulding  nature  has  ever  produced. 

"  Waves." 

The  form,  magnitude  and  a  velocity  of  waves,  are  subjects  admitting  of 
deep  mathematical  research  ;  and  are  rendered  the  more  interesting,  because 
certain  phenomena  of  sound  and  light  are  of  kindred  nature.  Here,  how- 
ever, they  must  be  treated  with  great  brevity. 

A  stone  thrown  into  a  smooth  pond,  causes  a  succession  of  circular  waves 
to  spread  from  the  spot  where  it  falls  as  a  common  centre.  They  become  of 
less  elevation  as  they  expand,  and  each  new  one  is  less  raised  than  the  pre- 
ceding, until  gradually  the  liquid  mirror  becomes  again  perfect  as  before. 
Several  stones  falling  at  the  same  time  in  different  places,  cause  crossing 
circles  which,  however,  do  not  disturb  the  progress  of  one  another — a  phe- 
nomenon seen  in  beautiful  miniature  at  each  leap  of  the  little  insects  which 
cover  the  surface  of  our  pools  in  the  calm  hours  of  summer.  The  rationale 


WAVES.  217 

of  the  formation  of  waves  in  such  cases  is  as  follows  :  When  the  stone  falls 
into  the  water,  because  the  liquid  is  incompressible,  a  part  of  it  is  displaced 
laterally,  and  becomes  an  elevation  or  circular  wave  around  the  stone.  This 
wave  then  spreads  outward  in  obedience  to  the  laws  of  fluidity,  already 
explained,  and  the  circle  is  seen  to  widen.  •  In  the  meantime,  where  the 
stone  descended,  a  hollow  is  left  for  a  moment  in  the  water,  but  owing  to 
the  surrounding  pressure,  is  soon  filled  up,  chiefly  by  a  sudden  rush  from 
below.  The  rising  water  does  not  stop,  however,  at  the  exact  level  of  that 
around,  but  like  a  pendulum  sweeping  past  the  centre  of  its  arc,  it  rises 
almost  as  far  above  the  level  as  the  depression  was  deep.  The  central  eleva- 
tion now  acts  as  the  stone  did  originally,  and  causes  a  second  wave,  which 
pursues  the  first ;  and  when  the  centre  subsides,  like  the  pendulum  still,  it 
sinks  again  almost  as  much  below  the  level  as  it  had  mounted  above  :  hence 
it  has  to  rise  again,  again  to  fall,  and  so  on  for  many  times,  sending  forth 
a  new  wave  at  each  alternation.  Owing  to  the  friction  among  the  particles 
of  the  water,  each  new  wave  is  less  raised  thten  the  preceding,  and  at  last 
the  appearance  dies  away. 

A  wave  passing  through  any  gap  or  opening,  spreads  from  it  as  a  new 
centre ;  and  a  wave  coming  against  a  perpendicular  surface  of  wall  or  rock, 
is  completely  reflected  from  this,  and  acquires  the  appearance  of  coming 
from  a  point  as  far  beyond  the  reflecting  surface,  as  its  real  origin  or  centre 
is  distant  on  the  side  where  it  is  moving. 

So  absolutely  level  is  a  liquid  surface,  and  so  sensitive  or  mobile,  that  the 
effect  of  any  disturbing  cause  is  perceived  at  great  distances.  A  boat  rowed 
across  a  still  lake,  ruffles  its  surface  to  a  great  extent ;  and  although  the 
widening  waves  become  at  last  such  gentle  risings  as  not  to*be  perceptible 
to  the  eye,  they  still  produce  a  rippling  noise  where  they  fall  among  the 
pebbles  on  shore.  In  seas  liable  to  sudden  but  partial  hurricanes,  the  roar 
of  breakers  on  distant  coasts  often  tells  of  the  storm  which  does  not  other- 
wise reach  them.  The  author  once,  in  the  eastern  ocean,  had  an  opportunity 
of  contemplating  waves  of  extraordinary  magnitude  rolling  along  during  a 
gloomy  calm,  and  therefore  with  unbroken  surface,  appearing  like  billows  of 
molten  lead.  At  that  very  time,  about  a  hundred  and  fifty  miles  to  the 
north-east,  four  of  the  finest  ships  of  the  India  Company  were  perishing  in  a 
storm. — In  the  polar  seas  which  are  comparatively  tranquil,  because  partially 
defended  from  the  wind  by  the  floating  islands  of  ice,  a  few  sudden  waves 
are  occasionally  observed,  and  quickly  all  is  calm  again.  Such  a  phenomenon 
announces,  that  the  occurrence  described  at  page  153  has  happened  some- 
where, of  an  island  of  ice  turning  over,  when  the  place  of  its  centre  of  gravity 
is  changed  by  partial  melting. 

The  common  cause  of  waves  is  the  friction  of  the  wind  upon  the'surface 
of  the  water.  Little  ridges  or  elevations  first  appear,  which,  by  continuance 
of  the  force,  gradually  become  loftier  and  broader,  until  they  are  the  rolling 
mountains  seen  where  the  winds  sweep  over  a  great  extent  of  water.  The 
heaving  of  the  Bay  of  Biscay,  or  still  more  remarkably,  of  the  open  ocean 
beyond  the  southern  capes  of  America  and  Africa,  exhibits  one  extreme,  and 
the  stillness  of  the  tropical  seas,  which  are  sheltered  by  near  encircling  lands 
exhibits  the  other.  In  the  vast  archipelago  of  the  east,  where  Borneo  and 
Java  and  Sumatra  lie,  and  the  Molucca  Islands  and  the  Philippines,  the  sea 
is  often  fanned  only  by  the  land  and  sea  breezes,  and  is  like  a  smooth  bed  in 
which  these  islands  seem  to  repose  in  bliss — islands  in  which  the  spice  and 
pefume  gardens  of  the  world  are  embowered,  and  where  the  bird  of  paradise 
has  its  home,  and  the  golden  pheasant,  and  a  hundred  other  birds  of  brilliant 


218  HYDRAULICS. 

plumage,  among  thickets  so  luxuriant,  and  scenery  so  picturesque,  that 
European  strangers  find  there  the  fairy  land  of  their  youthful  dreams. — One 
who  has  visited  these  islands  in  his  early  days,  may  perhaps  be  pardoned  for 
thus  adverting  to  their  beauties. 

In  rounding  the  Cape  of  Good  Hope,  waves  are  met  with,  or  rather  a 
swell,  so  vast,  that  a  few  ridges  and  a  few  depressions  occupy  the  extent  of  a 
mile.  But  these  are  not  so  dangerous  to  ships,  as  what  is  termed  a  shorter 
sea,  with  more  perpendicular  waves.  The  slope  in  the  former  is  compara- 
tively gentle,  and  the  rising  and  falling  are  much  less  felt ;  while  among  the 
latter,  the  sudden  tossing  of  the  vessel  is  often  destructive.  When  a  ship 
is  sailing  directly  before  the  wind,  over  the  long  swell  now  described,  she 
advances  as  if  by  leaps ;  for  as  each  wave  passes,  she  is  first  descending  head- 
long on  its  front,  acquiring  a  velocity  so  wild  that  she  can  scarcely  be  steered ; 
and  soon  after,  when  it  has  glided  under  her,  she  appears  climbing  on  its 
back,  and  her  motion  is  slackened  almost  to  rest,  before  the  following  wave 
arrives.  To  a  passenger  perched  at  such  a  time  on  the  extremity  of  the 
bowsprit,  and  looking  back  on  the  enormous  body  of  the  ship,  with  perhaps 
its  thousands  of  a  crew,  a  hundred  feet  behind  him,  heaved  by  those  billows 
as  a  cork  is  on  a  ruffled  lake,  the  scene  is  truly  sublime.  When  a  coming 
wave  lifts  the  stern  and  in  the  same  decree  depresses  the  bow,  he  is  deep  in 
the  hollow  or  valley  between  the  waves,  and  sees  only  the  ship  rushing  head- 
long down  towards  him  as  if  to  be  engulphed  -r  but  soon  after,  when  the  stern 
is  down,  and  the  bow  is  raised,  he  looks  from  his  station  in  the  sky  upon  an 
awful  scene  beneath  him  and  around. 

The  velocity  of  waves  has  relation  to  their  magnitude.  The  large  waves 
just  spoken  of,  proceed  at  the  rate  of  from  thirty  to  forty  miles  an  hour. — 
It  is  a  vulgar  belief  that  the  water  itself  advances  with  the  speed  of  the  wave, 
but  in  fact  the  form  only  advances,  while  the  substance,  except  a  little  spray 
above,  remains  rising  and  falling  in  the  same  place,  with  the  regularity  of  a 
pendulum.  A  wave  of  water,  in  this  respect,  is  exactly  imitated  by  the  wave 
running  along  a  stretched  rope  when  one  end  is  shaken  ;  or  by  the  mimic 
waves  of  our  theatres,  which  are  generally  undulations  of  long  pieces  of  car- 
pet moved  by  attendants.  But  when  a  wave  reaches  a  shallow  bank  or 
beach,  the  water  becomes  really  progressive,  for  then,  as  it  cannot  sink 
directly  downwards,  it  falls  over  and  forwards,  seeking  the  level. 

So  awful  is  the  spectacle  of  a  storm  at  sea,  that  it  generally  biases  the 
judgment;  and,  lofty  as  waves  really  are,  imagination  pictures  them  loftier 
still.  Now  no  wave  rises  much  more  than  ten  feet  above  the  ordinary  sea- 
level,  which,  with  the  ten  feet  that  the  surface  afterwards  descends  below 
this,  give  twenty  feet  for  the  whole  height,  from  the  bottom  of  any  water- 
valley  to  an  adjoining  summit.  This  is  easily  verified  by  a  person  who  tries 
at  what  heighten  a  ship's  mast  the  horizon  remains  always  in  sight  over  the 
top  of  the  waves — allowance  being  made  for  accidental  inclinations  of  the  ves- 
sel, and  for  her  sinking  in  the  water  to  considerably  below  her  water  line,  at 
the  time  when  she  reaches  the  bottom  of  the  hollow  between  the  two  waves. 
The  spray  of  the  sea,  driven  along  by  the  violence  of  the  wind,  is  of  course 
much  higher  than  the  summit  of  the  liquid  wave  ;  and  a  wave  coming  against 
an  obstacle,  or  entering  a  narrow  inlet,  may  dash  to  an  elevation  much  greater 
still.  At  Eddystone  light  house,  which  is  about  ninety  feet  high,  placed 
on  a  solitary  rock  ten  miles  from  land,  when  a  surge  breaks  which  has  been 
growing  under  a  storm  all  the  way  across  the  Atlantic,  it  often  dashes  to  100 
feet  above  the  lantern  at  the  summit. 

The  magnitude  of  waves  is  well  judged  of  when  they  are  seen  breaking 


WAVES.  219 

on  an  extended  shore  or  beach.  In  the  deep  sea  the  wave  is  only  an  eleva- 
tion of  the  water,  sloping  on  either  side ;  but  as  it  rolls  towards  the  shore, 
its  front  becomes  more  and  more  perpendicular,  until  at  last  it  curls  over 
and  falls  with  its  whole  weight,  and  when  several  miles  of  it  break  at  the 
same  instant,  its  force  and  noise  may  shake-  the  country  abroad. 

Along  the  east,  or  Corom-andel  Coast  of  India,  at  certain  season,  vast 
waves  are  constantly  breaking ;  and  as  there  are  no  good  harbours  there,  com- 
munication between  the  sea  and  land  is  rendered  impossible  to  ordinary  boats. 
The  natives  of  the  coast,  at  Madras,  for  instance,  have  hence  become  almost 
amphibious.  They  reach  ships  beyond  the  breakers  by  the  help  of  what  are 
called  catamarans,  consisting  of  three  small  logs  of  wood  tied  together.  On 
these  they  secure  themselves,  and  boldly  advance  up  to  the  coming  wall  of 
water,  which  they  shoot  into,  and  rise  to  the  smooth  surface  beyond  it,  like 
water-fowls  after  diving.  Boats  unsuited  to  the  breakers  often  perish  in  them. 
The  author  of  this  work  had  gone  on  shore  with  a  watering  party  on  the 
coast  of  Sumatra,  and  during  the  hours  spent  there,  a  swell  had  risen  in  the 
sea,  which  on  their  return  was  already  bursting  along  the  beach  and  across 
the  river's  mouth  in  lofty  breakers.  The  boat  in  which  he  happened  to  be, 
regained  the  high  sea  in  safety,  but  a  larger  boat  which  followed  at  a  short 
distance  was  overwhelmed,  and  an  officer  and  part  of  the  crew  perished. 

There  is  a  phenomenon  observed  at  the  mouths  01*  many  great  rivers,  called 
the  Boar,  which  has  resemblance  to  a  wave.  When  the  tide  returning  from 
sea  meets  the  outward  current  from  the  river,  and  both  have  the  force  which 
in  certain  situations  belongs  to  them,  the  stronger  mass  from  the  ocean  assumes 
the  form  of  an  almost  perpendicular  wall,  moving  inland  with  resistless  sweep. 
This  is  called  the  boar.  It  is  in  fact  the  great  sea-wave  of  the  tide,  produced 
twice  a-day  by  the  attraction  of  the  moon,  rolling  in  upon  the  land  and  inlets, 
where  contracting  channels  concentrate  its  mass.  In  the  different  branches 
of  the  Ganges  the  boar  is  seen  in  a  remarkable  degree.  Its  roaring  is  heard 
long  before  it  arrives.  Smaller  boats  and  skiffs  cannot  live  where  it  comes ; 
and  as  it  passes  the  city  of  Calcutta,  even  the  large  ships  at  anchor  there 
are  thrown  into  such  commotion,  as  sometimes  to  be  torn  away  from  their 
moorings. — The  nature  and  effects  of  this  boar  are  strikingly  illustrated 
upon  certain  coasts  where  extensive  tracts  of  sand  are  left  uncovered  at  low 
water.  In  such  situations,  of  which  there  are  many  on  the  western  shores 
of  Britain,  the  returning  tide  is  seen  advancing  with  steep  front,  and  with 
such  rapidity,  that  the  speed  of  a  galloping  horse  can  scarcely  save  a  person 
who  has  incautiously  approached  too  near.  Many,  every  year,  are  the 
victims  of  temerity  or  ignorance  on  these  treacherous  plains. 

In  the  end  of  the  year  1831,  on  the  low  flat  coast  of  the  Indian  peninsula, 
north  of  Madras,  one  great  wave  of  the  kind  now  described  was  produced 
during  a  very  high-spring  tide  of  midnight,  by  an  extraordinary  wind,  and 
spread  ten  miles  in  upon  the  inhabited  land.  It  had  retired  with  the  ebbing 
tide  before  morning,  but  the  next  day's  sun  disclosed  a  scene  of  devastation 
rarely  matched.  Amidst  the  total  wreck  of  the  villages  and  fields,  there  lay 
the  drowned  carcases  of  more  than  ten  thousand  human  beings,  mixed  with 
those  of  elephants,  horses,  bullocks,  wild  tigers  and  the  other  inhabitants 
of  the  land. 

It  has  been  proposed  lately  to  construct  sub-marine  boats,  or  vessels  cal- 
culated to  swim  so  deep  in  the  water  as  to  be  below  the  superficial  motion  of 
the  waves,  and  therefore  beyond  the  influence  of  storms  at  the  surface.  Such 
a  boat  has  been  tried  with  considerable  success;  and  man's  increasing  fami- 
liarity with  sub-marine  matters  since  the  invention  of  the  diving-bell,  may 


220  HYDRAULICS. 

ultimately  lead  to  improvements  rendering  the  sub-marine  vessel,  for  certain 
purposes,  commodious  and  safe. 

"  Fluids  resisting  the  motion  of  bodies  immersed  in   them,  or  themselves 
moving  forcibly  against  other  bodies.''     (See  the  Analysis.) 

The  same  force  is  required  to  give  or  to  take  away,  or  to  bend  motion,  in 
a  fluid,  as  in  an  equal  quantity  of  solid  matter.  A  pound  of  water  enclosed 
in  a  bladder  is  not  more  easily  thrown  to  a  given  height  than  a  pound  of  ice 
or  of  lead;  nor,  if  falling  into  the  scale  of  a  weighing  beam,  does  it  require 
less  as  a  counterpoise ;  nor,  if  made  to  revolve  at  the  end  of  a  sling,  does  it 
render  the  cord  less  tight. 

A  convenient  measure  of  the  force  of  moving  water  on  an  obstacle,  or  of 
the  resistance  of  still  water  to  a  moving  body,  exists  in  the  facts  already 
explained,  that  the  pressure  of  a  known  height  of  fluid  column  produces  from 
an  orifice  a  certain  velocity  of  jet,  while  conversely,  that  jet,  or  a  current  of 
equal  speed,  directed  against  the  orifice  supports  the  column.  The  impulse 
given  or  received,  therefore,  by  a  flat  surface  in  water,  such  as  the  vane  of 
a  water-wheel,  whether  that  of  a  steam-boat  pressing  against  the  water,  or 
that  of  a  corn-mill  pressed  by  it,  is  measured  by  the  weight  of  the  column 
alluded  to,  the  height  of  which  is,  according  to  the  velocity  and  the  breadth 
or  diameter,  according  to  the  breadth  or  extent  of  the  solid  surface  concerned. 
This  estimate  supposes  that  the  pressure  of  air  upon  the  surface  is  direct ;  if 
it  be  oblique,  there  is  a  diminution  according  to  the  rule  given  under  the 
head  of  "  resolution  of  forces." 

Many  persons  looking  carelessly  at  the  subject  of  fluid  resistance,  would 
expect  that  if  a  body,  as  a  boat,  moving  through  a  fluid  at  a  given  rate,  meets 
a  given  resistance,  it  should  just  meet  double  resistance  when  moving  twice 
as  fast.  Now  the  resistance  is  four  times  greater  with  a  double  rate. 

This  fact  is  but  another  example  of  a  principle  already  explained,  and 
when  more  closely  examined,  is  easily  understood.  A  boat  which  moves 
one  mile  per  hour,  displaces  or  throws  aside  a  certain  quantity  of  water,  and 
with  a  certain  velocity ; — if  it  move  twice  as  fast,  it  of  course  displaces  twice 
as  many  particles  at  the  same  time,  and  requires  to  be  moved  by  twice  the 
force  on  that  account;  but  it  also  displaces  every  particle  with  a  double 
velocity,  and  requires  another  doubling  of  the  power  on  this  account;  the 
power  then  being  doubled  on  two  accounts  becomes  a  power  of  four.  In  the 
same  manner  with  a  speed  of  three,  three  times  as  many  particles  are  moved 
and  each  particle  with  three  times  the  velocity;  therefore,  to  overcome  the 
resistence,  a  force  of  nine  is  wanted ;  for  a  speed  of  four,  a  power  of  sixteen ; 
for  a  speed  of  five,  a  power  of  twenty-five,  and  so  forth :  the  relations  being 
that  which  mathematicians  indicate  by  saying  that  the  resistance  increases  as 
the  square  of  the  speed.  The  corresponding  numbers,  up  to  a  speed  of  ten 
are  as  here  shown. 

Speed      ..123456789         10 

Correspondingj    j      £        9       16       25       36       49       64       81       100 
resistance     j 

Thus,  if  even  the  resistance  at  the  bow  of  a  vessel  was  all  that  had  to  be 
considered,  the  force  of  one  hundred  horses  would  only  drag  the  vessel  ten 
times  as  fast  as  the  force  of  one  horse.  But  there  is  another  important 
element  in  the  calculation,  viz  ;  the  lessening,  as  the  vessel's  speed  quick- 


WAVES.  221 

ens,  of  the  usual  water  pressure  on  the  stern, — which  pressure  while  she  is 
at  rest  is  equal  to  the  pressure  on  the  bow,  and  the  force  therefore  required 
to  produce  an  increased  velocity  is  still  considerably  greater  than  as  noted 
in  the  table. 

There  is  not  a  more  important  truth  in,  physics  than  the  law  of  fluid 
assistance  to  moving  bodies  here  treated  of;  .it  explains  so  many  phenomena 
of  nature,  and  becomes  a  guide  in  so  many  matters  of  art.  We  will  now 
set  forth  some  interesting  examples. 

It  explains  at  what  a  heavy  expense  of  coal  high  velocities  are  obtained  in 
steam-boats.  If  an  engine  of  about  50  horse  power  would  drive  a  boat  7 
miles  an  hour,  two  engines  of  50,  or  one  of  100  would  be  required  to  drive 
it  10  miles,  and  three  such  to  drive  it  12  miles,  even  supposing  the  increased 
resistance  to  the  bow,  as  already  stated,  to  be  the  measure  of  the  whole  work 
done,  which  it  is  not,  and  that  engines  worked  to  the  same  advantage  with  a 
high  velocity  as  with  a  low,  which  they  do  not. — For  the  same  reasons,  if  all 
the  coal  which  a  ship  could  conveniently  carry  were  just  sufficient  to  drive 
her  1,000  miles,  at  a  rate  of  12  miles  per  hour,  it  would  drive  her  more 
than  3,000  at  a  rate  of  7  miles  per  hour;  and  more  than»6,000  at  a  rate  of 
5  miles  per  hour.  This  is  a  very  important  consideration  for  persons 
concerned  in  steam  navigation  to  distant  parts. 

The  same  law  shows  the  folly  of  putting  very  large  sails  on  a  ship;  the 
trifling  advantage  in  point  of  speed  by  no  means  compensating  for  the  addi- 
tional expense  of  making  and  working  the  sails,  and  the  risk  of  accidents  in 
bad  weather.  The  ships  of  the  prudent  Chinese  have  not,  for  the  same 
tonnage,  one-third  so  much  sail  as  those  of  Europeans,  and  yet  they  move 
but  little  slower  on  that  account.  A  European  ship  under  jury-masts  does 
not  lose  so  much  of  her  usual  speed  as  most  people  would  expect. 

This  law  explains  also  why  a  ship  glides  through  the  water  one  or  two 
miles  an  hour  when  there  is  very  little  wind,  although  with  a  strong  breeze 
she  would  only  sail  at  the  rate  of  eight  or  ten  miles.  Less  than  the  100th 
part  of  that  force  of  wind  which  drives  her  ten  miles  an  hour,  will  drive  her 
one  mile  per  hour,  and  less  than  the  400th  part  will  drive  her  half  a  mile. 
Thus,  also,  during  a  calm,  a  few  men  pulling  in  a  boat  can  move  a  large 
ship  at  a  sensible  rate. 

These  considerations  show  strikingly  of  what  importance  to  navigation  it 
might  be  to  have,  as  a  part  of  a  ship's  ordinary  equipment,  one  or  two  water- 
wheels,  (  or  ready  means  of  forming  them,)  to  be  affixed  upon  the  ship's  side 
when  required,  like  the  paddle-wheels  of  a  steam-boat,  and  by  turning  which, 
the  crew  might  easily  deliver  themselves  from  the  tedium,  or  even  disastrous 
consequences  of  a  long  calm  at  sea. — This  idea  occurred  to  the  author  while 
in  a  ship  completely  becalmed  for  weeks  on  the  Line :  during  which  weari- 
some periods,  the  breezes  were  often  seen  roughening  the  water  a  mile  or 
two  farther  on ;  and  any  means  that  could  have  enabled  the  ship's  company 
to  advance  her  that  little  distance  might  have  saved  the  delay.  The  wheels 
might  be  driven  by  connection  with  the  capstan,  at  which,  under  such 
circumstances,  the  crew  would  most  willingly  work.  Delay  in  a  large  vessel 
often  costs  hundreds  of  pounds  per  day,  and  may  retard  the  execution  of 
important  projects. — But  the  propelling  of  a  ship  in  a  calm  seems,  by  no 
means,  the  most  important  purpose  which  such  wheels  might  serve.  If 
from  disease,  fatigue,  or  other  cause,  the  crew  were  inadequate  to  existing 
necessities,  two  wheels  affixed  to  the  extremities  of  an  axis  crossing  the  ship 
might  be  equivalent  in  many  cases  to  additional  hands,  or  to  a  steam-engine 
of  great  power ;  for  when  acted  upon  by  the  water  as  the  ship  sailed,  they 


222  HYDRAULICS. 

would  turn  with  the  force  of  water-wheels  on  shore,  and  might  be  made  to 
move  the  pumps;  to  hoist  the  sails,  and  to  do  any  work  which  a  steam-engine 
could  perform.  Many  a  gallant  vessel  has  perished  because  the  exhausted 
crew  could  not  longer  labour  at  the  pumps,  where  such  water-wheels  as  now 
contemplated,  or  a  wind-mill  wheel  in  the  rigging  would  have  performed  the 
duty  most  perfectly. 

The  law  that  resistance  to  a  body  moving  in  a  fluid  increases  in  a  greater 
proportion  than  in  speed  of  Ihe  body,  applies  where  the  fluid  is  aeriform, 
as  well  as  where  it  is  liquid. 

A  bullet  shot  through  the  air  with  a  double  velocity,  for  the  reason 
assigned  above,  experiences  four  times  as  much  resistance  in  front,  as  with 
a  single  velocity :  the  motion  is  retarded  also  by  the  diminution  of  the  usual 
atmospheric  pressure  of  15  Ibs.  per  inch  on  the  posterior  surface,  which 
diminution  is  proportioned  to  the  speed.  It  is  farther  true,  that  when  the 
velocities  of  bodies  moving  in  the  air  are  very  great,  the  resistance  increases 
in  a  still  quicker  ratio  than  in  liquids, — probably  because  the  compressibility 
of  air  allows  it  to  be  much  condensed  or  heaped  up  before  the  quick  moving 
body.  It  is  useless  to  discharge  a  cannon-ball  with  a  velocity  exceeding 
1,200  feet  in  a  second,  because  the  powerful  resistance  of  the  air  to  any 
velocity  beyond  that,  soon  reduces  it  to  that  at  least. 

The  rule  of  reciprocal  action  between  a  solid  and  fluid,  now  explained,  holds 
equally  when  the  fluid  is  in  motion  against  the  solid,  as  when  the  solid 
moves  through  the  fluid. 

If  a  ship  be  anchored  in  a  tide's  way,  where  the  current  is  four  miles  an 
hour,  the  strain  on  her  cable  is  not  one  fourth  part  so  great  as  if  the  current 
were  eight  miles. 

A  wind  moving  three  miles  an  hour  is  scarcely  felt :  if  moving  six  miles, 
it  is  a  pleasant  breeze ;  if  twenty  or  thirty  miles,  it  is  a  brisk  gale ;  if  sixty, 
it  is  a  storm;  and  beyond  eighty,  it  is  a  frightful  hurricane,  tearing  up 
trees  and  destroying  every  thing. 

Supposing  the  wind  to  move  one  hundred  miles  per  hour,  there  are  one 
hundred  times  as  many  particles  of  matter  striking  any  body  exposed  to  it, 
as  when  it  moves  only  one  mile  per  hour,  and  each  particle  strikes,  more- 
over, with  one  hundred  times  the  velocity  or  force,  so  that  the  whole  increase 
of  force  is  a  hundred  times  a  hundred,  or  ten  thousand.  This  explains  how 
the  soft  invisible  air  may  by  motion  acquire  force  sufficient  to  unroof  houses, 
to  level  oaks  which  have  been  stretching  their  roots  around  for  a  century, 
and  in  some  West  India  hurricanes,  absolutely  to  brush  every  projecting 
thing  from  the  surface  of  the  earth. 

The  law  of  rapidly  increasing  resistance  assigns  a  limit  to  many  velocities, 
both  natural  and  artificial. 

It  limits  the  velocity  of  bodies  falling  through  the  air.  By  the  law  of 
gravity,  a  body  would  fall  with  a  constantly  accelerating  speed,  but  as  the 
resistance  of  the  air  increases  still  more  quickly  than  the  speed  at  a  certain 
point,  this  resistance  and  the  gravity  balance  each  other,  and  the  motion 
becomes  uniform* 


ACTION    BETWEEN    FLUIDS    AND    SOLIDS.  223 

The  parachute,  by  means  of  which  a  person  may  safely  descend  to  the* 
earth  from  a  balloon  at  any  elevation,  furnishes  a  good  example.  The  con- 
trivance resembles  a  large  flat  umbrella.  The  aeronaut  attaches  himself 
underneath  it,  and  when  it  is  let  loose  from  the  balloon,  he  is  partly  sup- 
ported by  the  resistance  which  its  broad  expanse  experiences  in  falling 
through  the  air,  tind  falls,  therefore,  in  a  corresponding  degree  more  slowly. 
After  the  first  second  or  two,  for  the  reason  stated  above,  it  descends  with 
a  uniform  motion ;  and  its  breadth  is  generally  made  such,  as  to  allow  a 
velocity  of  about  eleven  feet  in  a  second,  or  that  which  a  man  acquires  in 
jumping  from  a  chair  two  feet  high. 

No  ship  sails  faster  than  fifteen  miles  in  an  hour. — And  it  is  because  the 
resistance  to  be  overcome  in  steam-carriages  on  rail-ways,  viz.,  their  friction, 
does  not  increase  with  their  velocity  like  the  fluid  resistance  to  steamboats, 
that  the  speed  of  the  former  may  so  much  exceed  that  of  the  latter. 

No  fish  swims  with  a  velocity  exceeding  twenty  miles  an  hour;  not  the 
dolphin,  when  shooting  ahead  of  our  swiftest  frigates,  nor  the  salmon,  when 
darting  forward  with  a  speed  which  lifts  him  over  the  water-fall. 

And  the  flight  of  birds  through  the  thin  air  has  a  limited  celerity.  The 
crow,  when  flying  homewards  against  the  storm,  cannot  face  the  wind  in  the 
open  sky,  but  skims  along  the  surface  of  the  earth  in  the  deep  valleys,  or 
wherever  the  swiftness  of  the  wind  is  retarded  by  terrestrial  obstructions. 
The  great  albatross,  stemming  upon  the  wing  the  current  of  a  gale  so  as  to 
keep  company  with  a  driving  ship  where  the  air  is  passing  at  the  rate  of  a 
hundred  miles  an  hour,  often  takes  shelter  momentarily  under  the  lee-side 
of  the  lofty  billows.  The  bird  called  the  stormy  petrel  abides  chiefly  in  the 
midst  of  the  Atlantic  Ocean,  but  the  irresistible  violence  of  the  wind  occa- 
sionally sweeps  it  from  the  waves,  and  causes  its  appearance  on  the  western 
shores  of  Europe.  Vessels  from  the  high  sea,  approaching  a  coast  from 
which  the  wind  blows,  generally  become  resting  places  to  exhausted  land 
birds  driven  off  the  shore  by  wind  which  they  have  not  had  strength  of  wing 
to  stem  ; — sad  evidences  of  the  myriads  which  are  constantly  perishing  where 
no  resting  place  is  found,  and  where  no  human  eye  notes  their  fate. 

The  action  or  resistance  between  a  meeting  fluid  and  solid,  is  influenced  by 
the  shape  of  the  solid. 

This  follows  from  what  has  already  been  said  of  direct  and  oblique  impulse. 
If  a  flat  surface  directly  opposed  to  the  fluid  experience  a  certain  resistance, 
a  projecting  surface  like  that  of  a  sphere  or  short  wedge  is  resisted  in  a  less 
degree,  and  a  concave  surface  in  a  greater.  The  explanation  is,  that  a  flat 
or  plane  surface  throws  the  particles  of  fluid  almost  directly  outwards  from 
its  centre  to  its  circumference,  and  therefore  with  greater  velocity,  while  the 
convex  or  wedge-like  surface,  although  displacing  them  just  as  far,  still  does 
so  more  slowly,  and  therefore  with  less  expenditure  of  force,  in  proportion 
to  the  obliquity  of  surface,  or  as  its  point  is  in  advance  of  its  shoulder  or 
broadest  part ;  and  a  concave  surface  must  give  to  some  of  the  particles  a 
forward  as  well  as  lateral  motion.  The  shape  of  the  hinder  part  of  a  solid 
moving  through  a  fluid  is  of  importance  for  corresponding  reasons. 

The  following  are  instances  of  projecting  or  wedge-like  surfaces,  intended 
to  diminish  the  resistance.  Fishes  are  wedge-like,  both  before  and  behind, 
their  form  being  modified,  however,  in  relation  to  other  objects  than  mere 
speed  of  motion.  Birds  are  so,  also  ;  and  they  stretch  out  their  necks  while 
flying,  so  as  to  make  their  form  perfect  for  dividing  the  air.  In  the  form 


224  HYDRAULICS. 

f)f  the  under  part  of  boats  and  ships,  men  have,  in  a  degree,  imitated  the 
shape  of  fishes.  The  light  wherries  which  shoot  about  upon  the  surface  of 
the  Thames,  appear  the  very  essence  of  what  imagination  can  picture  of  form 
combining  utility  and  grace.  There  are  boats  used  in  China  called  snake- 
boats,  which  are  only  a  foot  or  two  broad,  but  perhaps  a  hundred  feet  in 
length,  and  when  moved,  as  they  often  are,  by  nearly  a  'hundred  rowers, 
their  swiftness  is  extreme.  The  problem  of  which  it  is  the  object  to  assign 
for  a  ship's  hull  or.  bottom  the  best  possible  form  that  she  may  have  speed 
of  sailing,  is  not  yet  completely  solved;  so  that  a  kind  of  empiricism  prevails 
in  the  matter,  and  very  unexpected  results  often  arise.  Yet  the  subject 
merits  much  attention,  for  when  vessels  have  to  chase  and  to  flee,  speed 
becomes  of  the  greatest  importance;  and  at  all  times  the  sailor's  heart  swells 
with  delight  to  find  his  well-beloved  vessel  performing  well. 

The  following  instances  exhibit  the  mutual  influence  of  meeting  solids  and 
fluids,  where  the  surface  of  the  solid  is  plane  or  concave. — In  a  water-wheel, 
whether  the  water  be  moving  against  the  wheel,  as  is  the  case  where  a  stream 
acts  to  drive  machinery,  or  the. wheel  be  moving  against  the  still  water,  as 
in  the  case  of  the  paddle-wheels  of  a  steam-boat,  the  extended  faces  of  the 
vanes  or  float-boards  give  or  receive  a  powerful  impulse.  When  a  wheel 
with  float-boards  has  its  lower  part  merely  dipping  into  a  stream  of  Water, 
to  be  driven  by  the  momentum,  it  is  called  an  undershot-wheel;  when  the 
water  reaches  the  wheel  near  the  middle  of  its  height,  and  turns  it  by  falling 
on  the  float-boards  of  one  side  as  they  sweep  downwards  in  a  curved  trough 
fitting  them,  the  modification  is  called  a  breast-wheel ;  and  when  the  float- 
boards  are  shut  in  by  flat  sides,  so  as  to  become  the  bottoms  of  a  circle  of 
cavities  or  buckets  surrounding  the  wheel,  into  which  the  water  is  allowed 
to  fall  at  the  top  of  the  wheel,  and  to  act  by  its  weight  instead  of  its  momen- 
tum, the  modification  is  called  the  overshot-wheel.  To  have  a  maximum  of 
effect  from  wheels  moved  by  the  momentum  of  water,  they  are  generally 
made  to  turn  with  a  velocity  about  one-third  as  great  as  that  of  the  water ; 
and  wheels  moved  by  the  simple  weight  of  water  usually  have  their  circum- 
ference turning  with  a  velocity  of  about  three  feet  per  second.  The  subject 
of  water-wheels  is  one  of  the  most  important  in  practical  mechanics ;  for 
moving  water  performs  a  great  deal  of  labor  for  man. 

Oars  for  boats  are  made  flat,  and  often  a  little  concave,  that  the  mutual 
action  between  them  and  water  may  be  as  great  as  possible.  The  webbed 
feet  of  water-fowl  are  oars ;  in  advancing,  they  collapse  like  a  shutting  um- 
brella, but  open  outwards  in  the  thrust  backwards,  so  as  to  offer  a  broad 
concave  surface  to  the  water.  The  expanded  wings  of  birds  are  in  like 
manner  a  little  concave  towards  the  air  which  they  strike.  The  sails  of 
ships,  when  they  are  receiving  a  fair  wind,  are  left  slack  so  as  to  swell  and 
become  hollow. 

The  resistance  between  a  meeting  solid  and  fluid  being  nearly  proportioned 
to  the  breadth  of  the  solid,  it  follows  that  large  bodies,  because  containing 
more  matter  in  proportion  to  their  breadth  or  surface  than  smaller  bodies 
of  similar  form,  are  less  resisted  in  proportion  to  their  weights,  than 
smaller  bodies. 

The  science  of  measures  tells  us  that  a  bullet  or  other  solid  of  two  inches 
diameter,  has  eight  times  as  much  matter  in  it  as  a  similar  solid  of  one  inch 
diameter,  while  it  has  only  four  time  the  breadth  or  surface.  Thus,  by 
putting  eight  dice  or  little  cubes  together,  as  here  represented,  we  have  a 


ACTION    BETWEEN    FLUIDS    AND    SOLIDS.         225 

larger  cube,  of  which  compared  with  a  single  dice,  the  edge  is  evidently 
twice  as  long,  the  surface  four  times  as  great ;  and  the  quantity  of  matter 
eight  times  as  great; — again,  twenty -seven  dice  simi- 
larly put  together  form  a  cube  with  sides  three  times  Fig.  109. 
as  long,  and  the  surface  nine  times  as  great ;  and  sixty- 
four  dice  form  a  cube  with  sides  four  times  as  long, 
and  a  surface  sixteen  times  as  great.  All  solids  similar  M^TL- 
have  to  each  other  this  kind  of  relation,  which,  in  the 
language  of  the  science  of  quantity,  is  called  the  rela- 
tion of  cubes :  they  are  said  to  be  to  each  other  as  the 
cubes  of  any  of  their  corresponding  lines.    Hence  if  a 
bullet  of  eight  pounds,  and  a  bullet  of  one  pound  be  shot  off  with  equal 
velocity,  because  that  of  eight  pounds  has  only  half  as  much  surface  in  pro- 
portion to  its  weight,  and  therefore  to  its  moral  inertia  or  force,  as  the 
other,  it  will  go  much  farther  than  the  other. 

This  important  rule  explains  why  shells  and  large  shot  may  be  thrown 
four  or  five  miles,  while  smaller  cannon-balls,  musket-bullets,  pistol  and 
swan  shot,  and  the  common  small  shot  of  the  sportsman,  all  of  which  are 
generally  discharged  from  their  respective  pieces  with  the  same  commencing 
velocity,  have  a  shorter  range,  as  the  size  of  the  projectile  is  less.  Even 
water  is  sometimes  thrown  from  a  gun  or  powerful  syringe  to  stun  birds, 
that  they  may  be  obtained  with  uninjured  plumage ;  but  it  soon  divides  in 
the  air  so  minutely  that  it  reaches  only  a  short  distance. 

Water  falling  through  the  air  from  a  great  height,  goes  on  suffering  a 
gradual  division  into  smaller  and  smaller  portions,  which  at  last  may  be  said 
to  be  nearly  all  surface :  and  then  the  resistance  of  the  air  lets  them  fall 
very  slowly  indeed.  The  relation  of  the  size  and  resistance  is  well  shown 
by  the  difference  of  celerity  in  the  descent  of  a  minute  fog,  a  drizzling  mist, 
and  common  rain.  The  toy  called  the  iuater-hammcr,  is  merely  a  little 
water  enclosed  in  a  tube  exhausted  or  empty  of  air ;  and  when,  by  turning 
the  tube,  the  water  is  made  to  fall  from  one  end  to  the  other,  as  there  is  no 
air  to  impede  or  divide  it  in  its  descent,  it  falls  as  one  mass,  and  makes  a 
sharp  noise  like  the  blow  of  a  hammer. 

The  same  law  explains  why  a  spider's  thread  or  a  single  filament  of  silk 
floats  so  long  in  the  air  before  it  falls; — why  there  is  almost  constantly  sus- 
pended in  the  air,  wherever  active  man  resides,  that  immense  quantity  of 
very  minute  solid  particles,  which  when  rendered  visible  by  the  sun's  light 
passing  directly  through  them,  are  called  motes  in  the  sunbeam — particles  of 
which  are  constantly  settling  on  household  furniture,  and  rendering  necessary 
the  daily  operation  of  dusting  or  cleaning;  —  why  the  fine  dust  sent  aloft 
during  the  eruption  of  volcanoes  is  often  carried  by  the  wind  to  a  distance 
of  hundreds  of  miles; — why  in  the  deserts  of  Africa  the  strong  winds  often 
transport  fine  sand  from  place  to  place,  overwhelming  caravans,  and  forming 
new  mountains,  which  succeeding  blasts  are  again  to  lift; — why  in  the 
bottom  of  a  river,  or  in  a  tides-way,  fine  mud  is  found  where  the  current  is 
slow ;  sand  where  it  is  quicker ;  pebbles,  or  large  stones,  where  it  is  quicker 
still  j  while  in  rapids  and  water-falls,  only  massy  rocks  can  resist  the  fluid 
force.  Now  .rocks,  pebble,  sand  and  mud,  may  all  be  the  same  material  in 
portions  of  different  magnitude. 

This  law  explains  the  operation  of  levigating,  by  which  substances  inso- 
luble in  water  are  obtained  in  the  state  of  a  very  fine  powder.  Any  such 
substance  is  first  ground  or  powdered  in  the  ordinary  way,  and  mixed  with 
water.  The  grosser  parts  then  soon  fall  to  the  bottom,  while  the  fine  dust 

15 


226  HYDRAULICS. 

t. 

remains  longer  suspended.  This  is  afterwards  obtained  separately  by  pour- 
ing the  fluid  which  bears  it  into  another  vessel,  and  allowing  more  time  for 
the  slow  subsidence.  The  fine  powder  of  flint  used  in  the  manufacture  of 
porcelain  is  obtained  by  levigation  •  as  is  also  that  of  calamine  stone;  and 
other  powders  used  in  medicine  and  various  arts. 

This  law  farther  explains  how,  by  means  of  air  or  water,  bodies  of  differ- 
ent specific  gravities,  although  mixed  ever  so  intimately,  may  be  easily 
separated.  If  pieces  of  cork,  and  lead  be  let  fall  together  through  the  air, 
the  lead  will  reach  the  ground  first,  and  may  be  swept  away  before  the  cork 
arrives ;  but  in  a  vacuum  the  whole  would  reach  the  ground  at  the  same 
time  as  is  proved  by  the  common  experiment  of  the  guinea  and  feather 
falling  in  the  exhausted  receiver  of  an  air-pump.  Again,  when  a  mixture  of 
corn  and  chaff,  as  it  comes  from  any  threshing  machine,  is  showered  down 
from  a  sieve  in  a  current  of  air,  the  chaff  being  longer  in  falling,  is  carried 
farther  by  the  wind,  while  the  heavier  corn  falls  almost  perpendicularly. 
The  farmer,  therefore,  by  winnoioing  in  either  a  natural  or  artificial  current 
of  air,  readily  separates  the  grain  from  the  chaff;  and,  if  he  desire  it,  may 
even  divide  the  grain  itself  into  portions  of  different  quality.  Similar  to  the 
operation  of  separating  chaff  from  corn  by  wind,  is  that  of  separating  sand 
or  mud  from  gold-dust  by  water : — the  soil  containing  gold-dust  is  first 
spread  on  a  flat  surface,  over  which  a  current  of  water  is  then  made  to  pass  \ 
which  current  carries  away  the  lighter  rubbish,  and  leaves  the  gold.  If  a 
mass  of  metal  be  affixed  on  the  end  of  a  rod  of  wood,  the  rod  then,  whether 
simply  falling  through  the  air,  or  advancing  as  an  arrow,  will  follow  the 
heavier  metal  as  its  points.  The  cork  of  a  shuttlecock  is  always  foremost 
for  the  same  reason. 

The  instances  enumerated  under  this  head  serve  to  show  how  many  and 
varied  the  results  may  be  which  flow  from  a  single  principle. 


When  a  fluid  and  a  solid  meet  each  other  obliquely,  the  impulse  or  effect  is 
still  perpendicular  to  the  surface  of  the  solid,  as  if  they  met  directly,  but 
is  less  forcible  as  the  obliquity  of  the  approach  is  greater. 

Suppose  a  b  to  represent  the  upper  edge  of  a  smooth  board  or  of  any  flat 
polished  surface,  standing  in  a  current,  the  fluid  approaching  this  surface 
in  whatever  direction,  must  act  upon  it  as  if  approaching  perpendicularly, 
because  on  account  of  its  smoothness,  the  fluid  can  take 
Fig.  110.  no  hold  of  it  to  push  it  endways,  either  towards  a  or  b. 

But  the  impulse  of  a  stream  acting  on  the  surface  will 
be  less  forcible  if  the  surface  be  oblique  to  the  stream, 
both  because  less  fluid  will  touch,  and  because  the  velo- 
city of  the  effective  approach  will  be  less.  The  line  c  d 
marks  the  breadth,  and  therefore  force,  of  the  part  of 
a  stream  reaching  the  board  directly ;  and  the  shorter 
line/c  marks  the  smaller  breadth  that  can  touch  it,  of 
a  stream  coming  obliquely  in  the  direction  c  I :  in  the 
oblique  stream,  moreover,  if  the  line  c  b  mark  the  whole 
velocity,  the  shorter  line  c  a  marks  the  slower  rate  of 
the  direct  approach  of  any  one  particle  to  the  board. 
(This  subject  was  treated  of  at  page  57,  under  the  head 
of  Resolution  of  Forces.) 

Hence  the  wind  blowing  upon  the  sail  of  a  ship,  however  obliquely 


OBLIQUE    FLUID    ACTION. 


227 


Fig.  112. 


CL 


always  presses  it  directly  forward  or  perpendicularly  Fig.  111. 

to  its  surface,  but  acts  less  forcibly  as  the  obliquity  is 
•  greater.  If  the  wind  be  represented,  as  to  direction 
and  strength,  by  the  line  ed  approaching  the  sail  a  b, 
it  will  act  on  the  sail  as  if  it  came  from/*,  but  with 
the  smaller  force  f  d,  instead  of  the  whole  force  e  d. 
The  effect,  therefore,  is  the  same  as  if  the  sail  were 
pulled  by  the  rope  d  c.  We  see  in  this  how  a  ship 
can  be  made  to  sail  in  a  certain  degree  against  the 
wind  > — for  all  the  sails  being.adjusted  so  as  to  receive 
the  wind  in  the  direction  here  shown,  they  all  act  to 
produce  the  same  result  as  if  ropes  were  pulling  from 

each  in  the  direction  of  dc;  and  a  force  like/c?,  or  a  rope  like<ic,  urging  side- 
ways as  well  as  forwards — as  instanced  in  the  tow-rope  of  a  canal  boat — 
makes  the  vessel  advance  rapidly  forward,  but  scarcely  at  all  sideways,  be- 
cause the  form  of  vessels  causes  them  to  pass  forward  at  least  twenty  times 
more  easily  with  their  sharp  bow  than  sideways  with  the  long  keel;  and 
therefore  a  force  urging  equally  sideways  and  forwards  makes  a  ship  advance 
twenty  miles  in  the  direction  of  her  keel,  that  is  forwards,  for  one  mile  which 
she  deviates  sideways.  The  deviation  sideways,  which,  in  sailing  vessels, 
must  take  place  to  a  certain  extent  whenever  the  wind  is 
at  all  oblique,  is  called  the  lee-way. 

A  vessel  having  to  sail  from  b  to  a,  while  the  wind 
blows  directly  against  her  course,  or  from  a  to  b,  is 
obliged  to  sail  dose  to  the  wind,  as  represented  in  fig.  112, 
first  perhaps  to  e,  as  represented  by  this  figure,  with  the 
left  or  larboard  side  to  the  wind,  then  to  tack  as  it  is 
called,  or  turn  around,  at  e,  and  to  sail  to  d}  with  the 
right  or  starboard  side  in  the  wind ;  then  to  go  on  the  lar- 
board tack  again  to  c,  and  thence  to  the  port  at  a. 

In  making  way  against  a  contrary  wind,  the  sails  of  a 
ship  are  pointed  so  nearly  edgewise  to  the  wind,  that 
unless  very  flat,  a  great  portion  of  their  surface  becomes  useless.  The 
Chinese  manner  of  rigging  is,  in  this  respect  at  least,  superior  to  the  Euro- 
pean ;  for  in  it  bamboo  reeds  attached  across  the  sails,  render  them  as  flat 
as  boards.  When  a  Chinese  ship  has  her  sails  pointed  edgeways  to  a  spec- 
tator, he  only  sees  the  masts  which  support  them. 

The  reason  why  a  ship  of  several  masts  generally  sails  faster  when  the 
wind  is  more  or  less  from  a  side,  than  when  directly  astern,  is,  that  in  the 
former  case  all  the  sails  are  acting,  although  individually  not  to  the  best 
advantage,  while  in  the  latter  the  sails  in  front  are  becalmed  by  those  behind 
them.  A  ship  with  a  side-wind  may  move  faster  than  the  wind  itself,  as  is 
often  true  of  the  outer  extremities  of  the  wind-mill's  vanes.  A  correspond- 
ing relation  of  motions  is  observed  when  a  slippery  wedge  is  forced  out  two 
or  three  inches  laterally  from  its  place,  by  a  weight  which  descends  only  one 
inch  perpendicularly. 

The  law  now  under  consideration  explains  the  action  of  the  rudder  of 
ships, — that  contrivance,  by  which  a  single  steersman  can  direct  the  course 
of  an  enormous  vessel  through  rocks  and  shoals  more  steadily  and  safely 
than  an  adroit  charioteer  can  guide  his  tiny  vehicle  on  a  common  road.  The 
helm  or  rudder  is  a  flat  projection  from  the  stern-post  of  the  ship,  turning  on 
strong  hinges,  in  the  manner  of  a  door  or  gate,  and  moved  by  a  beam  or  lever 


.-''€. 


b 


228  HYDRAULICS 

called  the  tiller,  which  proceeds  from  it  forwards  to  where  the 
steersman  stands.  In  small  vessels  the  tiller  is  above  the  de'ck 
and  the  steersman  applies  his  hand  directly  to  it ;  but  in  large 
ships  it  is  below,  and  is  moved  by  ropes,  rising  from  it  to  the 
wheel  on  the  deck,  where  the  steersman  stands,  with  the  com- 
pass before  him.  While  the  rudder  points  directly  astern,  as 
to  a,  like  a  continuation  of  the  keel  and  stern  post,  it  does  not 
affect  the  vessel's  course ;  but  if  it  be  inclined  ever  so  little  to 
one  side,  as  to  b  on  the  left  or  larboard  side,  the  water  imme- 
diately acts  on  it  in  the  direction  of  c  b.  perpendicular*  to  its 
surface,  and  pushes  the  stern  to  the  right  or  starboard  side, — 
an  action  equivalent  to  pulling  to  the  left  or  larboard. 
It  is  possible  to  make  a  ship  or  boat  steer  itself,  by  placing  a  powerful 
vane  on  the  mast-head,  and  connecting  it  with  the  tiller-ropes  by  two  pro- 
jecting arms  from  its  axis.  If  it  were  desired  to  make  the  ship  sail  directly 
before  the  wind,  the  tiller-ropes  would  be  fixed  to  the  arms  of  the  vane  so 
that  the  helm  should  be  in  the  middle  position,  when  the  vane  was  pointing 
directly  forward  :  should  the  vessel  then  from  any  cause  deviate  from  her 
course,  the  vane,  by  its  changed  position  with  respect  to  her,  would  have 
produced  a  corresponding  change  in  the  position  of  her  helm,  just  .such  as 
to  bring  her  back  to  her  course.  Again,  it  is  evident  that,  by  adjusting  such 
a  vane  and  rudder  to  each  other  in  different  ways,  any  other  desired  course 
might  be  obtained,  and  which  would  alter  only  with  the  wind.  The  vane, 
to  have  the  necessary  power,  would  require  to  be  of  large  size ;  it  would  be 
a  wide  hoop,  for  instance,  with  canvas  stretched  upon  ifc ;  and  the  rudder,  to 
turn  with  little  force,  might  be  hung  on  an  axis  passed  nearly  through  its 
middle,  instead  of,  as  usual,  by  hinges  at  one  edge.  Cases  have  occurred 
where  shipwrecked  persons  might  have  sent  intelligence  of  their  disaster  to 
a  distant  coast,  by  a  small  vessel,  or  even  a  block  of  wood  fitted  up  in  this 
way.  The  method  admits  also  of  other  applications,  particularly  in  war. 

As  fluids  act  on  surfaces,  in  a  direction  perpendicular  to  them,  the  water 
on  the  right  side  of  a  ship's  bow  is  always  pressing  it  towards  the  left  side, 
but  owing  to  the  equivalent  and  contrary  pressure  there,  the  ship  holds  her 
course  evenly  between  the  two,  or  straight  forward.  When  a  ship,  however, 
owing  to  the  side  wind,  lies  over,  or  heels,  as  it  is  called,  that  side  of  the  bow 
which  sinks  most  is  more  pressed  than  the  other ;  and  were  there  not  a 
counteracting  inclination  of  the  rudder  then  made,  constituting  what  is 
called  weather-helm,  the  ship's  head  would  come  round  to  the  wind.  Now 
ships  so  rarely  have  the  wind  exactly  astern,  that  to  diminish  the  almost 
constant  necessity  for  weather-helm,  the  mast  or  masts,  and  consequently 
the  mass  of  the  sails  are  placed  more  towards  the  bow  than  the  stern. 

Again,  because  the  bow  of  a  ship  is  oblique  downwards  as  well  as  side- 
ways, the  water,  when  she  moves,  is  constantly  tending  to  lift  the  bow ; 
hence  when  the  vessel  is  dragged  by  a  low  horizontal  rope,  as  in  the  case  of 
a  boat  attached  to  a  sailing  ship's  stern,  or  is  moved  by  paddle-wheels  like 
steam-boats,  the  bow  rises  much  out  of  the  water,  and  the  stern  sinks  in  the 
hollow  or  furrow  of  the  track :  but  when  she  is  driven  by  sails,  as  these  are 
high  on  the  mast,  and  are  acting  therefore  on  a  long  lever  to  depress  the 
bow,  the  two  opposite  tendencies  just  balance  each  other,  and  the  vessel  sails 
evenly  along. 

The  form  of  the  fore  part  of  a  ship  has  less  influence  upon  her  speed  of 
sailing  than  the  form  of  the  hind  part,  called  the  run,  from  the  middle  to  the 
stern.  When  a  ship  is  at  rest;  there  is  of  course  as  much  forward  pressure 


OBLIQUE    FLUID    ACTION.  229 

of  water  about  the  stern  as  of  backward  pressure  on  the  bow ;  but  when  she 
sails,  she  is  running  away  from  the  propelling  pressure,  and  is  increasing 
the  resisting  pressure.  A  gradual  tapering  of  the  hind  part,  therefore,  or  a 
fine  run,  as  it  is  called,  which  allows  the  water  to  apply  itself  readily  to  it, 
as  it  passes  along,  must  influence  much  the  rate  of  sailing.  The  fore  part 
of  any  mass  drawn  through  the  water,  however  blunt  or  square,  becomes  in 
effect  sharp  or  rounded  by  a  quantity  of  water  which  it  pushes  on  before  it. 
A  tree,  or  the  tapering  mast  of  a  ship,  can  be  drawn  through  the  water 
more  easily  with  the  large  end  foremost  than  in  a  contrary  way. 

The  common  windmill  furnishes  another  illustration  of  the  action  of  fluids 
on  oblique   surfaces.     The  face  of  the  windmill 
is  turned  directly  to  the  wind,  but  the  four  flat  Fig.  114. 

vanes  or  sails,  of  which  the  great  wheel  consists, 
are  individually  oblique.  Thus  the  edge  a  of  the 
vane  a  e,  is  more  forward  as  regards  the  coming 
wind  or  a  spectator  in  front,  than  the  edge  e,  and  m 
the  action  of  the  wind,  therefore,  being  perpen- 
dicular to  the  oblique  surface  a  e,  pushes  it  in  a 
degree  towards  a.  The  same  remark  applies  to 
each  of  the  other  vanes  where  the  edges  b  c  and 
d  are  in  'front  and  those  marked  by  the  fainter 
lines  are  behind ;  so  that  each  vane  produces  an 
equal  effect  in  turning  the  wheel.  The  law  of  the  "  decomposition  offerees," 
explained  in  page  57,  tells  in  what  proportion  the  force  of  the  wind  is  exerted 
to  push  the  wheel  backwards  against  its  supports,  and  to  turn  it  round. 

Windmills  were  first  used  in  Europe  in  the  fourteenth  century,  and  they 
are  still  of  great  importance  in  countries  where  there  are  no  waterfalls,  and 
little  fuel  for  steam-engines.  In  some  of  the  richest  European  landscapes, 
every  height  is  crowned  by  its  busy  windmill,  grinding  corn,  or  sawing 
wood,  or  pressing  oil-seeds ;  and  over  the  plains,  similar  wheels  are  pump- 
ing water  for  domestic  use,  or  incessantly  draining  the  land. 

The  smoke-jack  of  our  chimneys  is  a  small  windmill,  driven  by  t  h 
ascending  current  of  air  in  the  chimney. 

The  feathering  of  an  arrow  acts  in  part  on  the  principle  of  the  windmill. 
The  feathery  projection  from  the  shaft  is  not  quite  straight,  but  winds  round 
it  a  little,  like  the  thread  of  a  screw ;  and  the  arrow,  therefore,  constantly 
turns  as  it  flies,  and  goes  straight  to  its  object  although  the  shaft  itself  be 
bent,  because  any  deviation  is  constantly  correcting  itself. 

The  rifle-barrel  in  fire-arms  has  spiral  furrows  or  threads  along  its  interior 
surface,  so  that  the  bullet  in  passing  out  receives  a  turning  motion  corres- 
ponding to  that  of  an  arrow,  and  producing  similar  results.  A  bullet  which 
receives  any  other  turning  motion  than  round  the  line  of  its  course — and 
most  bullets  from  a  common  barrel  do  acquire  such,  owing  to  the  irregularity 
of  their  form,  or  unequal  friction  at  the  mouth  of  the  piece — is  sure  to 
deviate  from  its  course,  because  unequally  pressed  or  resisted  by  the  atmo- 
sphere. A  good  rifle  fixed  to  its  place  will  send  a  succession  of  shots  through 
the  hole  made  in  the  target  by  the  first  shot,  at  the  distance  of  200  yards. 
Duels  have  been  fought  with  rifles,  and  the  parties  having  fired  at  the  same 
moment,  have  been  corpses  the  moment  after. 

It  might  be  supposed  that  a  wheel  which  the  wind  turned  by  direct  action 
on  the  rim,  as  water  turns  common  water-wheels,  would  be  preferable  to  the 
windmill-wheel  above  described,  which  is  turned  by  oblique  action  on  the 
face  :  accordingly,  a  wheel  like  a  water-wheel,  only  with  broader  vanes,  has 


230  HYDRAULICS. 

been  placed  in  a  house  or  cover,  so  that  only  one  side  at  a  time  was  exposed 
to  the  wind  ; — but  it  is  a  powerless  machine.  The  oblique  vane  wheel  may 
apply  to  use  only  half  or  less  of  the  force  of  the  air  which  reaches  it,  but 
its  wide  expanse  receives  a  stream  of  air  of  thirty  feet  in  diameter,  while  an 
ordinary  window  would  admit  that  required  for  a  wheel  of  equal  size  of  the 
other  construction. 

There  are  some  situations  where  it  would  be  an  advantage  to  have  water- 
wheels  like  the  common  windmill-wheel,  viz  ,  where  the  stream  is  sluggish, 
and  is  deep  enough  to  allow  a  large  wheel  to  be  wholly  immersed. 

A  small  wheel  of  this  sort,  with  broad  oblique  vanes,  has  been  used  as  a 
means  of  ascertaining  the  rate  of  a  ship's  sailing.  It  is  allowed  to  drag 
astern  in  the  water ;  and  the  number  of  revolutions  made  in  a  given  time 
marks  the  ship's  speed. 

A  windmill-wheel  made  to  turn  during  a  calm  by  force  applied  to  its  axle, 
would  be  pressed  endways,  or  in  the  direction  of  its  axle,  just  as  if  wind 
were  blowing  upon  it,  owing  to  the  reaction  of  the  still  air,  through  which 
its  oblique  vanes  were  made  to  sweep.  Such  a  form  of  wheel  fitted  to  work 
in  water,  and  called  a  water-screw,  has  been  applied  at  the  bow  or  stern  of 
steamboats  to  propel  them  in  canals  where  there  was  no  room  for  side 
wheels.  But  as  from  the  obliquity  of  the  surfaces  only  apart  of  the  applied 
power  becomes  propulsive — the  remainder  being  wasted  in  the  lateral  strain 
or  twisting  of  the  water — the  method  is  not  applicable  to  general  purposes. 
Two.  small  windmill-wheels  placed  horizontally  one  above  the  other,  on 
the  same  axis,  and  made  to  turn  in  opposite  ways  by  springs  or  otherwise, 
would  rise  in  the  air,  carrying  a  certain  load  with  them,  and  would  consti- 
tute, therefore,  a  flying-machine. 

The  effect  of  a  single  oar  projecting  from  the  stern,  used  to  propel  a  boat 
or  vessel,  in  the  manner  called  sculling,  is  referable  to  the  law  now  under 
consideration.  The  oar  or  scull  rests  on  a  round-headed  prop  or  nail  at  the 
stern,  and  is  made  to  vibrate  from  side  to  side.  In  all  positions  it  has  the 
surface  which  pressess  the  water  turned  obliquely  backwards;  hence  the 
reaction  of  the  water  drives  the  boat  forward.  In  China,  large  vessels  are 
moved  by  a  single  sculling  oar,  which  half  the  ship's  company  may  be  urging 
at  the  same  time.  A  sculling  oar  may  be  regarded  as  a  single  vane  of  such 
a  propelling-wheel  or  water-screw  as  above  described,  made  to  sweep  across, 
behind  the  vessel,  alternately  to  the  right  and  to  the  left. 

The  action  of  a  fish's  tale  and  of  the  bending  of  an  eel  or  snake  in  water, 
partly  resembles  that  of  the  sculling  oar.  Many  people  believe  that  the  tail 
of  the  fish  is  only  the  rudder  of  the  body,  and  that  the  fins  give  it  forward 
motion — as  is  true  of  a  bird's  tail  and  wings — but  the  fish's  tail  is  in  fact 
the  great  instrument  of  motion,  while  the  fins  serve 
chiefly  to  steady  and  direct  the  motion. 

A  paper  kite  rising  in  the  air  is  another  example 
belonging  to  this  place.  Its  cord  d  is  attached  to  it 
above  the  middle  of  its  loop,  and  therefore  so  as  to 
make  it  present  always  an  oblique  surface  to  the  wind ; 
and  by  the  action  of  the  wind,  perpendicular  to  its 
surface,  it  rises  as  if  pushed  up  in  the  direction  c  a, 
or  as  if  drawn  up  in  the  direction  of  a  b.  A  kite 
might  be  made  large  enough  to  lift  a  man.  Cats 
have  been  sent  up  at  kites'  tails,  and  have  fallen 
down  safely  under  parachutes  from  the  greatest  eleva- 
tions. It  might  be  safer  for  a  man  to  rise  at  a  kite's 


LIFTING    OF    WATER.  231 

tail  to  reconnoitre  an  enemy's  position,  or  to  survey  an  unknown  country, 
than  under  a  balloon,  as  was  practiced  by  the  French  during  the  revolutionary 
wars.  He  might  have  the  security  of  a  parachute,  and  the  power  of  regulat- 
ing the  obliquity  of  attachment  of  the  rope,  so  as  to  command  his  ascent  or 
descent  at  pleasure.  An  exhibition  was  made  in  October,  1827,  between 
Bath  and  London,  of  a  car  drawn  along  the  highway  by  kites.  That  they 
might  ascend  to  a  great  elevation,  where  the  wind  is  generally  stronger  than 
below,  they  were  attached  to  each  other  in  a  row,  so  that  the  second  kite 
mounted  as  if  its  cord  were  held  by  a  hand  at  the  first,  the  third  as  if  rising 
from  the  second,  and  so  forth.  The  projector  of  this  novelty  hoped  that  he 
had  pointed  out  a  most  valuable  means  of  travelling  across  extensive  plains, 
sandy  deserts,  tracks  of  snow,  &c.?  and  in  all  cases,  nearly  with  the  speed  of 
the  wind. 

"  Fluids  lifted  in  opposition  to  gravity."     (See  the  Analysis.) 

F  Water,  as  we  -have  seen  in  former  parts  of  this  work  is  to  the  living  uni- 
verse, in  some  degree  what  the  blood  is  to  the  animal  body,  and  a  constant 
supply  and  circulation  are  required.  This  supply  has  been  provided  for  to 
an  extraordinary  extent,  by  the  operation  of  natural  causes ;  but  for  many 
purposes  of  human  society,  water  is  still  required  where  none  naturally 
exists.  A  great  variety  of  means  have  been  employed  in  raising  it,  some 
of  which,  sufficient  to  illustrate  the  whole,  are  now  to  be  considered. 

Water  may  be  raised  in  a  bucket  which  is  attached  to  a  rope  to  be  pulled 
up  by  the  hand. — The  rope  carrying  the  bucket  may  be  drawn  up  more 
easily  by  being  wound  round  a  barrel  or  axle  turned  by  a  winch. — There 
may  be  a  succession  of  buckets  on  a  rope,  rising  one  after  the  other,  and 
when  emptied,  descending  again  on  the  opposite  side  of  the  wheel  or  axle 
which  lifts  them  :  the  rope  to  which  they  are  attached  being  a  circle  or  end- 
less  rope,  and  constituting  with  them  what  is  called  the  bucket-machine. — 
Instead  of  buckets  on  such  an  endless  rope  or  chain  there  may  be  a  succes- 
sion of  flat  pieces  of  wood,  which,  on  being  drawn  up  through  a  large  tube 
or  barrel,  like  loose-fitting  pistons,  will  raise  a  copious  stream  of  water ;  this 
is  the  contrivance  called  the  chain-pump. — Or  simply  an  endless  rope  of  hair, 
very  rough,  passing  round  one  wheel  above,  another  below,  may  be  whirled 
quickly  by  turning  the  upper  wheel,  so  that  a  mass  of  water  adhering  by 
friction  to  its  rising  half,  shall  be  thrown  into  a  reservoir  at  the  top  where  it 
passes  over  the  upper  wheel :  several  such  ropes  may  be  joined  side  by  side 
to  increase  the  effect. — But  the  most  important  of  all  water-raising  engines 
are  the  lifting  and  forcing  pumps,  already  described  at  pages  171  and  172. 
They  are  used  to  draw  from  wells,  to  drain  mines,  to  send  a  supply  over 
cities,  to  pump  ships,  to  throw  water  for  extin- 
guishing fires  and  for  many  other  purposes.  Fig.  116. 

A  stream  of  water  passing  through  a  garden, 
or  in  the  midst  of  fields,  may  have  beauty  with 
little  utility,  unless  it  can  be  employed  to  irrigate 
the  vegetable  creation  around.  In  the  fields  and 
gardens  of  Persia,  where  the  heat  of  the  sun  is 
very  intense,  the  streams  are  caused  by  their  own 
action,  to  lift  a  part  of  their  water  into  elevated 
reservoirs,  from  which  it  again  flows  in  sloping 
channels  to  wherever  it  is  required,  A  large 
water-wheel  is  placed  so  that  the  stream  may  turn 


232 


HYDRAULICS. 


it,  and  around  its  circumference  buckets  are  attached  to  be  filled  as  they 
sweep  along  below,  and  to  be  emptied  into  a  reservoir  as  they  pass  above—- 
or instead  of  buckets,  the  spokes  of  the  wheels  themselves  are  made  hollow, 
and  curved  as  here  represented,  so  that  their  extremities  dip  into  the  water 
at  each  evolution,  they  receive  a  quantity  of  it,  which  runs  along  them  as 
they  rise,  and  is  discharged  into  a  reservoir  at  the  centre.  These  are  usually 
called  Persian  wheels,  but  they  are  as  commonly  employed  on  the  banks  of 
the  Nile  and  elsewhere  as  in  Persia. 

A  pipe  wound  like  a  screw  upon  a  sloping  barrel,  and  made  to  dip  its  lower 
mouth  into  water  at  each  revolution  of  the  barrel,  will  also  raise  water ;  the 
lower  portions  of  the  turning  pipe  will  always  be  full  of  it,  and  it  will  be 

rising  in  them  to  the  top,  as  if  on 

Fig.  117.  an  inclined   plane.     Archimedes 

was  the  inventor  of  this  beautiful 
water-screw,  and  his  name  has  re- 
mained to  it.  It  may  be  turned  by 
the  hand  or  by  a  passing  stream 
wftich  acts  on  the  vanes  of  a  water- 
wheel  fixed  upon  it. 

Water  may  be  raised  by  produc- 
ing centrifugal  force  at  the  upper  end  of  a  bent  pipe  which  dips  into  a  res- 
ervoir.    Supposing  the  pipe  to  be  bent  as  here  represented, 
Fig.  118.      and  the  horizontal  arm  a  to  turn  like  the  spoke  of  a  wheel, 
round  the  upright  portion  as  the  axis, — if  the  pipe  be  once 
filled  with  water,  and  be  turned  with  sufficient  speed,  it  will 
continue  to  throw  out  a  constant  stream  from  the  end  a.    To 
increase  the  discharge  there  may  be  several  horizontal  arms 
from  one  large  upright  pipe,  and  emptying  themselves  into  a 
circular  trough  or  reservoir;  and  to  prevent  the  necessity  of 
refilling  the  apparatus  after  every  interruption  of  its  motion, 
a  valve  opening  upwards  must  be  placed  at  the  bottom.    This 
contrivance  has  been  called  the  centrifugal  pump,  because  the 
water  is  raised  at  b  as  in  a  pump,  by  the  pressure  of  the  atmosphere,  to  sup- 
ply the  place  of  that  which  is  thrown  out  from  a  by  the  centrifugal  force. 
The  velocity  of  rotation  must  bear  proportion  to  the  height  of  the  discharging 
aperture  a,  above  the  surface  of  the  water  in  the  reservoir. 

It  had  long  been  observed  in  household  experience  and  elsewhere,  that 
while  water  is  running  through  a  pipe,  if  a  cock  at  the  extremity  be  suddenly 
shut,  a  shock  and  noise  are  produced  there.  The  reason  is,  that  the  forward 
motion  of  the  whole  water  contained  in  the  pipe  being  instantly  arrested,  and 
the  momentum  of  a  liquid  being  as  great  as  if  a  solid,  the  water  strikes  the 
cock  with  as  much  force  as  if  it  were  a  long  bar  of  metal  or  a  rod  of  wood 
having  the  same  weight  and  velocity  as  the  water.  Then  as  a  fluid  presses 
equally  in  all  directions,  a  leaden  pipe  of  great  length  may  be  widened,  or 
even  burst  in  this  experiment. — Lately  this  forward  pressure  of  an  arrested 
stream  has  been  used  as  a  force  for  raising  water,  and  the  arrangement  of 
parts  contrived  to  render  it  available  has  been  called,  on  account  of  the  shocks 
produced,  the  water-ram.  The  ram  may  be  described  as  a  sloping  pipe  in 
which  the  stream  runs,  having  a  valve  at  its  lower  end,  to  be  shut  at  inter- 
vals to  arrest  the  stream,  and  having  a  small  tube  rising  from  near  that  end 
towards  a  reservoir  above,  to  receive  a  portion  of  the  water  at  each  interrup- 
tion. Now  water  allowed  to  run  for  one  second,  in  a  pipe  ten  yards  long, 


LIFTING    OF    WATER. 


233 


Fig.  119. 


O, 


two  inches  wide  and  sloping  six  feet,  acquires 
momentum  enough  to  drive  about  half  a  pint, 
on  the  shutting  of  the  cock,  into  a  tube  lead- 
ing to  a  reservoir  forty  feet  high.  Such  an 
apparatus,  therefore,  with  the  valve  shutting 
every  second,  raises  about  sixty  half-pints  or 
four  gallons  in  a  minute.  The*  valve  is  so 
contrived  that  the  steam  works  it  as  desired — 
In  this  figure,  which  represents  the  lower  end 

of  the  water-ram,  a  is  the  opening  by  which      _ 

the  stream  escapes  from  it,  and  the  valve  or 

flap  seen  below  the  opening  is  that  which  by  suddenly  shutting  arrests  the 
stream.  The  valve  is  made  so  heavy,  that  the  stream  must  run  for  a  certain 
time  to  acquire  force  enough  to  shut  it :  and  in  the  instant  of  its  shutting, 
a  little  of  the  advancing  water  passes  upwards  through  the  valve  b  towards 
the  reservoir.  The  water  in  the  main  pipe  then  becoming  stagnant  again, 
no  longer  has  power,  by  its  weight  alone,  to  keep  the  valve  a  shut :  this, 
therefore,  falls  open,  and  the  stream  begins  again,  again  to  be  arrested  as 
before  ;  and  as  long  as  the  supply  of  water  lasts,  the  action  of  the  apparatus 
continues.  The  action  of  a  water-ram  has  been  compared  to  the  beating  of 
an  animal's  pulse,  The  upright  tube  has  usually  a  reservoir  at  the  bottom, 
where  it  first  receives  the  water,  constituting  there  an  air-vessel  5,  (described 
at  page  161)  which,  by  the  air's  elasticity,  converts  the  interrupted  jets  first 
received,  into  a  nearly  uniform  current  towards  the  reservoir.  The  supply 
of  air  to  this  vessel  is  manifested  by  the  contrivance  called  a  sJiiftmg-valve. 
In  the  preceding  examination  of  the  doctrines  of  fluidity,  we  have  had  to 
touch  on  many  of  those  phenomena  of  nature  and  art  which  are  the  most 
important  to  man ;  yet  we  have  seen  how  beautifully  simple  and  intelligible 
they  are  all  rendered  when  referred,  by  a  methodical  arrangement,  to  a  few 
heads  dependent  on  the '"  fundamental  truths."  Each  one  of  the  many 
particulars  belonging  to  this  department,  and  which,  when  now  explained, 
appears  so  simple  and  obvious,  has  yet  been  a  distinct  step  in  the  slow 
progress  of  discovery  or  invention,  and  probably,  when  first  made,  has  filled 
some  ingenious  mind  with  intense  and  purest  delight. 


234  ACOUSTICS. 

PART  III. 

(CONTINUED.) 


SECTION  IV.— ACOUSTICS, 
OR  PHENOMENA  OF  SOUND  AND  HEARING. 


ANALYSIS   OP   THE   SECTION. 

1.  SOUND  is  heard  when  any  sudden  shock  or  impulse  is  given  to  the  air, 
or  to  any  other  body  which  is  in  contact  directly  or  indirectly  with  the  ear. 

2.  If  such  impulses  be  repeated  at  very  short  intervals,  the  ear  cannot  attend 
to  them  individually,  but  hears  them  as  a  CONTINUED  SOUND,  which  is 
UNIFORM,  or  what  is  called  a  TONE,  if  the  impulse  be  similar  and  at  equal 
intervals,  and  is  GRAVE  or  SHARP,  according  as  they  are  few  or  many  in 
a  given  time  /  and  all  continue^  sound  is  but  a  repetition  of  impulses. 

3.  When  the  number  of  impulses  in  a  giv£n  time  producing  some  uniform 
continued  sound  has  a  simple  relation,  as  of  half,  third,  fourth,  &c.,  to 
the  number  producing  some  other  such  sound,  which  is  heard  either  simul- 
taneously with  it,  or  a  little  before  or  after,  the  ear  is  generally  much  and 
pleasingly  affected  by  the  circumstance  ;  and  the  sounds  are  said  to  have 
MUSICAL  RELATION  to  each  other,  or  to  be  ACCORDANT,  while  all  others 
are  termed  DISCORDANT. 

4.  The  shock  which  causes  the  sensation  of  sound  SPREADS  or  is  propagated 
in  all  bodies,  somewhat  as  a  wave  spreads  in   water,  uith  decreasing 
strength  as  the  distance  increases,  but  with  a  velocity  nearly  uniform,  and 
which  in  air  is  about  1,1.^2  feet  per  second. 

5.  Sound  is  REFLECTED  from  smooth  surfaces,  and  hence  arise  many  curious 
and  pleasing  effects  called  ECHOES,  &c. 

6.  The  structure  of  the  ear  illustrates  the  law  of  sound. 


EARLY  inquiries  into  nature  had  remarked  that  in  most  instances  of  noise 
or  sound  there  was  present  a  shock  or  trembling  of  the  sounding  body,  often 
visible,  but  sometimes  only  sensible  to  the  touch,  or  discoverable  by  other 
means ;  it  was  noted  for  instance,  in  the  string  of  a  harp,  in  the  reed  of  a 
hautboy,  in  the  prongs  of  a  tuning-fork,  in  the  lip  of  a  bell;  but  it  was 
reserved  for  the  moderns  to  understand  fully,  that  the  animal  organ  called 
the  ear,  is  merely  a  structure  of  parts  admirably  adapted  to  be  affected  by 
the  concussions  or  tremblings  of  things  around ;  and  that  sounds  in  all  their 
varieties  are  merely  such  motions,  affecting  the  ear  through  the  medium  of 


CONTINUED    SOUND.  235 

the  air  which  surrounds  it,  or  of  some  other  body,  or  series  of  bodies  reach- 
ing from  the  trembling  thing  to  the  ear. 

The  delicacy  and  complexity  of  an  organ  destined  to  feel  and  to  distin- 
guish such  slight  and  varying  influences,  and  the  vast  importance  of  it  to 
man,  as  that  which  makes  him  capable  of  using  language,  besides  being  his 
ever-watchful  monitor  of  surrounding  occurrences,  and  the  channel  by  which 
the  fascination  of  music  enters,  render  this  subject,  to  all  who  love  to  read 
in  nature  the  attributes  of  its  author,  a  most  favourite  study. 

Because  all  the  bodies  around  us  are  immersed,  in  common  with  ourselves, 
in  the  ocean  of  air  which  covers  the  earth,  we  are  much  more  frequently 
warned  of  the  shocks  and  tremblings  of  which  we  have  been  speaking,  by 
their  effect  on  the  air,  than  in  any  other  way;  hence  the  early  prejudice  that 
air  was  necessary  to  sound,  and  hence,  in  part,  the  reason  why  the  doctrines 
of  sound  have  generally  been  accounted  a  part  of  pneumatics.  We  shall 
now  find,  however,  that  all  bodies  are  more  or  less  fitted  to  convey  these 
tremblings,  and  that  air  in  many  cases  is  neither  the  quickest  nor  the  best 
conductor.  Although  our  notions  on  the  subject  are  thus  corrected,  it  is 
still  convenient  to  study  the  theory  of  sound  as  a  part  of  Pneumatics. 

1.  "  Sound  is  heard  wTien  any  sudden  shock  or  impulse  occurs  in  a  body 
having  communication  through  the  air  or   otherwise,   with   the   ear." 
(Read  the  Analysis.) 

Common  instances  of  a  single  impulse  are — the  blow  of  a  hammer — the 
clap  of  hands — the  crack  of  a  whip — a  pistol-shot — any  explosion — the 
thunder-clap. 

The  loudness  of  sound  conveyed  by  the  air  depends  on  the  air's  density.  A 
bell  enclosed  in  the  receiver  of  an  air-pump  is  heard  less  and  less  distinctly 
as  the  air  is  exhausted,  and  in  a  vacuum  is  not  heard  at  all. — Even  the  blow 
of  a  hammer,  in  a  vacuum,  is  not  heajrd  if  care  is  taken  to  prevent  the  shock 
from  being  communicated  through  neighbouring  solid  bodies^ — In  the  thin 
air  surrounding  a  lofty  mountain-top  the  report  of  a  pistol  is  much  less  loud, 
and  human  voices  are  weaker. — In  the  condensed  atmosphere  of  a  diving- 
bell  a  whisper  is  loud. — When  volcanoes  and  various  other  resemblances 
to  the  constitution  of  our  earth  were  first  discovered  in  the  moon,  some 
persons  fancied  that  during  the  stillness  of  night  we  should  hear  the  thunder 
there: — but  supposing  the  thunder  to  happen,  and  to  be  ever  so  loud,  it 
could  not  be  heard  on  earth,  because  there  is  no  medium  to  bear  thither  the 
pulses  of  sound — there  is  a  vacuum  between. 

2.  Impulses  quickly  repeated  cannot  be  individually  attended  to  ~by  the  ear, 
and  hence  they  appear  as  one  continued  sound,  of  which  the  pitch  or  tone 
depends  upon  the  number  occurring  in  a  given  time  ;  and  all  continued 
sound  is  but  a  repetition  of  impulses.     (Read  the  Analysis.) 

If  a  wheel  with  teeth  be  made  to  turn  and  to  strike  any  elastic  plate,  as 
a  piece  of  quill,  with  every  tooth,  it  will,  when  moved  slowly,  allow  every 
tooth  to  be  seen  and  every  blow  to  be  separately  heard ;  but  with  increasing 
velocity  the  eye  will  lose  sight  of  the  individual  teeth,  and  the  ear  ceasing 
to  perceive  the  separate  blows,  will  at  last  hear  only  a  smooth  continued 
sound  called  a  tone,  of  which  the^character  will  change  with  the  velocity  of 
the  wheel. 

In  like  manner  the  vibrations  of  a  long  harp-string,  while  it  is  very  slack, 
are  separately  visible,  and  the  pulses  produced  by  it  in  the  air  are  separately 


236  ACOUSTICS. 

audible;  but  as  it  is  gradually  tightened,  its  vibrations  quicken,  so  that, 
•where  it  is  moving,  the  eye  soon  sees  only  a  broad  shadowy  bellying  line ; 
and  the  distinct  sound  which  the  ear  lately  perceived,  seeming  now  to  run 
together  on  account  o'f  the  shortness  of  the  intervals,  are  felt  as  one  uniform 
continued  tone,  which  constitues  the  note  or  sound  then  belonging  to  the 
string. 

Again,  if  a  current  of  air  passing  through  a  tube  or  opening,  be  in  any 
way  interrupted  at  regular  and  very  short  intervals,  as  by  a  little  stop-cock 
placed  in  the  opening,  of  which  cock  the  plug,  instead  of  being  only  partially 
turned  by  a  cross  handle,  as  in  a  common  beer-cock,  has  a  wheel  fixed  upon 
it,  so  that  any  desired  rapidity  of  rotation  may  be  given  to  it, — then  at  every 
time  when  the  passage  for  air  becomes  open,  there  will  be  a  certain  shock 
given  to  the  air  around,  and  the  repetition  of  such  shocks  will  constitute  a 
musical  tone.  This  apparatus  can  produce  all  tones,  and  it  enables  us  with 
great  precision  to  ascertain  the  number  of  pulses  required  to  constitute  any 
given  tone. 

It  is  the  elasticity  of  any  string  used  to  produce  a  tone  which  causes  the 
repetition  of  the  percussion,  and  therefore  the  continuance  of  the  sound, 
thus  : — the  string  having  been  pulled  at  its  middle  to  one  side,  and  then  let 
go,  is,  owing  to  its  elasticity,  carried  back  quickly  to  the  straight  position ; 
but  by  the  time  that  it  has  reached  this,  it  has  acquired  a  momentum  which, 
like  the  momentum  of  a  vibrating  pendulum,  carries  it  nearly  as  far  beyond 
the  middle  station  as  the  distance  whence  it  came : — it  has  to  return, 
therefore,  by  its  elasticity,  from  this  second  deviation,  in  the  same  way ;  but 
still  passing  the  middle  as  before,  it  has  again  to  return ;  and  thus  continues 
vibrating  uniformly  as  a  pendulum  does,  until  the  resistance  of  the  air  and 
friction  gradually  bring  it  to  rest.  A  large  vibration  of  any  string,  like  a 
large  oscillation  of  a  pendulum,  occupies  very  nearly  the  same  time  as  a 
smaller,  because  the  farther  that  the  string  is  displaced  or  bent,  the  more 
forcibly,  and  therefore  quickly,  is  it  pulled  back  again  by  its  elasticity ; 
hence  the  uniformity  of  the  tone  produced  by  a  musical  string  is  not  injured 
by  the  different  force  with  which  the  finger  of  the  player  may  touch  the 
string.  According,  however,  as  the  vibrations  of  a  string  are  more  extensive 
or  quicker,  the  impulses  given  to  the  air  are  more  sharp  or  forcible,  and 
hence  the  sound  becomes  louder.  And  this  explains  why  sharp  sounds  are 
generally  also  loud.  Vibrations  which  are  comparatively  few  and  slow, 
strike  the  ear  very  gently,  as  in  the  flapping  of  a  pigeon's  wing,  or  in  the 
play  of  a  switch. 

The  most  familiar  instance  of  sounding -vibration  is  that  of  an  elastic  cord 
extended  between  two  fixed  points,  as  in  stringed  instruments  of  music  ;  but 
because  elastic  bodies  generally,  when  by  any  force  their  natural  form  is  for 
a  time  altered,  recover  it  when  allowed,  not  by  a  first  effort,  but  like  the 
string  of  a  pendulum,  after  a  series  of  oscillations,  almost  all  such  bodies 
repeat  many  times  an  impulse  once  given  to  them,  and  thus  may  become  the 
means  of  producing  a  continued  sound. — If  a  solid  rod  of  steel,  glass,  or  any 
other  elastic  substance,  be  firmly  fixed  at  one  end  and  left  free  at  the  other, 
and  if  that  other  be  then  pulled  a  little  to  one  side  of  its  station  or  rest,  and 
suddenly  let  go,  it  will  immediately  seek  its  station  again,  but  by  the  momen- 
tum acquired  in  the  approach,  will  go  beyond  it :  it  will  then  return  as  be- 
fore, but  again  to  pass,  and  so  will  continue  to  vibrate  with  diminishing  force 
for  considerable  time. — A  boy  at  school,  thus,  sticks  the  point  of  his  pen- 
knife into  the  bench,  and  by  one  touch  makes  it  produce  a  continued  uniform 
sound  of  considerable  duration. — The  prongs  of  a  tuning-fork,  or  of  the  com- 


CONTINUED    SOUND.  237 

mon  sugar  tongs,  vibrate  and  sound  in  the  same  way.  In.  the  musical  snuff- 
boxes and  chimney-clocks,  the  sounds  are  produced  by  the  vibration  of  little 
rods  of  steel,  fixed  by  one  end,  in  a  row,  like  the  teeth  of  a  comb,  and 
touched  by  small  pins  or  points  projecting  from  a  turning  barrel.  Any 
elastic  flap,  as  of  metal  or  of  tough  wood,  placed  over  an  opening,  so  as  to 
stand  away  from  it  a  little  when  not  pressed  by  passing  air,  but  to  close  the 
opening  if  so  pressed,  becomes  a  sounding  reed  when  air  is  gently  forced 
through  the  opening  :  thus,  the  air  pressing  on  the  flap  to  close  them  causes 
a  momentary  interruption  of  the  current,  but  the  flap  immediately  recoiling 
from  the  blow,  as  well  as  by  reason  of  its  own  elasticity,  again  opens  the 
passage,  and  the  continued  rapid  alteration  of  the  shutting  and  opening  pro- 
duces the  tone.  The  reed  of  a  clarionet  is  a  thin  plate  of  elastic  wood,  made 
to  vibrate  in  this  way.  The  drone  of  the  bag-pipe  and  the  common  straw- 
pipe,  are  reeds  of  nearly  the  same  kind.  The  Chinese  organ,  and  the  sweet 
instrument  lately  introduced  under  the  name  of  .^Eolian,  have  reeds  which 
differ  from  these,  by  beating  through  the  opening  instead  of  merely  on  its 
face.  Elastic  rods  simply  resting  on  supports  at  both  ends,  or  suspended  by 
their  middle,  will  also  vibrate;  a  musical  instrument  is  thus  made  of  pieces 
of  glass  laid  upon  two  strings,  and  struck  by  a  cork  hammer :  in  the  island 
of  Java,  a  rude  instrument  of  the  same  kind  is  made  of  blocks  of  hard 
elastic  woodF.  A  portion  of  a  hollow  sphere  of  elastic  metal  very  readily 
takes  on  a  vibration,  during  which  its  form  is  constantly  changing  from  the 
perfect  round  to-  the  oval,  and  conversely ;  there  are  consequently  repeated 
percussion  of  the  air,  and  a  continued  sound,  and  the  thing  is  called  a  bell. 
A  bell  admits  a  great  variety  of  shapes,  and  may  be  made  of  any  elastic  sub- 
stance, as  metal,  glass,  earthenware,  (buyers  ring  earthenware  to  ascertain 
its  soundness,)  and  even  of  hard  wood.  The  Chinese  gong  is  a  metallic  vessel 
shaped  like  a  common  sieve,  having  a  manner  of  vibration  very  peculiar, 
and  producing  sounds  that  are  rousing  and  sublime.  The  drum  has  a  tense 
elastic  membrane  on  which  the  blows  of  the  drum-stick  are  received :  its 
tone  ceases  quickly,  because  the  motion  of  so  broad  a  surface  is  much' 
resisted  by  the  air.  In  the  flute,  flageolet,  common  organ-pipes,  &c.,  the 
air  is  forced  through  narrow  passages,  and  is  divided  by  sharp  edges,  in  such 
a  way  as  to  suffer  repeated  but  perfectly  regular  condensations  or  interrup- 
tions surncient  to  affect  the  ear*;  and  hence  the  endless  variety  of  sweet 
continued  sounds  which  these  contrivances  are  known  to  produce. 

To  the  production  of  a  tone,  it  is  of  no  consequence  in  what  way  the 
pulses  of  the  air  are  caused,  provided  they  follow  with  sufficient  regularity ; 
witness,  in  addition  to  some  of  the  instances  given  above,  the  pure  sound 
produced  by  the  motion  of  a  fly's  wing — supposed  by  many  to  be  the  voice 
of  an  insect.  The  clacking  of  a  corn-mill,  and  the  noise  of  a  stick  pulled 
along  a  grating,  are  not  tones,  because  the  pulses  follow  too  slowly. 

Where  a  continued  sound  is  produced  by  impulses  which  do  not,  like 
those  of  an  elastic  body,  follow  in  regular  succession,  the  effect  ceases  to  be 
a  clear  uniform  sound  or  tone,  and  is  called  a  noise.  Such  is  the  sound  of 
a  saw  or  grind-stone — the  roar  of  the  waves  breaking  on  a  rocky  shore,  or 
of  a  violent  wind  in  a  forest — the  roar  and  crackling  of  houses  or  of  a  wood 
in  flames — the  mixed  voices  of  a  talking  multitude — the  diversified  sounds 
of  a  great  city,  including  -the  rattling  of  wheels,  the  clanking  of  hammers, 
the  voices  of  street-criers,  the. noises  of  manufactories,  &c. ;  which  rough 
elements,  however,  at  last  mingle  so  completely  that  the  combined  result 
has  often  been  called  "  the  hum  of  men/'  from  analogy  to  the  smooth 
mingling  miniature  sounds  which  constitute  the  hum  of  a  bee-hive. 


238  ACOUSTICS. 

"  Grave  and  sharp  sounds."      (Read  the  Analysis.) 

The  difference  of  sounds,  which  depends  on  the  different  number  of  vibra- 
tions of  the  sounding  body  in  a  given  time,  divides  them  into  those  called 
bass,  low,  or  grave  notes,  for  comparatively  few  and  slow  vibrations ;  and 
those  called  high,  shrill,  or  sharp,  for  vibrations  more  numerous  and  quick. 

The  frequency  of  vibrations  in  strings  increases  with  their  shortness,  light- 
ness and  tension — for  if  a  string  be  long  or  heavy,  there  is  a  greater  mass  of 
matter  to  be  moved,  and  hence  a  slower  motion;  and  if  a  string  be  slack, 
the  force  of  elasticity  which  pulls  it  from  any  deviation  back  to  the  straight 
line  is  so  much  the  less.  It  is  found  that  a  string  taken  of  half  the  length 
or  of  one-fourth  the  weight,  or  of  quadruple  the  tension  of  another  string, 
vibrates  twice  as  fast  on  any  one  of  these  accounts. 

These  truths  are  familiarly  illustrated  in  the  violin.  The  low  or  bass 
string  is  thick  and  very  heavy  from  being  covered  with  metallic  wire,  and  the 
others  gradually  diminish  in  magnitude  and  weight,  up  to  the  smallest  or 
treble.  The  strings  are  tuned  to  each  other  by  being  attached  by  one  end 
to  moveable  pins,  which,  when  tuned,  increase  or  diminish  their  tension ;  and 
the  sound  produced  by  each  may  be  afterwards  varied  to  a  certain  extent, 
by  the  performer  pressing  different  parts  of  it  with  the  finger  against  the 
board,  so  as  to  shorten  the  vibrating  portion.  • 

An  analogous  law,  as  to  the  influence  upon  tone,  of  weight  and  dimen- 
sions, holds  with  respect  to  bells,  glasses,  reeds,  &c.,  and  enables  us  to  use 
these  also  in  the  construction  of  musical  instruments. 

3.  "  When  the  number  of  impulses  producing  some  continued  sound  has  a 
simple  rotation,  as  of  half,  third,  fourth,  &c.,  to  the  number  producing 
some  other  sound,  which  is  heard  either  simultaneously,  or  a  little  before 
or  after  it,  the  ear  is  much  and  pleasingly  affected  ;  and  the  sounds  are 
said  to  have  musical  relation  to  each  other,  or  to  be  accordant,  while  all 
others  are  termed  discordant."  (Read  the  Analysis.) 

Understanding  now  that  all  continued  uniform  sounds  are  produced  by  a 
repetition  of  similar  beats  or  vibrations,  we  perceive  that  in  the  series  from 
grave  to  sharp,  there^must  be  such  as,  with  respect  to  the  number  of  beats 
in  a  given  time,  are  related  to  each  other,  as  the  numbers  1,  2,  3,  4,  &c.,  or, 
which  is  the  same  thing,  as  10,  20,  30,  &c.  Now  as  between  two  sounds, 
one  of  which  has  20  beats  while  another  has  10,  there  will  be  a  coincidence 
by  every  second  beat  of  the- quicker,  and  between  sounds  whose  beats  are  to 
each  other  as  30  to  20,  there  must  be  a  coincidence  at  every  third  beat  of 
the  quicker,  and  so  forth,  we  should  naturally  expect  the  ear  to  be  differently 
affected  by  such  correspondence  than  when  the  coincidence  is  either  less 
frequent,  or  is  irregular.  Accordingly  we  find  that  all  sounds  which  have 
such  simple  relations  to  each  other,  are  remarkably  agreeable  to  the  ear, 
either  when  heard  together,  or  in  close  succession ;  while  those  in  which  the 
coincident  beats  are  farther  apart,  are  heard  with  indifference,  or  are  felt  to 
be  positively  harsh  or  disagreeable.  It  is  in  fact  offering  itself  to  be  noticed 
here,  that  the  coincident  or  double  pulses  of  any  two  concordant  sounds  be- 
come the  cause  of  elements  of  a  third  sound,  perfectly  distinct  from  them, 
but  which  is  always  heard  with  them,  and  is  called  their  grave  harmonic  or 
resultant:  it  is  the  same  as  a  simple  sound  having  as  many  vibrations  in  a 
given  time  as  there  are  coinciding  beats  between  the  two  other  sounds. 

If  a  long  musical  string  be  made  to  sound,  and  the  number  of  its  vibrations 
in  a  given  time  be  ascertained,  we  find  that  if  only  half  of  it  be  allowed  to 


MUSIC.  239 

vibrate  at  a  time,  as  when  a  finger  presses  its  middle  against  a  board,  that 
half  will  vibrate  twice  as  fast ;  and  similarly,  a  third  part  three  times  as 
fast;  a  fourth  part  four  times  as  fast;  and  so  on,  producing  the  sounds  or 
tones  most  nearly  related  to  each  other.  A  fine  illustration  of  this  is  afforded 
by  the  string  of  a  violoncello,  when  made  to  vibrate  by  a  bow  moved  very 
gently  across  it,  near  the  bridge ;  for  it  often  divides  itself  spontaneously 
into  two,  three  or  four,  &c.,  equally  vibrating  parts  or  bellies,  with  points 
of  rest  between  them  called  knots :  when  this  happens,  there  are  heard  not 
only  the  sound  or  note  belonging  to  the  whole  length  of  the  string,  but, 
also,  more  feebly,  the  subordinate  notes  belonging  to  its  half,  third,  or 
fourth,  &c.,  according  to  circumstances,  beautifully  mingling  with  the  first 
sound,  and  forming  with  it  a  rich  harmony.  Often  in  such  a  case  the  sub- 
ordinate sounds  swell  with  such  force  as  to  overpower  for  a  time  the  funda- 
mental note :  but  any  one  such  sound  is  rarely  of  long  duration.  The  same 
harmonic  sounds  may  be  produced  still  more  certainly,  while  drawing  the 
the  bow  across  the  string,  by  touching  the  string  lightly  with  the  finger,  at 
one  of  the  points  where  we  wish  it  to  divide.  Even  a  tune  may  be  so 
played. 

The  sounds  thus  belonging  to  a  single  cord  or  string,  and  produced  by  its 
spontaneous  division  into  different  numbers  of  equal  parts,  constitute,  when 
heard  together  or  in  succession,  what  may  be  called  the  simple  music  of 
nature  herself.  It  is  produced  pleasingly,  as  just  described,  by  the  single 
string  of  a  violoncello ;  but  in  the  most  perfect  manner  by  the  instrument 
called  the  ^Eolian  harp. 

The  JEolian  harp  is  a  long  box  or  case  of  light  wood,  with  harp  or  violin 
strings  extended  on  its  face.  These  are  generally  tuned  in  perfect  unison 
with  each  other,  or  to  the  same  pitch,  as  it  is  expressed,  except  one  serving 
as  a  bass,  which  is  thicker  than  the  others,  and  vibrates  only  half  as  fast; 
but  when  the  harp  is  suspended  among  trees,  or  in  any  situation  where  the 
fluctuating  breeze  may  reach  it,  each  string,  according  to  the  manner  in 
which  it  receives  the  blast,  sounds  either  entire,  or  breaks  into  some  of  the 
simple  divisions  above  described  j  the  result  of  which  is  the  production  of 
the  most  pleasing  combination  and  succession  of  sounds  that  ear  has  ever 
listened  to,  or  fancy  perhaps  conceived.  After  a  pause  this  fairy  harp  may 
be  heard  beginning  with  a  low  and  solemn  note,  like  the  bass  of  distant 
music  in  the  sky  :  the  sound  then  swells  as  if  approaching,  and  other  tones 
break  forth,  mingling  with  the  first,  and  with  each  other ;  i-n  the  combined 
and  varying  strain,  sometimes  one  clear  note  predominates  and  sometimes 
another,  as  if  single  musicians  alternately  led  the  band  :  and  the  concert 
often  seems  to  approach  and  again  to  recede,  until  with  the  unequal  breeze 
it  dies  away,  and  all  is  hushed  again. — It  is  no  wonder  that  the  ancients, 
who  understood  not  the  nature  of  air,  nor  consequently  even  of  simple 
sound,  should  have  deemed  the  music  of  the  ^Eolian  harp  supernatural,  and, 
in  their  warm  imaginations,  should  have  supposed  that  it  was  the  strain  of 
invisible  beings  from  above,  come  down  in  the  stillness  of  evening  or  night 
to  commune  with  men  in  a  heavenly  language  of  soul  intelligible  to  both. 
But  even  now  that  we  understand  it  well,  there  are  few  persons  so  insensible 
to  what  is  delicate  and  beautiful  in  nature,  as  to  listen  to  this  wild  music 
without  emotion ;  while  the  informed  ear  finds  it  additionally  delightful,  as 
affording  an  admirable  illustration  of  those  laws  of  sound  which  human 
ingenuity  at  last  has  traced. 

And  the  simple  scale  of  sound,  called  a  chord,  which  nature  thus  gives 
by  the  spontaneous  dividing  of  a  single  string,  has  considerable  vacancies  in 


240 


ACOUSTICS. 


it,  human  taste  or  feeling,  long  before  there  was  any  theory  of  music,  had 
joined  to  it  the  notes  of  two  additional  strings,  one  sharper  or  more  acute 
than  it,  and  the  other  more  grave ;  of  which  additional  notes,  while  part 
agreed,  or  were  in  unison  with  certain  notes  of  the  principal  chord,  the 
remainder  just  served  to  fill  up  its  larger  intervals,  and  to  complete  a  scale 
of  nearly  uniform  interval — as  three  ladders  having  unequal  intervals 
between  their  steps,  might  still,  if  placed  together,  complete  a  stair  of  easy 
ascent.  The  relation  between  these  strings  or  chords  is  such,  that  the  prin- 
cipal beats  thrice  or  twice  of  the  low  chord,  and  the  high  chord  beats  thrice 
for  twice  of  the  principal :  and  in  the  complete  scale  of  notes,  the  principal 
is  five  notes  above  the  lower  and  five  notes  below  the  higher.  So  truly 
natural  is  the  scale  thus  formed,  that  it  has  arisen  in  all  nations,  however 
remote  and  unconnected ;  and  an  untutored  individual  in  attempting  to  raise 
his  voice  by  regular  steps,  falls  into  it  almost  as  readily  as  the  learned  pro- 
fessor. The  scale  has  eight  steps  or  notes  between  any  tone,  and  the  tone 
above  it  vibrating  twice  as  fast,  or  the  tone  below  it  vibrating  half  as  fast  j 
these  two  tones  or  notes  being  hence  called  the  octaves  above  and  below  the 
note  with  which  they  are  compared,  and  the  intermediate  notes  which  fill  up 
either  octave  from  the  fundamental  note  are  distinguished  by  the  names  of 
second,  third,  fourth,  &c.,  in  ascending  or  descending.  The  numbers  which 
express  the  relations  of  beats  among  the  notes  of  an  octave  are  easily  found, 
from  our  knowing  the  relative  number  of  beats  in  the  notes  of  any  one 
simple  chord,  and  the  relation  as  above  described  of  the  -three  chords  form- 
ing the  compound  scale.  The  following  table  exhibits  these  numbers  or 
the  arithmetical  expression  for  the  notes  of  an  octave,  as  well  as  the 
corresponding  lengths  of  a  given  string  required  to  produce  them,  and  the 
English  designation  of  the  notes  by  letters,  and  the  continental  designation 
by  names,  these  names  being  the  first  syllables  of  certain  verses  sung  by 
learners. 


Number  of  vibra-  ) 

1 

I 

« 

I 

1 

§ 

¥ 

2 

tions*        .     .  j 

Length  of  string 

1 

! 

4 

5 

| 

i 

-§ 

T85 

a 

English  characters 

C 

1) 

E 

F 

G 

A 

B 

c 

Continental  names 

ut 

re 

mi 

fa 

sol 

la 

SI 

ut 

The  musical  scale,  however  far  extended,  is  a  repetition  of  similar  octaves, 
so  that  any  note  in  it  vibrates  just  twice  as  often  as  the  corresponding  note 
in  the  octave  below,  and  half  as  often  as  that  in  the  octave  above.  The 
lowest  note  which  is  perceptible  to  the  human  ear  has  about  thirty  beats  in 
a  second,  and  the  highest  about  thirty  thousand  :  and  there  is  included  be- 
tween these  two,  a  range  of  nearly  ten  octaves.  To  certain  ears  the  extremes 
of  this  range  are  totally  inaudible,  as  if  their  power  did  not  reach  so  far. 
Some  persons  do  not  hear  at  all  the  sharp  note  of  the  grasshopper,  while 
some  are  equally  insensible  to  the  lowest  tones  of  an  organ  or  piano ;  and 
yet  to  all  the  perception  of  intermediate  sounds  may  be  equally  perfect. 
Few  musical  instruments  comprehend  more  than  six  octaves,  and  the  human 
voice  in  general  has  only  from  one  to  three,  the  female  voice  being  in  pitch 
an  octave  higher  than  the  male. 

If  the  intervals  in  the  musical  scale  were  all  equal,  a  performer  might 
choose  indifferently  any  note  as  a  fundamental  or  key-note,  and  would  only 
have  to  attend  to  the  number  of  intervals  above  and  below  it;  but,  in  fact, 
the  relation  of  the  three  constituent  chords  is  such  that  the  third  and  seventh 
intervals,  in  ascending  from  a  key-note,  are  only  about  half  as  large  as  the 
others.  It  is  owing  to  this  circumstance  that  in  changwy  the  key  on  any 


MUSIC.  241 

instrument,  certain  notes  belonging  to  other  keys  are  half  a  note  too  low  or 
too  high,  that  is,  too  flat  or  too  sharp,  and  must  he  changed  accordingly. 
And  hence,  when  an  instrument  is  to  be  used  to  play  on  all  keys,  its  larger 
intervals  must  be  divided  into  two  parts.  The  fact  of  these  unequal  inter- 
vals, ill  understood,  is  what  gives  an  appearance  of  great  complexity  and 
difficulty  to  musical  science. 

Melody,  in  music,  is  when  notes,  having  the  simple  numerical  relations 
of  beat  which  we  have  been  describing,  are  played  in  succession  ',  harmony 
is  when  two  or  more  such  notes  are  sounded  together.  The  effect  of  both 
is  delightfully  increased  by  what  is  called  measure,  viz.,  making  the  dura- 
tion of  the  notes  or  strains  correspond  with  certain  regular  divisions  of  time. 
This  gives  to  the  ear  a  prescience,  to  a  certain  degree,  of  what  is  coming, 
with  the  pleasure  of  having  expectation  realized,  as  happens  similarly  from 
the  metre  and  rhyme  of  poetry ',  it  moreover  enables  the  memory  to  retain 
musical  combinations  of  sound — for  the  airs  of  the  ^olian  harp,  which 
observe  no  time,  cannot  be  learned  or  repeated.  The  music  of  a  single 
drum  is  that  of  time  only. 

Melody,  harmony,  time  and  varying  intensity  of  sound,  are  the  four  con- 
stituents of  music,  and  it  seems  that  almost  every  state  of  mind  has,  in  some 
combination  of  these,  an  appropriate  expression,  intelligible  to  the  general 
feelings  of  the  human  race.  The  exact  relation  between  the  movements  of 
the  animal  spirits,  as  it  has  been  expressed,  or  the  fluctuating  stream  of 
feeling,  and  the  varying  flow  of  sound  in  a  musical  composition  is  not  clearly 
understood,  but  the  fact  of  their  correspondence  and  its  consequences  are 
most  remarkable.  Under  many  circumstances,  the  association  between  the 
feeling  and  expression  is  so  strong,  that  the  latter  is  often  spontaneously  be- 
traying itself; — witness  the  almost  constant  humming  or  low  song  of  some 
contented  beings — the  singing  and  whistling  of  careless  childhood,  or  of  the 
light-hearted  rustic  living  among  the  beauties  of  nature — the  heart-rousing 
strain  of  the  hunter  or  warrior — and  the  tender  expression  of  many  of  the 
modifications  of  anxiety  and  sorrow.  The  musical  sensibilities  are  by  no 
means  limited  to  the  human  race,  for  there  is  no  expression  more  exquisite 
than  in  the  song  of  the  nightingale  during  the  evenings  of  spring,  or  of  the 
thrush  and  blackbird,  in  the  same  season,  amid  the  quiet  retreats  of  our 
woodlands, — the  music  of  which  untutored  songsters  is  made  up  of  the  same 
elements  as  our  own. 

The  accompaniment  of  an  air  afforded  to  a  singer  by  one  or  more  instru- 
ments, and  which  is  so  pleasing,  is  chiefly  the  sounding,  simultaneously,  in 
a  subdued  manner,  some  other  notes  of  the  chords  to  which  the  several  vocal 
notes  belong.  Duetts  and  more  complicated  concert-pieces  have  their  origin 
from  the  same  source :  and  highly  cultivated  musical  sense  can  even  follow 
and  enjoy  several  melodies  played  together. 

Musical  notes,  by  whatever  instrument  produced,  have  to  each  other  the 
same  numerical  relations,  in  the  beats  or  vibrations  which  constitute  them. 
The  different  qualities  of  tone,  therefore,  from  different  instruments,  can 
only  depend  on  the  peculiarities  of  the  single  beat,  as  to  whether  they  are 
sharp  or  soft,  strong  or  weak,  &c.  Such  is  the  extraordinary  nicety  of  per- 
ception which  the  human  ear  possesses  in  this  respect,  that  it  cannot  only 
distinguish  different  kinds  of  instruments,  as  a  flute  and  clarionet,  playing  the 
same  note,  but  different  instruments  of  the  same  kind,  even  to  the  extent 
for  instance,  of  recognizing  each  one  of  a  hundred  voices  singing  the  same 
air.  One  of  the  greatest  charms  of  concert  music  is,  that  the  voice  and  the 
different  instruments  may  take  up  separately,  parts  of  the  strain  suited  to 

16 


242  ACOUSTICS. 

their  individual  expression — the  flute  and  clarionet,  for  instance,  breathe  soft- 
ness; the  trumpet  and  drum  arouse ;  the  harp  rolls  forth  its  brilliant  chord; 
the  violin  leads  the  flowing  sounds  through  rapid  and  endless  variety;  and 
so  of  the  rest. 

That  there  might  be  correspondence  in  instruments  when  played  together 
and  a  k*nown  pitch  when  played  apart,  it  became  necessary  to  fix  on  some 
tone  or  number  of  vibrations  as  a  point  of  comparison.  Hence,  tuning-forks 
have  been  made  of  steel,  with  length  of  prongs  calculated  to  produce  a  certain 
note.  The  note  is  usually  the  fourth,  A  or  la  from  the  bass  of  the  piano- 
forte, and  vibrates  about  430  times  in  a  second  ; — and  when  the  note  of  the 
game  name  on  any  instrument  is  tuned  in  unison  with  this,  the  other  notes 
can  be  easily  adjusted  according  to  the  harmonic  relations  above  explained. 

Almost  every  substance  or  contrivance  that  can  produce  a  uniform  con- 
tinued sound  may  enter  into  the  composition  of  a  musical  instrument;  hence 
the  almost  endless  variety  which  the  world  has  seen,  The  chief  classes  of 
instruments  are  stringed  instruments,  wind  instruments  and  bells  or  rods. 

Of  the  stringed  instruments  we  may  mention  the  harp,  the  lyre  or  lute, 
the  guitar,  the  viol  of  all  sizes,  and  piano-forte.  The  harp,  lyre  and  lute 
were  the  inventions  of  antiquity,  and  have  brought  down  with  them  to  the 
present  times  a  thousand  delightful  associations.  They  awakened  to  inspi- 
ration the  bards  and  poets  of  the  young  world,  and  they  were  the  beloved 
companions  of  many  of  the  noblest  spirits  of  succeeding  times.  Their 
great  charm  appears  to  have  been  in  their  power  to  heighten  the  emotions 
produced  by  music's  twin  sister,  poetry;  and  the  combined  effects  seem  to 
have  been  magical. — The  other  instruments  mentioned  are  of  comparatively 
modern  invention,  particularly  the  piano-forte;  and  their  perfection  has 
assisted  in  carrying  the  combination  of  musical  sound  to  degrees  of  com- 
plexity and  difficulty  of  which  antiquity  dreamt  not.  It  is  a  question,  how- 
ever, whether  the  style  of  much  of  the  music  now  in  vogue  does  not  prove 
rather  a  degeneracy,  than  a  desirable  refinement  of  musical  taste.  Music  is 
a  language  of  nature,  intelligible  at  once  to  all  susceptible  minds,  and,  in  a 
degree  even  to  inferior  animals;  but  modern  art  is  attempting  to  make  of  it 
an  artificial  and  conventional  language,  in  which  there  may  be  fashion  and 
change.  The  ornaments  and  accompaniments  are  now  often  so  overwhelm- 
ing, that  the  melody,  in  which  the  idea  and  sentiment  really  reside,  is 
masked  and  almost  lost ;  and  an  unpractised  ear,  particularly  if  listening  to 
an  organ,  often  discovers  only  an  unmeaning  succession  of  chords.  And 
when  a  singer,  abandoning  the  natural  simplicity  of  melody,  strains  to  exe- 
cute with  the  voice  the  complicated  movements  which  belong  properly  to 
instrumental  accompaniments,  the  attempt  destroys  the  poetry,  by  either 
rendering  the  words  inaudible,  or  by  sacrificing  their  natural  expression  to 
some  supposed  appropriate  expression  of  the  ornamental  music.  These 
considerations  may  account  in  part  for  the  insensibility  of  so  many  highly- 
endowed  persons  to  wliat  is  now  called  excellent  music.  Some  of  the 
tricks  on  the  voice  and  on  instruments,  at  present  so  common,  are,  to 
natural  or  graceful  music,  what  tumbling  or  rope  dancing  are  to  natural 
or  graceful  gesture.  And  when  we  hear  noted  professors  avow  their  in- 
ability to  sing  a  simple  ballad,  or  to  play  an  unadorned  melody,  must  we 
not  conclude  that  the  natural  sense  of  music  has  left  them,  as  the  relish 
for  simple  but  the  most  invigorating  fare  has  left  the  morbid  epicure? 

The  guitar,  as  affording  an  accompaniment  to  vocal  music,  has  many 
advantages.  It  is  not  too  loud,  yet  the  strains  are  very  distinct ;  it  admits  of 
most  touching  expression ;  it  is  very  easily  learned  by  any  one  who  should 


MUSIC.  213 

attempt  to  learn  ^nusic  ;  it  is  portable  and  cheap.  The  great  facility  of  ac- 
companiment on  it  depends  on  this,  that  the  player  is  able  by  one  position 
of  the  hand  to  touch  the  strings  so  that  the  sound  of  all  the  six  shall  belong 
to  the  same  chord  : — three  positions  of  the  hand,  therefore,  for  one  key,  pro- 
duce all  the  notes  and  chords  which  a  simple  accompaniment  requires;  and 
the  hand  soon  falls  into  these  so  readily,  that  the  player  is  hardly  sensible 
of  exerting  volition. 

Among  wind  instruments  are  ihe  flute,  the  flageolet,  the  organ,  the  clario- 
net, the  hautboy,  the  horn,  the  trumpet,  &c.  The  pitch  or  tone  of  a  tubular 
wind  instrument,  just  as  of  a  musical  string,  has  relation  to  its  length  ;  and 
the  vibrations  causing  the  sound  seem  to  be  waves  or  condensations  of  air 
passing  from  the  mouth  to  the  extremity  of  the  tube  ;  being  more  frequent, 
therefore,  as  the  tube  is  shorter; — when  the  bottom  of  the  tube  is  closed, 
the  wave  has  to  come  back  again,  and  thus  renders  the  note  twice  as  grave. 
It  appears,  also,  that  on  blowing  more  strongly,  the  air  in  the  tube  divides 
into  separate  vibrating  portions,  as  a  string  may  divide  to  produce  its  har- 
monic sounds,  and  produces  thus  all  the  harmonic  sounds  belonging  to  the 
fundamental  note  at  the  tube.  By  blowing  into  a  common  G-erman  flute, 
for  instance,  it  is  possible  to  produce  five  ascending  harmonics  without  mov- 
ing the  fingers  at  all.  The  music  of  a  trumpet  is  limited  to  these  five  notes 
of  the  same  chord ;  but  in  the  flute  and  other  instruments  with  holes,  the 
effective  length  of  the  tube  is  calculated  from  the  upper  end  to  the  nearest 
hole  left  open  ;  and  each  length  has  its  harmonics.  —»If  a  tuning  fork,  Jew's- 
harp,  or  any  such  sounding  body,  be  held  at  the  open  end  of  a  tube  or  other 
empty  space  of  dimensions  calculated  to  produce  a  frequency  of  undulation, 
in  its  contained  air,  according  with  the  pulses  of  the  sounding  body,  then 
the  tube  or  space  will  immediately  give  out  its  own  beautiful  tone ;  and  if 
the  space  be  enlarged  or  diminished  in  a  double,  treble  or  any  other  simple 
proportion — as  a  tone  may  be,  by  a  piston  moved  up  or  down  in  it — then 
will  its  note  become  the  fifth,  octave,  twelfth,  &c.,  above  or  below  the  origi- 
nal tone,  although  that  tone  continues  unchanged.  The  tones  of  the  Jew's- 
harp  are  well  known  to  depend  altogether  on  the  varying  dimensions  of  the 
player's  mouth ;  but  to  obtain  perfect  music  from  it,  three  harps  at  least,  to 
be  substituted  one  for  the  other  during  the  performance,  are  required  to 
produce  the  notes  of  the  three  constituent  chords  of  the  common  musical 
scale. — In  wind-instruments  with  reeds,  the  tone  depends  on  the  stiffness, 
weight,  length,  &c.,  of  the  vibrating  plate  or  tongue  of  the  reed,  as  well 
as  on  the  dimensions  of  the  tube  or  space  with  which  it  may  be  connected. 
This  truth  is  well  illustrated  in  that  instrument,  the  .ZEolian,  already  men- 
tioned, which,  in  improved  and  varied  forms,  promises  to  become  common, 
and  one  of  the  most  expressive  wind-instruments. — The  sounds  of  the  human 
voice  are  the  sweetest  of  all,  and  are  produced  by  the  vibrations  of  two  de- 
licate membranes  situated  at  the  top  of  the  windpipe,  with  a  slit  or  opening, 
called  the  glottis,  left  between  them,  for  the  passage  of  the  air.  The  tones 
of  the  voice  are  grave  or  acute,  according  to  the  varying  tension  of  these 
membranes,  and  to  the  size  of  the  opening. — In  the  organ  there  is  a  pipe 
for  each  note,  and  wind  is  admitted  from  the  bellows  to  the  pipes,  by  the 
action  of  the  keys,  like  the  keys  of  a  piano-forte.  The  organ  may  be 
played  also  very  perfectly  by  a  barrel,  made  to  turn  slowly  under  the  keys, 
and  to  lift  them  in  passing,  by  pins  projecting  from  it  at  the  required  situa- 
tions. Very  complicated  pieces  of  music  are  thus  set  on  barrels,  but  by  a  great 
cost  of  study  and  labour,  and,  therefore,  of  money ;  now  a  plain  barrel,  made 
to  turn  near  the  keys  of  an  organ  during  performance  on  it  by  the  hands, 


244 


ACOUSTICS. 


Fig.  120. 


might  be  made  to  record,  with  mathematical  accuracy,  every  touch  of  the 
most  finished  player,  by  receiving  marks  of  some  kind  from  the  keys  as  they 
were  lifted ;  and  to  repeat  with  absolute  accuracy,  therefore  any  performance, 
however  delicate  and  exquisite,  it  would  only  be  farther  necessary  to  drive 
pins  into  the  barrel  where  the  marks  remained,  and  afterwards  make  these 
pins  lift  the  keys. 

Bells  are  often  conjoined  in  sets,  having  the  musical  relation,  and  to  some 
persons  their  music  is  very  agreeable.  There  are,  in  the  tolling  of  a  single 
bell,  a  loudness  and  a  solemnity  rendering  it  a  fit  accompaniment  of  funeral 
rites. 

The  Chinese  gong  partakes  of  the  nature  both  of  the  bell  and  of  a  great 
drum,  and  has  something  in  its  sound  which  is  singularly  affecting.  In  its 
own  country  it  bears  a  part  in  one  of  the  most  imposing  ceremonies  which 
man  has  ever  imagined.  On  certain  festivals  as  the  sun  is  sinking  in  the 
west,  the  whole  population  of  China,  a  host  of  more  than  a  hundred  millions, 
issues  forth  under  the  single  canopy  of  heaven,  to  testify,  amid  the  thunder 
of  gongs  and  the  continued  discharge  of  fire  works,  that  adoration  and  grati- 
tude towards  the  Deity  which  human  nature,  in  all  ages  and  climes,  has  felt 
to  be  due,  and  has  eagerly  sought  to  express,  however  blind  as  to  the  sublime 
simplicity  of  religious  truth. 

Bells  or  goblets  of  glass  sound  still  more  perfectly  than  those  of  metal, 
and  by  gentle  friction  on  their  edges  with  a  bow  or  the  wetted  finger,  their 
tones  may  be  continued  for  any  length  of  time,  and  may  be  made  to  swell 

and  diminish  like  a  human  voice  or  the 
notes  of  a  violin.  A  set  of  glasses,  there- 
fore, attuned  to  each  other,  according  to 
the  harmonic  scale,  becomes,  for  certain 
species  of  music,  the  most  perfect  of  all 
instruments.  It  is  in  fact  an  ^Eolian  harp 
at  command.  Dr.  Franklin,  who  first 
constructed  a  set,  doubled  the  long  line  of 
glasses  upon  itself,  and  placed  the  half- 
notes  as  outside  rows.  The  author  of  this 
work,  however,  during  some  experiments 
on  sound,  found  the  zig-zag  arrangement 
here  represented  to  possess  certain  advan- 
tages. The  small  open  circles  represent 
the  mouths  of  the  glasses  standing  in  a  box 
a  b  c,  and  the  relation  of  the  glasses  to  the 
written  musical  notes  is  shown  by  the 

common  music  lines  and  spaces  which  connect  them.  The  learner  discovers 
immediately  that  one  row  of  the  glasses  produces  the  notes  written  upon  the 
lines,  and  the  other  row  the  notes  written  between  the  lines ;  and  he  is  men- 
tally master  of  the  instrument  by  simple  inspection.  This  arrangement  also 
renders  the  performance  easy,  for  the  notes  most  commonly  sounded  in  suc- 
cession are  contiguous :  and  the  relations  of  the  notes  forming  a  tune  are  so 
obvious  to  the  eye,  that  the  theory  of  musical  combination  and  accompani- 
ment is  learned  at  the  same  time.  The  set  of  glasses  here  represented  has 
two  octaves,  and  with  the  additional  flat  seventh  and  fourteenth,  seen  at  a  and 
c,  which,  when  required,  may  be  substituted  for  the  corresponding  glasses  in 
the  rows,  it  is  capable  of  playing  the  greater  part  of  our  simple  melodies. 
All  the  half-notes,  if  desired,  may  be  placed  in  outside  rows.  The  player 
stands  at  the  side  of  the  box  between  a  and  b}  and  has  the  notes  ascending 
towards  the  right  hand,  as  in  a  piano-forte. 


MUSICAL    EAR.  245 


Musical  ear. 

Philosophers 'have  not  yet  been  able  to  account  for  a  remarkable  difference 
among  individuals,  as  regards  their  perception  of  the  musical  relations  of 
sounds.  Many  persons,  without  understanding  any  thing  of  acoustics,  or 
having  studied  music  as  a  science  can  tell  instantly  whether  various  notes 
heard  together  or  in  succession,  have  the  mutual  relations  which  we  call 
musical — and  which  we  now  know  to  depend  on  the  comparative  numbers 
of  beats  in  a  given  time ;  and  they  quickly  recognize  and  learn  to  repeat 
tunes,  and  to  sing  a  fit  second  or  bass  to  the  performance  of  another ; — while 
there  are  persons,  again,  with  an  equally  perfect  sense  of  hearing,  who  can 
neither  know  if  an  air  be  played  in  tune,  nor  what  air  it  is,  nor  can  they  ever 
sing  alone  or  accompany.  The  former  class  of  persons  are  said  to  have  a 
musical  ear,  and  the  latter  to  want  it;  and  although  cultivation  will  raise 
mediocrity  to  considerable  expertness,  it  cannot  bestow  the  faculty  where 
originally  deficient.  On  this  subject  there  is  a  very  common  misconception, 
which  becomes  a  source  of  great  mortification  on  one  side,  and  of  arrogance 
on  the  other,  viz  ,  that  the  possession  of  a  musical  ear,  or  the  power  of  dis- 
tinguishing notes,  is  the  indication  of  all  the  finer  qualities  of  the  mind 
while  the  want  of  it  proves  an  opposite  deficiency;  and  Shakspeare's  opinion 
of  him  "  that  hath  no  music  in  himself,"  is  often  triumphantly  cited  as  ap- 
plicable to  all  who  want  the  distinguishing  ear.  The  truth,  however,  is,  that 
many  who  possess  this  characteristic  in  a  remarkable  degree,  are  deficient  in 
almost  all  else  that  humanity  reveres, — witness  the  weak  minds  and  disor- 
derly lives  of  so  many  professed  musicians, — while  many,  again,  who  have 
it  not,  are  otherwise  examples  of  excellence,  and  exquisitely  sensible  to  other 
beauties  and  harmonies  of  nature.  They  may  not  be  deaf,  for  instance,  to 
the  general  music  of  spring,  when  all  nature  bursts  forth  in  voice  of  rejoicing, 
nor  to  the  awful  music  of  the  storm — they  may  feel  as  touching  music  the 
silence  of  a  lone  wood,  contrasted  with  the  unceasing  din  of  multitudes — or 
even  the  stillness  of  night  in  a  great  city,  where  the  astronomer,  contempla- 
ting the  wondrous  spheres  above,  hears  only  the  tongues  of  passing  time  in 
the  church  towers,  or  the  call  of  watchmen,  faintly  sounding  in  the  distance. 
In  fine,  many  distinguished  poets  and  philosophers  have  had  no  musical  ear. 
— That  the  charm  of  music  is  often  as  much  from  early  associations  as  from 
peculiar  aptitude  in  the  individuals,  is  proved  by  the  effects  so  well  known 
of  the  Swiss  airs,  when  heard  by  native  Swiss  in  foreign  lands ;  and,  indeed, 
of  the  national  melodies  of  all  countries,  whose  people  are  happy,  and  mix 
song  with  their  usual  occupations, — it  not  being  in  nature,  that  at  any  period 
of  life,  or  in  any  clime,  a  man  should  cease  to  deem  those  modulations  lovely, 
which  recall  the  ecstatic  emotions  of  his  infancy  and  childhood ;  modula- 
tions learned  in  general  from  a  parent's  voice,  perhaps  an  excellant  mother's, 
whose  affection  was  so  long  around  him  as  a  shield ;  whose  tears  fell  to 
chide  his  errors,  and  to  reward  where  there  was  promise  of  virtue ;  whose 
steady  judgment  was  his  guide,  whose  faultless  life  was  his  example, 
and  who  in  all  things  to  him  was  a  personification  of  God's  goodness  on 
earth. 

It  is  the  prejudice  of  which  we  are  now  speaking  with  respect  to  musical 
ear  and  musical  taste,  that  in  the  present  day,  condemns  many  young  women, 
possessed  of  every  species  of  loveliness  and  talent,  except  that  of  note-dis- 
tinguishing, to  waste  years  of  precious  time  in  an  attempt  to  aquire  this 
talent  in  spite  of  nature ;  but  when  they  have  succeeded  as  far  as  they  can, 


246  ACOUSTICS. 

they  have  only  the  merit  of  being  machines,  upon  which  tunes  are  set  as 
upon  a  barrel-organ,  and  of  which  the  performance  is  often  far  from  being 
pleasing  to  good  judges.  Such  persons  when  liberty  comes  to  them  with  age 
or  marriage,  generally  abandon  the  offensive  occupation ;  but  tyrant  fashion 
will  force  their  daughters  to  run  the  same  course.  The  waste  of  time  now 
spoken  of,  is  only  one  of  many  evil  consequences  which  arise  from  the  pre- 
vailing false  notions  with  respect  to  music  :  a  subject  which,  however  inter- 
esting, cannot  be  farther  pursued  in  this  place. 

" The  trembling  which  causes  the  sensation  of  sounds  spreads  in  all  bodies, 
solid  or  fluid."     (Read  the  Analysis.) 

As  air  consists  of  material  particles  held  far  apart  from  each  other  by  the 
repulsion  of  heat  among  them,  we  can  conceive  how  an  impulse  given  to  a 
certain  portion  of  the  particles  is  transmitted  to  those  beyond,  by  the  increase 
of  repulsion  as  they  approximate ;  and  from  the  second  layer  in  the  same 
manner  to  the  third,  and  so  on.  And  as  in  fluids  the  particles  all  mutually 
rest  against,  or  repel  each  other,  we  can  conceive  why  a  motion  produced  in 
any  part  of  a  mass  should  be  felt  in  every  direction.  The  explosion  of  gun- 
powder, in  which  there  is  a  sudden  formation  of  a  quantity  of  air,  gives  a 
shock  all  round  which  spreads  a  spherical  wave  to  a  great  distance. 

Although  material  particles  in  the  form  of  liquid  or  solid  are  much  nearer 
to  each  other  than  in  the  form  of  air,  we  still  have  many  proofs,  as  stated  at 
page  30,  that  they  are  not  in  absolute  contact,  and  we  therefore  see  the  rea- 
son why  the  impulses  producing  sound  should  be  transmitted  through  a 
liquid  or  solid  in  the  same  manner  as  through  air,  and  even,  by  reason  of 
the  greater  proximity  "of  the  particles,  more  quickly  and  forcibly  than  in  air. 

Instances  of  air  carrying  sound  were  given  at  page  2.35. — As  farther  ex- 
amples we  may  cite  the  cases  of  what  are  called  sympathetic  sounds.  Every 
elastic  body  being  sonorous,  that  is  to  say,  being  fitted  to  tremble  when  struck, 
with  a  certain  frequency  of  oscillation,  depending  on  its  weight  and  shape, 
&c.,  if  the  air  around  it  be  made  to  tremble  by  any  cause,  with  the  velocity 
which  it  is  fitted  to  take  on  or  produce,  it  immediately  begins  to  tremble  in 
unison  with  the  air;  and  its  motion  or  sound  may  continue  after  the  original 
cause  has  ceased. — Thus  almost  any  sound  produced  near  a  piano-forte  whose 
dampers  are  raised,  finds  a  responsive  string,  and  if  bits  of  paper  are  strewed 
upon  the  strings  generally,  those  falling  on  the  strings  which  return  unisons 
or  octaves  to  the  sounding  body  are  soon  shaken  off,  while  the  others  remain. 
A  harp  or  guitar  in  a  room  with  talking  company,  is  often  mingling  a  note 
with  their  conversation. — A  wine-glass  or  goblet  may  be  made  to  tremble, 
and  if  on  a  table  at  all  inclined,  even  to  fall,  by  a  person  sounding  on  a  vio- 
loncello near  it,  the  note  accordant  to  its  own. 

Sounding  bodies  vibrate  much  more  quickly,  or  have  sharper  tones,  if 
placed  in  light  hydrogen,  than  in  common  air,  and  more  quickly  in  common 
air  than  in  any  of  the  heavier  gases : — because  the  lighter  the  surrounding 
fluid,  the  less  is  the  resistance  to  a  body  moving  in  it.  Thus  also  a  bell  will 
ring  under  water,  but  with  a  much  graver  sound  than  in  the  air. 

That  water  is  a  vehicle  of  sound,  is  proved  by  the  fact  last  mentioned, — 
by  the  distinctness  with  which  the  blows  of  workers  around  a  diving-bell  are 
heard  above, — by  the  fact  that  fishes  hear  very  acutely,  &c. 

And  the  following  are  instances  of  sound  conveyed  by  solids. — A  scratch 
of  a  pin  at  one  end  of  a  wooden  log  is  distinctly  heard  by  a  person  applying 
his  ear  at  the  other  end,  although  through  the  air  it  is  not  at  all  audible  even 
to  the  person  who  makes  it. — Savages  often  discover  the  proximity  of  ene- 


SPREADING    OF    SOUND.  247 

mies  or  of  prey,  by  applying  an  ear  to  the  ground  and  hearing  the  tread.— 
The  approach  of  horsemen  at  night  is  easily  discovered  in  the  same  way. — . 
The  report  of  a  cannon  placed  on  the  ice  is  carried  much  farther  by  the  ice, 
than  by  the  air  around. — In  the  military  operation  of  mining,  or  cutting  away 
under  ground  for  the  purpose  of  entering  a  citadel,  or  blowing  up  fortifications, 
the  approach  of  the  enemy  is  often  discovered  by  the  subterranean  sound  of 
the  pioneer's  tools. — The  awful  muttering  of  earthquakes  is  merely  the  sound 
of  subterranean  explosions,  conveyed  from  amazing  distances,  by  the  solid 
earth 

A  superstitious  man  sleeping  in  the  upper  story  of  a  lofty  house  had  for 
some  time  heard,  during  the  stillness  of  the  night,  a  singular  beating  noise 
near  the  head  of  his  bed.  There  was  no  adjoining  house  beyond  the  wall, 
nor  was  there  any  thing  going  on  near  him  in  his  own  house  to  account  for 
it,  and  he  at  last  deemed  it  supernatural.  Accident  at  last  discovered  that 
in  a  hovel  built  at  the  bottom  and  outside  of  the  wall  against  which  his  bed 
stood,  there  was  a  wooden  clock  hanging,  of  which  the  sound,  while  all  else 
was  still,  became  audible  aloft. 

It  is  easy  to  ascertain  whether  a  kettle  boils,  by  putting  one  end  of  a  stick 
or  poker  on  the  lid,  and  the  other  end  to  the  ear ;  the  bubbling  of  the  water 
then  appears  as  loud  as  the  rattling  of  a  carriage  in  the  street. — A  slight 
blow  given  to  a  steel  poker  or  common  triangle,  of  which  an  end  is  held  to 
the  ear,  produces  a  sound  which  is  even  painfully  strong. 

The  readiness  with  which  solids  receive  and  transmit  sound  is  farther  per- 
ceived in  the  fact,  that  a  small  musical  box,  while  held  in  the  hand,  is 
scarcely  audible,  but  when  pressed  against  a  table,  or  a  door,  will  rival  a 
little  harp.  The  vibration  communicated  from  the  box  pervades  the  whole 
of  the  wood,  and  the  extended  surface  then  acting  on  the  air  increases  the 
effect.  The  construction  of  violins,  harps,  guitars,  &c.,  and  of  sounding 
boards  generally,  is  governed  by  the  same  law.  In  the  dancing-master's 
kit  or  small  fiddle,  which  he  carries  in  his  pocket,  there  may  be  the  same 
strings  and  the  same  bow  as  for  a  violin,  but  it  has  very  little  sound,  because 
the  extent  of  its  surface  is  so  small.  'A  heavy  piece  of  metal  called  a 
sourdine,  when  fixed  upon  the  bridge  of  a  violin,  damps  the  sound,  because 
it  is  a  dead  mass  resisting  the  motion  of  the  elastic  wood. 

The  fact  of  solids  conveying  sounds  so  much  more  perfectly  than  air  has 
lately  been  applied  to  useful  purposes  in  medicine.  Dr.  Laennec,  of  Paris, 
proposed  some  years  ago  to  listen  to  what  was  going  on  in  the  interior  of  the 
body,  and  of  the  chest  particularly,  by  applying  one  end  of  a  wooden 
cylinder,  which  he  called  a  stethoscope  or  chest  inspector,  to  the  surface,  and 
resting  the  ear  against  the  other  end.  The  results  of  this  happy  thought 
have  been  important. 

The  actions  going  on  in  the  chest  are,  the  entrance  and  exit  of  the  air  in 
respiration,  the  voice,  and  the  motion  of  the  blood  in  the  heart  and  blood- 
vessels;— and  so  perfectly  do  all  these  declare  themselves  to  a  person  listen- 
ing through  the  stethoscope,  that  an  ear  once  familiar  with  the  natural  and 
healthy  sounds,  instantly  detects  certain  deviations  from  them.  Hence  this 
instrument  becomes  a  means  of  ascertaining  certain  diseases  in  the  chest 
almost  as  effectually  as  if  there  were  convenient  windows  for  visual  inspec- 
tion ;  and  when  it  is  considered  that  a  fourth  or  fifth  part  of  the  inhabitants 
of  Europe  die  of  diseases  of  the  chest,  such  as  inflammations,  abscesses, 
consumption,  dropsical  collections,  aneurism,  and  various  affections  of  the 
heart  and  blood-vessels,  each  of  which  requires  an  appropriate  treatment, 
the  importance  of  such  a  means  may  be  judged  of.  By  many  medical  men 


248  ACOUSTICS. 

this  instrument  was  at  first  ridiculed  as  quackery  and  nonsense,  and  many 
have  yet  to  learn  the  use  of  it.  May  not  both  of  these  facts  be  attributed 
to  the  error  which  has  existed  in  medical  education,  of  leaving  so  many 
practitioners  without  that  knowledge  of  the  general  laws  of  nature,  which 
should  enable  them  to  appreciate  at  once  any  means  likely  to  be  useful  in 
their  art,  from  whatever  quarter  offered  ? 

"  Velocity  of  sound."     (See  the  Analysis.) 

The  velocity  of  light  is  such,  that  for  any  distance  on  earth  its  passage 
may  be  regarded  as  instantaneous.  The  velocity  of  sound  is  very  much  less. 
—•-If  a  woodman  be  observed  at  his  occupation  on  the  hill,  his  axe  is  seen 
to  fall  a  considerable  time  before  the  sound  of  his  blow  reaches  the  specta- 
tor's ear. — The  flash  of  a  gun  fired  at  a  distance  is  seen  long  before  the 
report  is  heard. 

Most  accurate  experiments  have  been  made  to  ascertain  the  velocity  with 
which  sound  travels  in  the  atmosphere;  and  it  is  found  to  be  1,142  feet  per 
second,  or  a  mile  in  about  four  seconds  and-a-half;  varying  little  either  with 
the  density  or  temperature  of  the  air. 

By  noting  then  how  long  the  flash  of  a  gun  is  seen  before  the  report 
reaches  the  ear,  we  learn  the  distance  of  the  ship  or  battery  from  which  the 
gun  is  fired.  A  chasing  ship  may  thus  often  discover  whether  she  be  nearing 
or  not  the  object  of  her  pursuit.  In  the  same  manner  the  distance  of 
thunder  may  be  ascertained  :  and  the  reason  of  the  long-continued  roll  of 
thunder  is,  that  although  the  lightning  darts  instantly  through  the  chains  of 
clouds,  perhaps  of  miles  in  length,  the  claps  or  explosion  at  each  interruption 
of  the  chain  are  only  heard  successively,  as  the  sound  arrives  at  the  ear. 
The  pulse  at  the  wrist  of  a  healthy  man  is  a  convenient  measure  of  time 
for  ascertaining  distances  by  the  motion  of  sound, — each  beat  making 
nearly  a  second,  and  therefore  indicating  a  distance  of  nearly  a  quarter  of 
a  mile. 

A  line  of  muskets  fired  at  the  same  instant  cannot  appear  a  single  report 
to  any  person  who  is  not  in  the  centre  of  a  circle,  of  which  the  line  forms 
a  part. 

An  extended  orchestra  of  musicians  cannot  be  heard  equally  well  from  all 
situations  near  them. 

Wind  affects  the  velocity  of  sound  just  as  a  current  in  water  affects  the 
motion  of  a  sailing  ship. 

Sound  decreases  in  intensity  from  the  centre  where  it  originates,  according 
to  the  same  law  as  gravitation  or  light ;  that  is  to  say,  at  double  distance  it 
is  only  one-fourth  part  as  strong,  at  triple,  a  ninth,  and  so  on. 

By  confining  it,  however,  in  tubes,  which  prevent  it  spreading,  its  force 
diminishes  much  less  rapidly,  and  it  will,  therefore,  extend  to  much  greater 
distances. — In  many  manufactories,  and  even  private  dwellings  now,  there 
are  pipes  for  the  conveyance  of  sound  leading  to  all  parts ;  so  that  on  ring- 
ing a  bell  to  attract  attention,  verbal  orders  may  be  given  through  them  to 
great  distances. 

Sound  travels  in  water  four  times  quicker,  and  in  solids  fr-om  ten  to 
twenty  times  quicker,  than  in  air.  The  blow  of  a  hammer  given  to  a  wall 
by  a  person  at  one  end,  may  be  heard  twice  by  a  person  at  the  other,  viz., 
almost  immediately  by  an  eur  applied  to  the  wall,  and  a  little  after  through 
the  air. 

"  Reflection  of  sound."     (Read  the  Analysis.) 
As  a  wave  of  water  turns  back  at  a  smooth  wall  or  obstacle,  so  that  at  any 


REFLECTION    OF    SOUND.  249 

distance  after  the  reflection,  it  appears  what  it  would  have  been  at  the  same 
distance  beyond  the  wall,  only  moving  in  an  opposite  direction  ;  so  the 
pulses  or  waves  of  sound  are  regularly  reflected  from  flat  surfaces,  and 
produce  what  is  called  an  echo.  Such  flat  surfaces  of  nature's  works  are 
found  only  among  the  rocks  and  hills;  and  hence  arose  the  beautiful  fiction 
of  the  ancient  poets,  that  echo  was  a  nymph  who  dwelt  concealed  among 
the  rocks.  Science  has  now  disclosed  the  secret  of  the  viewless  echo ;  but 
who  does  not  vividly  recollect  the  wonder  and  delight  with  which  he  has 
listened  in  the  morning  of  his  days,  to  his  shrill  call  returned  to  him  from 
some  bold  precipice,  across  the  plain  or  river,  or,  perhaps,  sent  down  to  him 
again  from  the  vaulted  roof  of  ocean's  caves  ! 

The  quickness  with  which  an  echo  is  returned  to  the  spot  where  the 
sound  originates,  depends,  of  course,  upon  the  distance  of  the  reflecting  sur- 
face ;  and,  as  sound  travels  1,142  feet  in  a  second,  a  rock  at  half  that  distance 
returns  a  sound  exactly  in  one  second.  The  number  of  syllables  that  can 
be  pronounced  in  a  second,  will,  in  such  a  case,  be  repeated  distinctly,  while 
the  end  of  a  longer  phrase  would  mix  with  the  commencement  of  the  echo. 
The  breadth  of  a  river  may  easily  be  ascertained  where  there  is  an  echoing 
rock  on  the  farther  shore.  A  perpendicular  mountain's  side,  or  sublime 
cliffs,  such  as  in  many  parts  skirt  the  British  coasts,  return  an  audible  echo 
of  artillery,  or  of  thunder,  to  a  distance  of  many  miles. 

If  two  bold  faces  of  rock  or  wall  be  parallel  to  each  other,  a  sound  pro- 
duced between  them  is  repeated  often,  playing  like  a  shuttlecock  between 
them,  but  becoming  more  faint  each  time  until  it  is  heard  no  more.  In 
some  situations,  particularly  when  the  sound  plays  thus  above  the  smooth 
surface  of  water,  a  pistol-shot  may  be  counted  forty  times. 

The  resonance  of  enclosed  spaces  depends  on  this  continued  reverberation. 
It  often  increases  the  effect  of  music  by  converting  a  simple  melody,  which 
is  a  succession  of  notes,  into  a  harmonized  piece,  where  each  note  is  accom- 
panied by  some  accordant  tones ;  and  a  young  flute-player  is  often  first 
charmed  with  his  own  music  when  he  finds  himself  performing  a  duet  with 
echo  in  a  cave  or  under  a  spacious  arch  : — but  resonance  injures  the  distinct- 
ness of  speech,  so  as  even  in  some  ill-contrived  halls  of  assembly  or  theatres, 
to  render  the  articulation  unintelligible.  Small  rooms  or  near  surfaces  give 
no  perceptible  echo,  because  the  interval  of  time  between  the  original  sound 
and  its  repetition  is  too  short  for  the  ear  to  appreciate. 

It  is  worthy  of  remark,  that  every  apartment  or  confined  space,  has  a 
certain  musical  note  proper  to  it,  the  pitch  of  which  depends  upon  the 
number  of  pulses  or  repetitions  of  a  sound  produced  there  in  a  given  time 
by  the  returns  from  its  walls.  The  velocity  of  sound  being  uniform,  this 
number  must  depend  on  the  size  of  the  apartment. 

There  is  a  curious  effect  of  echo  which  both  illustrates  the  nature  of  the 
phenomenon,  and  proves  that  a  tone  or  musical  sound  is  merely  a  repetition 
of  pulses  following  each  other  very  quickly.  Iron  railings  are  generally 
formed  of  square  bars,  of  which  any  side  is  a  plane  surface,  and  may  produce 
an  echo.  Now  a  sound  such  as  the  sharp  blow  of  a  hammer,  occurring  near 
the  end  of  such  a  railing,  is  echoed  to  a  corresponding  place  on  the  other  side 
by  every  bar  in  it ;  and  as  the  echoes  do  not  return  all  at  once,  but  in  regular 
succession,  according  to  the  increasing  distances  of  the  bars,  the  consequent 
regular  succession  of  slight  pulses,  with  uniform  and  small  intervals,  affects 
the  ear,  not  as  the  echo  of  a  single  blow,  but  as  a  continued  musical  tone, 
the  pitch  of  which  depends  on  the  distance  of  the  bars  from  each  other. 
The  writer  of  this  had  observed,  in  passing  on  horseback  along  a  particular 


250 


ACOUSTICS. 


Fig.  121. 


a     I 


portion  of  road,  where  there  was  first  a  piece  of  wall  and  then  two  pieces  of 
paling  with  rails  or  bars  of  different  width', —  that  there  was  from  the  wall  a 
clear  echo  of  the  horse's  cantering  feet,  and  afterwards  opposite  the  palings 
a  ringing  sound  for  every  step  of  the  horse.  He  had  first  concluded  that 
the  road  there  was  singularly  hard,  although  it  did  not  appear  so,  and  he 
slackened  the  horse's  pace  to  save  his  feet,  until,  observing  one  day  that  the 
ringing  sound  was  of  different  pitch  opposite  the  two  pieces  of  pailing,  and 
so  as  to  correspond  with  the  different  width  of  the  bars,  the  true  explanation 
occurred  to  him  that  the  sound  was  an  echo  of  the  nature  above  described. 
That  an  echo  may  be  perfect,  the  surface  producing  it  must  be  smooth, 
and  of  some  regular  form  ;  for  the  wave  of  sound  rebounds  according  to  the 
same  law  as  a  wave  of  water,  or  a  ray  of  light,  or  an  elastic  ball,  &c.,  as 
explained  at  page  65,  viz.,  perpendicularly  to  the  surface,  if  it  fall  perpen- 
dicular, but  if  it  fall  obliquely  on  one  side,  departing  with  an  equal  degree 
of  obliquity  on  the 'other.  To  express  this  very  important  law  shortly,  we 

say  that  "  the  angle  of  reflection  is  equal  to 
the  angle  of  incidence." — According  to  this 
law,  any  irregular  surface  must  break  an  echo ; 
and  if  the  regularity  be  very  considerable, 
there  can  be  no  distinct  or  audible  reflection 
at  all.  A  regular  concave  surface,  on  the 
contrary,  as  e  g,  may  concentrate  sound,  and 
bring  all  which  falls  upon  it,  as  from  a  I  c  dt 
to  the  same  centre  or  focus,  as  at/",  so  as  to 
produce  there  a  very  powerful  effect. 

We  thus  see  the  reason  why  echo  is  much 

less  perfect  from  the  front  of  a  house  which  has  windows  and  doors,  than 
from  the  plane  end,  or  any  plane  wall  of  the  same  magnitude, — and  why 
the  resonance  of  a  room  is  so  irregular  and  indistinct  when  the  room  contains 
curtains,  carpets  and  other  furniture,  or  a  crowded  assembly.  Halls  for 
music  have  generally  plane  bare  walls.  Theatres  for  the  drama,  again,  have 
boundaries  broken  in  all  ways  by  rows  of  boxes,  and  various  ornaments. 

The  concentration  of  sound  by  concave  surfaces  produces  many  curious 
effects  both  in  nature  and  in  art. 

There  are  remarkable  situations  where  the  sound  from  a  cascade  is  con- 
centrated by  the  surface  of  a  neighbouring  cave  so  completely,  that  a  person 
accidentally  bringing  his  ear  into  the  focus,  is  suddenly  astounded,  as  if  the 
universe  were  crushing  around  him.  A  chair  placed  in  the  cave,  so  that  a 
person  sitting  down  in  it  must  bring  his  ear  into  the  focus,  insures  the  success 
of  the  sometimes  amusing  experiment. 

The  centre  of  a  circle  is  the  focus  in  which  sound  issuing 
from  it  is  again  collected  after  reflection  ;  hence  the  powerful 
echo  near  the  centre  of  a  round  apartment.  An  oval  has  two 
centres  or  foci — one  towards  each  end,  as  a  and  b — and  the 
nature  of  the  curve  is  such,  that  sound,  or  light,  or  heat, 
issuing  around  from  either  of  the  foci,  as  a,  by  obeying  the 
law  of  reflection  above  stated,  is  all  directed  from  the  various 
points,  as  at  c  d  e,  &c.,  to  the  other  focus,  as  at  b.  Hence 
a  person  uttering  a  whisper  in  one  focus  of  an  oval  room  is 
very  audible  to  the  other,  although  he  may  not  be  heard  by 
persons  placed  between.  Such  a  room  may  be  called  a  whispering  gallery. 
Concave  surfaces  facing  each  other,  as  two  alcoves  in  a  garden,  or  covered 
recesses  on  opposite  sides  of  a  street  or  bridge,  will  enable  persons  seated  in 


Fig    122. 


KEFLECTION.  251 

i 

their  foci  to  converse  by  whispers  across  louder  noises  in  the  space  between, 
and  without  themselves  being  overheard  in  that  space. 

The  reason  why  a  tube  conveys  sound  so  far,  is,  that  its  sides  confine  or 
repress  by  a  continued  reflection,  the  advancing  sound  which,  in  the  open 
air,  would  quickly  spread  laterally  and  be  dissipated.  And  the  reason  that 
the  plane  surface  of  a  smooth  wall,  or  of  water,  &c.,  also  conveys  sound  so 
far,  is,  thatit  similarly  prevents  the  lateral  spreading  and  dissipation,  although 
only  on  one  side. — Persons  far  apart  may  converse  along  a  smooth  wall. — 
The  barking  of  dogs  and  the  clear  voice  o£  a  street-crier,  in  a  town  situated 
on  the  border  of  a  lake,  may  be  heard  across  the  water  in  a  calm  evening,  at 
a  distance  of  more  than  five  miles — the  sound  of  bells,  of  course,  is  audible 
much  farther. — And  in  the  stillness  of  night,  even  the  splashing  oars  of  a 
boat  will  announce  its  approach  to  persons  waiting  at  a  great  distance. 

If  a  sound-reflecting  surface  be  curved  inwards,  that  is,  be  concave,  it  not 
only  prevents  the  spreading  of  any  sound  which  passes  along  it,  but  is  con- 
stantly condensing  the  sound  by  driving  the  external  part  inwards.  Hence, 
in  a  circular  space,  such  as  a  gallery  under  a  dome,  persons  close  to  the 
wall 'may  whisper  to  each  other  at  all  distances. 

An  ear-trumpet  is  a  tube  wide  at  one  end  where  the  sound  enters,  and 
narrow  at  the  other  where  the  ear  is  applied  :  its  sides  are  so  curved  that, 
according  to  the  law  of  reflection,  all  the  sound  which  enters  is  brought  to  a 
focus  in  the  narrow  end.  It  thus  increases  many  fold  the  intensity  of  a 
sound  which  reaches  the  ear  through  it,  and  enables  a  person  who  has  be- 
come deaf  to  common  conversation,  to  mix  again  with  pleasure  in  society, 
The  concave  hand  held  behind  the  ear  answers  in  some  degree  the  purpose 
of  an  ear- trumpet,  and  in  a  very  large  theatre  is  sometimes  useful  even  to 
persons  of  quick  hearing.  A  notorious  instance  of  a  sound-collecting  surface 
was  the  ear  of  Dionysimj  in  the  dungeons  of  Syracuse  :  the  roof  of  the  prison 
was  so  formed  as  to  collect  the  words  and  even  whispers  of  the  unhappy 
prisoners,  and  to  direct  them  along  a  hidden  conduit  to  where  the  tyrant  sat 
listening.  The  wide-spread  sail  of  a  ship,  rendered  concave  by  a  gentle 
breeze,  is  also  a  good  collector  of  sound.  It  happened  one  day  onboard  a  ship 
sailing  along  the  coast  of  Brazil,  far  out  of  sight  of  land,  that  the  persons 
walking  on  deck,  when  passing  a  particular  spot,  heard  very  distinctly,  during 
an  hour  or  two,  the  sound  of  bells,  varying  as  in  human  rejoicings.  All  on 
board  came  to  listen,  and  were  convinced,  but  the  phenomenon  was  most 
mysterious.  Months  afterwards  it  was  ascertained,  that  at  the  time  of  obser- 
vation the  bells  of  the  city  of  St.  Salvador,  on  the  Brazilian  coast,  had  been 
ringing  on  the  occasion  of  a  festival :  their  sound,  therefore,  favoured  by  a 
gentle  wind,  had  travelled  over  perhaps  100  miles  of  smooth  water,  and  had 
been  brought  to  a  focus  by  the  concave  sail  in  the  particular  situation  on 
the  deck  where  it  was  listened  to.  It  appears  from  this  that  a  machine 
might  be  constructed  having  the  same  relation  to  sound  that  a  telescope 
has  to  light. — A  friend  of  the  author,  on  the  18th  of  June,  1814,  while 
sitting  near  the  wall  of  his  garden,  situated  near  Dover,  heard  distinctly  the 
firing  of  the  cannon  at  the  battle  of  Waterloo. 

The  speaking-trumpet  is  made  according  to  the  same  law  of  reflected 
sound,  with  the  view  of  directing  the  strength  of  the  voice  to  a  particular 
point.  The  sea  captain  uses  it  to  hail  ships  at  a  distance,  or  to  send  his 
orders  aloft,  where  the  unaided  voice  would  be  lost  in  the  noise  of  the  wind 
and  waves.  A  similar  form  of  mouth  is  used  for  the  bugle  horn  and 
common  trumpet,  and  fits  them  to  sound  the  note  of  command  amid  the 
uproar  of  contending  armies. 


252  A  C  0  U-S  T  I  C  S  . 

Some  amusing  effects  have  been  produced  by  operating  on  sounds  with 
tubes  and  concave  surfaces.  What  was  termed  the  invisible  girl,  was  a 
contrivance  where  the  questions  of  vistors  were  caught  by  a  concealed  con- 
cave, and  carried  to  the  director  who  sat  at  a  distance ;  and  his  replies,  as 
in  the  whispering  gallery,  become  audible  to  the  inquirers  alone. 

The  concave,  undulating,  and  perfectly  polished  surface  of  many  sea- 
shells,  fits  them  to  catch,  mix,  and  return  the  pulses  of  sounds  that  happen 
to  be  trembling  about  them,  so  as  to  produce  that  curious  resonance  from 
within  which  closely  resembles  the  sound  of  the  distant  ocean — so  closely, 
that  the  spirited  boy,  after  studying  the  interesting  stories  of  voyagers  which 
paint  dangers  to  be  nobly  braved,  and  charm  of  nature  to  be  seen  in  distant 
lands,  often  feeds  his  imagination  with  this  voice  of  a  shell,  and  fancies 
himself  already  riding  among  the  billows. 

The  animal  ear, 

so  admirably  adapted  to  perceive  the  evanescent  tremblings  of  the  air,  has 
of  course  a  structure  in  nice  relation  to  their  nature  as  now  explained.  The 
parts  of  the  ear,  and  the  progress  of  the  sound  to  the  sentient  nerve,  may 
be  simply  described  as  follows : 

1st.     There  is  external  to  the  head,  a  wide-mouthed  tube  or  ear-trumpet 
a,  for  catching  and  concentrating  the  waves  of 
Fig.    123.  sound.     It  is  moveable  in  many  animals,  so 

that  they  can  direct  it  to  the  place  from  which 
the  sound  comes. 

2d.  The  sound  concentrated  at  the  bottom  of 
the  ear-tube,  falls  upon  a  membrane  stretched 
across  the  channel,  like  the  parchment  of  an 
ordinary  drum,  over  the  space  called  the  tym- 
panum or  drum  of  the  ear  b,  and  causes  the 
membrane  to  vibrate.  That  its  motion  may 
be  free,  the  air  contained  within  the  drum  has 

free  communication  with  the  external  air,  by  the  open  passage  /,  called  the 
Eustachian  tube,  leading  to  the  back  of  the  mouth.  A  degree  of  deafness 
ensues  when  this  tube  is  obstructed,  as  by  wax ;  and  a  crack  or  sudden 
noise,  with  immediate  return  of  natural  hearing,  is  generally  experienced 
when,  in  the  effort  of  sneezing  or  otherwise,  the  obstruction  is  removed. 

3d.  The  vibrations  of  the  membrane  of  the  drum  are  conveyed  farther  in. 
wards,  through  the  cavity  of  the  drum,  by  chain  of  four  bones  (not  here 
represented  on  account  of  their  minuteness,)  reaching  from  the  centre  of  the 
membrane  to  the  oval  door  or  window  leading  into  the  labyrinth  e. 

4th.  The  labyrinth,  or  complex  inner  compartment  of  the  ear,  over  which 
the  nerve  of  hearing  is  spread  as  a  lining,  is  full  of  water;  and  therefore  by 
the  law  of  fluid  pressure  (see  page  128,)  when  the  force  of  the  moving  mem- 
brane of  the  drum,  acting  through  the  chain  of  bones,  is  made  to  compress 
the  water,  the  pressure  is  felt  instantly  over  the  whole  cavity,  as  in  a  hydro- 
static press. — The  labyrinth  consists  of  the  vestibule  e,  the  three  semi-circular 
canals  c,  imbedded  in  the  hard  bone,  and  a  winding  cavity,  called  the 
cochlea  d,  like  that  of  a  snail-shell,  in  which  fibres,  stretched  across  like 
harp-strings,  constitute  the  lyra. — The  separate  uses  of  these  various  parts 
are  not  yet  perfectly  known.  The  membrane  of  the  tympanum  may  be 
pierced,  and  the  chain  of  bones  may  be  broken  without  entire  loss  of  hearing. 
Considerable  diversity  of  form  and  dimension  is  found  in  different  animals. 


THE    EAR.  253 

The  bone  containing  the  cavities  of  the  ear  is  the  hardest  in  the  body, 
and  is  the  first  formed. 

The  ear  has  the  power  of  judging  of  the  direction  in  which  sound  comes. 
A  person  in  a  thicket,  listening  to  the  song  of  various  birds,  although  they 
be  concealed  from  his  eye  by  the  luxuriance  of  the  vernal  foliage,  still  judges 
correctly  by  the  ear  in  what  tree  every  little  songster  is  concealed. — The 
same  truth  is  strikingly  exemplified  in  the  fact  that,  when  horses  or  mules 
march  in  company  at  night,  those  in  front  direct  their  ears  forward  ;  those 
in  the  rear,  backwards ;  and  those  in  the  centre,  laterally  or  across ; — the 
whole  troop  seeming  to  be  actuated  by  one  feeling,  which  watches  the 
common  safety. 

The  intensity  of  sound  is  to  the  ear  a  measure  of  distance.  In  a  windy 
night,  the  sound  of  a  distant  bell  may  be  brought  so  quickly,  that  it  has  not 
yet  had  time  to  spread  and  be  weakened  ;  and  a  person  is  often  roused  from 
a  reverie  by  its  unusual  loudness  and  apparent  nearness. — When  a  stormy 
wind  blows  directly  upon  a  coast,  and  rolls  the  great  waves  in  upon  the 
sandy  beach  or  among  the  rocks,  the  countryman  living  far  inland  hears  the 
uproar,  as  if  the  ocean  had  burst  its  barriers,  and  were  pouring  in  upon  the 
land.  The  scene-contrivers  at  our  theatres  heighten  the  illusion  of  an 
approaching  procession,  by  letting  the  accompanying  music  be  first  heard 
from  a  closed  chamber  or  in  a  feeble  tone,  and  afterwards  with  gradually 
increasing  loudness.  To  the  imagination,  already  excited  perhaps  to  the 
highest  pitch  by  the  drama  of  some  divine  mind,  the  advancing  host  is  thus 
most  vividly  portrayed  ;  and  when  at  last,  with  the  thunder  of  drums  and 
trumpets  from  the  front  of  the  stage,  the  troop  also  appears,  the  effect  is  com- 
plete. It  is  the  varying  loudness  of  the  JEolian  harp  which  produces  the 
feeling  that  the  heavenly  choir  is  sometimes  approaching  and  sometimes 


[For  an  account  of  the  Doctrines  of  Fluidity  in  relation  to  animals,  see 
Part  V.  Sec.  II.] 


254  IMPONDERABLE    SUBSTANCE. 


PART    IT. 

DOCTRINES  OF  IMPONDERABLE  SUBSTANCE. 


To  minds  beginning  this  study,  it  may  facilitate  the  conception  of  a  sub- 
stance which  is  without  weight,  or  at  least  is  imponderable  by  human  art, 
to  consider  the  nature  of  air.  Until  lately  men  were  so  imperfectly 
acquainted  with  the  constitution  of  the  universe  around  them,  that  a  person 
placed  in  an  apartment  which  offered  to  view  nothing  but  the  naked  walls, 
would  have  said  that  it  was  empty,  meaning  literally  what  he  said ;  and 
even  when  advertised  that  there  was  air  in  the  room,  he  would  still  have  been 
far  from  possessing  a  clear  notion  that  it  was  full  of  aerial  fluid  just  as  an 
open  vessel  immersed  in  the  sea  is  full  of  water,  and  that  if  air  were  not 
allowed  to  escape  from  it,  even  so  small  a  body  as  an  apple  could  not  be 
pressed  into  it  additionally  by  less  force  than  fifty  or  sixty  pounds.  This  truth, 
however,  is  now  clearly  understood,  and  daily  exemplified  in  easy  pneumatic 
experiments,  and  in  no  way  more  strikingly  than  by  the  recent  adoption  of 
the  substance  of  air  in  place  of  feathers,  as  stuffing  for  beds  and  pillows. 
An  air-tight  bag  or  sack  suspended  by  its  lip  in  the  air,  and  held  quite  open 
by  a  hoop  near  its^niouth,  would  appear  empty,  but  if  then  firmly  closed  above 
the  hoop,  it  would  have  imprisoned  its  fill  of  air,  just  as  a  bag  similarly  man- 
aged underwater  would  imprison  its  fill  of  water ;  and  while  in  some  respects 
the  air  would  be  softer  and  locally  more  yielding  than  feathers,  its  entire  mass 
would  be  much  less  compressible.  Now  this  air,  when  weighed  by  means  which 
modern  science  has  furnished,  is  found  in  a  cubic  foot  to  contain  somewhat 
more  than  an  ounce,  and  by  strongly  pressing  it,  or  by  causing  it  to  combine 
chemically  with  some  other  substance,  we  can  reduce  it  to  a  very  small  bulk, 
either  with  the  form  of  a  liquid  or  of  a  solid  j  proving  how  small  a  quantity 
of  ponderable  matter,  under  certain  circumstances,  will  occupy  great  space. 
And  common  air  is  by  no  means  the  lightest  known  substance,  which  as 
powerfully  resists  the  intrusion  of  other  bodies  where  it  exists.  Hydrogen 
gas,  for  instance,  of  the  same  space-occupying  force,  weighs  only  a  fourteenth 
part  as  much,  and  therefore  a  few  drachms  of  it  confined  in  a  bag  or  bed  as 
broad  as  the  foundation  of  a  house,  would  support  a  house  or  cask  as  large 
as  a  house  filled  with  water  to  a  height  of  thirty  feet,  the  gas  itself  being 
then  eighty  thousand  times  lighter  than  its  bulk  of  gold  ; — and  if  the  pressure 
on  it  were  diminished,  it  would  readily  expand  to  a  volume  a  thousand  times 
as  great,  and  would  still  be  exerting  a  considerable  outward  elasticity.  Again, 
a  mixture  of  oxygen  and  hydrogen  gases,  while  uniting  with  explosive  force 
to  form  water,  dilates  for  the  time,  even  under  the  great  pressure  of  the 
atmosphere,  to  a  bulk  about  twenty  times  greater  than  the  gases  have  while 
separate. 

The  mind,  pursuing  the  idea  of  such  expansion  or  occupancy  of  space  by 
a  small  quantity  of  matter,  and  reflecting  on  the  wonderful  divisibility  of 
matter  or  minuteness  of  the  ultimate  atoms,  as  explained  in  Part  I.  of  this 
work,  might  almost  admit  as  a  possible  reality  Newton's  hypothetical  illustra- 


IMPONDERABLE    SUBSTANCE.  255 

tion  of  that  divisibility,  viz.,  that  even  one  ounce  of  substance  uniformly  dis- 
tributed over  the  vast  space  in  which  our  solar  system  exists,  might  leave 
no  quarter  of  an  inch  without  its  particle.  Now  a  fluid  in  any  degree 
approaching  in  rarity  to  this,  although  it  might  press,  resist,  communicate 
motion,  and  have  other  influences  in  common  with  more  ponderable  matter, 
would  have  neither  weight  nor  inertia  discoverable  by  means  at  present 
known  to  man.  While  we  are  contemplating,  then,  or  modifying  the  agen- 
cies of  what  causes  the  phenomena  of  heat  and  cold,  of  light  and  darkness, 
of  electricity  in  its  forms  of  thunder  and  lightning,  of  galvanism,  or  of  mag- 
netism, in  a  word,  the  most  striking  phenomena  of  nature,  we  may  be  dealing 
with  matter  of  the  subtle  constitution  now  spoken  of.  And  as  in  the  terres- 
trial atmosphere  there  are  at  least  two  fluids  present,  $&.,  oxygen  and  nitrogen, 
of  distinct  nature,  so  in  a  more  subtle  ether,  filling  all  space,  there  may  be 
various  ingredients. 

A  majority  of  philosophers  now  incline  to  the  opinion  here, sketched,  that 
there  is  at  least  one  such  subtle  fluid  or  ether  occupying  completely  the 
space  of  the  universe,  and  tending  to  uniform  diffusion  by  reason  of  a  strong 
mutual  repulsion  of  its  particles,  which  fluid  pervades  denser  material  sub- 
stances somewhat  as  water  pervades  a  sponge  or  a  mass  of  sand,  being 
attracted  in  a  peculiar  way  by  each  substance,  and  which  fluid  may  or  may 
not  have  weight  and  inertia.  They  believe  farther  that  the  phenomena 
above  alluded  to,  and  which  human  art  can  exhibit  with  highest  beauty,  or 
with  awful  intensity,  are  produced  by  the  motion  of  other  affections  of  that 
fluid,  as  the  sensation  of  sound  in  all  its  varieties  is  produced  in  the  delicate 
structure  of  the  ear  by  a  certain  motion  in  the  air,  or  in  any  other  body 
having  communication  with  the  ear  j  or  as  the  sensation  of  jar  is  perceived 
by  a  hand  held  to  one  end  of  a  log  of  wood  when  a  blow  is  given  to  the  other 
end.  Some  philosophers  again  suppose  that  the  causes  of  the  phenomena 
are  material  particles  projected  through  space,  somewhat  as  sand  might  be 
scattered  by  an  explosion,  and  which  particles  are  present  only  when  the 
effects  are.  apparent.  Some  combine  these  two  hypotheses.  And  some  hold 
all  the  phenomena  of  heat  to  be  mere  motions  in  the  common  matter  of  the 
bodies  in  which  the  heat  exists. 

We  mention  these  hypotheses,  not  with  the  view  of  entering  upon  a  minute 
examination  of  their  respective  merits,  or  even  of  asserting  that  any  one  of 
them  is  true,  but  merely  to  make  the  reader  aware  of  the  directions  which 
inquirers'  minds  have  taken  in  pursuing  the  investigation.  .  To  understand 
the  subjects  as  far  as  men  yet  usefully  understand  them,  and  sufficiently  for 
a  vast  number  of  most  useful  purposes,  it  is  only  necessary,  as  in  other  de- 
partments of  science,  to  classify  important  phenomena,  so  that  their  nature 
and  resemblances  may  be  clearly  perceived.  When,  in  treating  of  the  human 
mind,  we  speak  of  its  retaining  an  idea,  or  being  depressed,  or  being  heated 
with  passion,  &c.,  we  speak  of  subjects  sufficiently  definite,  although  we  may 
have  no  hypothesis  as  to  the  intimate  nature  of  the  phenomena : — and  in  the 
same  manner  may  we  speak  of  the  accumulation,  radiation,  or  other  affec- 
tions of  heat  and  light.  We  know  nothing  of  the  cause  even  of  gravity,  the 
grandest  influence  in  nature,  but  we  can  calculate  its  effects  with  admirable 
precision. 


256  HEAT. 

PART     IV. 

/ 

(CONTINUED.) 
SECTION  I.— ON  HEAT. 

ANALYSIS   OP   THE   SECTION. 

Heat  (  by  some  called  Caloric  )  may  be  strikingly  referred  to  as  that  which 
causes  the  difference  between  winter  and  summer,  between  tropical  gardens 
and  polar  wastes.  Its  inferior  degrees  are  denoted  by  the  term  COLD.  It 
cannot  be  exhibited  apart,  nor  prove  to  have  weight  or  inertia,  and  the 
change  of  its  quantity  in  bodies  is  most  convenient //  estimated  by  the  con- 
comitant change  of  their  bulk',  any  substance  so  circumstanced  as  to  allow 
this  to  be  accurately  measured  constituting  a  THERMOMETER. 

Heat  diffuses  itself  among  neighbouring  bodies  until  all  have  the  same  tem- 
perature, that  is,  until  all  similarly  affect  a  thermometer.  It  spreads 
partly  through  their  structure,  or  by  conduction,  as  it  is  called,  with  a 
slow  progress,  different  for  each  substance,  and  in  jl.uids  modified  by  the 
motion  of  their  particles  ;  and  it  spreads  partly  also  by  being  shot  or 
radiated  like  light  from  one  body  to  another,  through  transparent  media 
or  space,  with  readiness  affected  by  the  material  and  state  of  the  giving 
and  receiving  surfaces. 

Heat,  by  entering  bodies,  expands  them,  and  through  a  range  which  in- 
cludes, as  three  successive  stages,  the  forms  of  SOLID,  LIQUID  and  AIR  or 
GAS ',  becoming  thus  in  nature  the  grand  antagonist  and  modifier  of  that 
attraction  which  holds  corporeal  particles  together,  and  which,  if  acting 
alone,  would  reduce  the  whole  material  universe  to  one  solid  lifeless  mass. 
Each  particular  substance,  according  to  the  nature,  proximity •,  &c.,  of 
its  ultimate  particles,  takes  a  certain  quantity  of  heat  (  said  to  mark  its 
capacity  ),  to  produce  in  it  a  given  change  of  temperature  or  calorific  ten- 
sion ',  undergoing  expansion  then  in  a  degree  proper  to  itself,  and 
changing  its  form  to  liquid  and  air  at  points  of  temperature  proper  to 
itself ;  the  expansion  in  bodies  generally  increasing  more  rapidly  than 
the  temperature,  because  the  cohesion  of  their  particles  lessens  with  in- 
crease of  distance  ;  being  remarkably  gre-tter  therefore  in  liquids  than  in 
solids;  and  in  air  than  in  liquids;  and  the  rate  of  expansion,  moreover, 
being  much  quickened  as  the  bodies  approach  their  points  of  changing 
foim  to  liquid  or  air,  to  produce  which  changes,  a  large  quantity  of  heat 
enters  them,  but  in  the  new  arrangement  of  particles  and  increased 
volume  of  the  mass,  it  becomes  hidden  from  the  thermometer,  and  is 
therefore  called  LATENT  HEAT.  For  any  given  substance  the  changes  of 
form  happens  so  constantly  at  the  same  temperature,  that  they  mark  fixed 
points  in  the  general  scale  of  temperature,  and  enable  us  to  regulate  and 


CAUSE  OF  SEASONS  AND  CLIMATES.      25 

compare   thermometers.       Heat,  by  expanding   different  substances  une- 
qually, influences  much  their  chemical  combination. 

Heat  influences  also  the  functions  of  vegetable  and  animal  life.  The  great 
source  of  heat  is  the  fun  ;  but  electricity,  combustion  and  other  chemical 
action,  condensation,  friction,  and  the  actions  of  life,  are  also  excitants* 


K  Heat  may  be  strikingly  referred  to  as  that  which  causes  the  difference 
between  winter  and  summer?  between  the  gardens  of  the  equator  and 
polar  wastes.7'  (See  the  Analysis,  page  256.) 

In  the  winter  of  climates,  where  the  temperature  is  for  a  time  below  the 
freezing  point  of  water,  the  earth  with  its  waters  is  bound  up  in  snow  and 
ice,  the  trees  and  shrubs  are  leafless,  appearing  everywhere  like  withered 
skeletons,  countless  multitudes  of  living  creatures,  owing  either  to  the  bitter 
cold  or  deficiency  of  food,  are  perishing  in  the  snows — nature  seems  dying 
or  dead ;  but  what  a  change  when  spring  returns,  that  is,  when  heat  returns  ! 
The  earth  is  again  uncovered  and  soft,  the  rivers  flow,  the  lakes  are  again 
liquid  mirrors,  the  warm  showers  come  to  foster  vegetation,  which  soon 
covers  the  ground  with  beauty  and  plenty.  Man,  lately  inactive,  is  recalled 
to  many  duties;  his  water-wheels  are  everywhere  at  work,  his  boats  are 
again  on  the  canals  and  streams,  his  busy  fleets  of  industry  are  along  the 
shores ',  winged  life  in  .new  multitudes  fill  the  sky,  finny  life  similarly  fills 
the  waters,  and  every  spot  of  earth  teems  with  vitality  and  joy.  .  Many 
persons  regard  these  changes  of  season  as  if  they  came  like  the  successive 
position  of  a  turning-wheel,  of  which  one  necessarily  brings  the  next;  not 
adverting  that  it  is  the  single  circumstance  of  change  of  temperature  which 
does  all.  But  if  the  colds  of  winter  arrive  too  early,  they  unfailingly  pro- 
duce the  wintery  scene,  and  if  warmth  come  before  its  time  in  spring,  it 
expands  the  bud  and  the  blossom,  which  a  return  of  frost  will  surely  destroy. 
A  seed  sown  in  an  ice-house  never  awakens  to  life. 

Again,  as  regards  climates,  the  earthly  matters  forming  the  exterior  of 
our  globe,  and,  therefore,  entering  into  the  composition  of  soils,  are  not 
different  for  different  latitudes, — at  the  equator,  for  instance,  and  near  the 
poles.  That  the  aspect  of  nature  then,  in  the  two  situations,  exhibits  a  con- 
trast more  striking  still  than  between  summer  and  winter,  is- merely  to  an 
inequality  of  temperature,  which  is  permanent.  Were  it  not  for  this,  in  both 
situations  the  same  vegetables  might  grow,  and  the  same  animals  might  find 
their  befitting  support.  But  now,  in  the  one,  namely,  where  the  heat  abounds, 
we  see  the  magnificent  scene  of  tropical  fertility ;  the  earth  covered  with 
luxuriant  vegetation  in  endless,  lovely  variety,  and  even  the  hard  rocks  fes- 
tooned with  green,  perhaps  with  the  vine,  rich  in  its  purple  clusters.  In  the 
midst  of  this  scene,  animal  existence  is  equally  abundant,  and  many  of  the 
species  are  of  surpassing  beauty — the  plumage  of  the  birds  is  as  brilliant  as 
the  gayest  flowers.  The  warm  air  is  perfume  from  the  spice-beds,  the  sky 
and  clouds  are  often  dyed  in  tints  as  bright  as  freshest  rainbow,  and  happy 

*  It  i?  to  be  remarked  here,  that  many  phenomena  in  which  heat  plays  an  important 
part,  have  been  already  described  in  preceding  chapters  of  this  work  ; — for  instance,  the 
action  of  the  steam-engine,  the  phenomena  of  winds,  many  facts  in  {meteorology,  Ac., 
under  the  head  of  Pneumatics.  In  a  separate  treatise  on  heat,  these  could  not  with  pro- 
priety have  been  omitted  ;  but  in  a  comprehensive  system  of  science  like  the  present,  they 
find  their  fit  place,  where,  being  surrounded  by  subjects  resembling  them  in  more  intri- 
cate particulars,  they  can  be  more  concisely  and  clearly  explained. 


258  HEAT. 

human  inhabitants  call  the  scene  a  paradise.  Again,  where  heat  is  absent, 
we  have  the  dreary  spectacle  of  polar  barrenness,  namely,  bare  rock  or 
mountain,  instead  of  fertile  field ;  water  everywhere  hardened  to  solidity ; 
no  rain,  nor  cloud,  nor  dew;  few  motions  but  drifting  snow;  vegetable  life 
scarcely  existing,  and  then  only  in  sheltered  places  turned  to  the  sun — and 
instead  of  the  palms  and  other  trees  of  India,  whose  single  leaf  is  almost 
broad  enough  to  cover  a  hut,  there  are  bushes  and  trees,  as  the  furze  and  fir, 
having  what  may  be  called  hairs  or  bristles,  in  the  room  of  leaves.  In  the 
winter  time,  during  which  the  sun  is  not  seen  for  nearly  six  months,  new 
horrors  are  added,  viz.,  the  darkness  and  dreadful  silence,  the  cold  benumb- 
ing all  life,  and  even  freezing  mercury — a  scene  into  which  man  may  pene- 
trate from  happier  climes,  but  where  he  can  only  leave  his  protecting  ship 
and  fires  for  short  periods,  as  he  might  issue  from  a  diving-bell  at  the  bottom 
of  the  ocean.  That  in  these  now  desolate  regions,  heat  only  is  wanted  to 
make  them  like  the  most  favoured  countries  of  the  earth,  is  proved  by  the 
recent  discoveries  under  ground  of  the  remnant  of  animals  and  vegetables 
formerly  inhabiting  them,  which  now  can  live  only  near  the  equator.  While 
winter,  then,  or  the  temporary  absence  of  heat,  may  be  called  the  sleep  of 
nature,  the  more  permanent  torpor  about  the  poles  appears  like  its  death ; 
and  when  we  farther  reflect,  that  heat  is  the  great  agent  in  numberless  im- 
portant processes  of  chemistry  and  domestic  economy,  and  is  the  actuating 
principle  of  the  mighty  steam-engine  which  now  performs  half  the  work  of 
society,  how  truly  may  heat,  the  subject  of  our  present  chapter,  be  con- 
sidered as  the  life  or  soul  of  the  univer&e. 

"  Heat  cannot  be  exhibited  in  a  separate  state,  nor  proved  to  have  weight  or 
inertia."      (Read  the  Analysis,  page  256.) 

Although  heat  is  known  to  be  abundant  in  the  sunbeam,  and  to  radiate 
around  from  a  blazing  fire,  we  cannot  otherwise  arrest  or  detect  it  in  its  pro- 
gress than  by  allowing  it  to  enter,  and  remain  in  some  ponderable  substance. 
We  know  hot  iron,  or  hot  water,  or  hot  air,  but  nature  no  where  presents  to 
us,  nor  has  art  succeeded  in  showing  us,  heat  alone. 

If  we  balance  a  quantity  of  ice  in  a  delicate  weigh-beam,  and  then  leave 
it  to  melt,  the  equilibrium  will  not  be  in  the  slightest  degree  disturbed.  Or 
if  we  substitute  for  the  ice,  boiling  water  or  red- hot  iron,  and  leave  this  to 
cool,  there  will  be  no  difference  in  the  result.  If  we  place  a  pound  of  mer- 
cury in  one  scale  of  the  weigh-beam  and  a  pound  of  water  in  the  other,  and 
then  either  heat  or  cool  both  through  the  same  number  of  therinometric 
degrees,  although  about  thirty  times  more  heat  (as  will  be  explained  below) 
enters  or  leaves  the  bulky- water  than  the  dense  mercury,  they  will  still 
remain  equivalent  weights. 

Again,  a  sunbeam,  with  its  intense  light  and  heat,  after  being  concen- 
trated by  a  powerful  lens  or  mirror,  may  be  made  to  fall  upon  the  scale  of 
a  most  delicate  balance,  but  will  produce  no  depressing  effect  on  the  scale, 
as  would  follow  if  what  constitutes  the  beam  had  the  least  forward  motal 
inertia  or  momentum. 

Such  are  the  facts  which  have  led  certain  inquirers  to  deny  the  material 
or  separate  existence  of  heat,  and  to  hold  that  it  is  merely  motion  of  one 
kind  among  the  material  particles  of  bodies  generally,  as  sound  is  motion 
of  another  kind  among  the  same  particles.  The  following  facts  they  consider 
to  have  the  same  bearing  in  the  argument :  Heat  can  be  produced  without 
limit  by  friction,  as — when  savages  light  their  fires  by  rubbing  together  two 


CAUSE  OF  SEASONS  AND  CLIMATES.      259 

pieces  of  wood — when  Count  Rumford  made  great  quantities  of  water  boil, 
by  causing  a  blunt  borer  to  rub  against  a  mass  of  metal  immersed  in  water — 
when  Sir  Humphrey  Davy  quickly  melted  pieces  of  ice  by  rubbing  them 
against  each  other  in  a  room  cooled  below  the  freezing  point,  &c.  Intense 
heat  is  produced  by  the  explosion  of  gunpowder  or  other  fulminating  mix- 
ture, yet  it  cannot  be  conceived  to  have  existed  in  the  small  bulk  of  the 
powder  before  the  explosion.  Other  inquirers,  on  the  contrary,  have  deemed 
to  be  proofs  of  the  separate  materiality  of  heat  such  facts  as  now  follow ; — 
that  it  is  radiated  through  the  most  perfect  vacuum  which  we  can  produce, 
and  even  more  readily  than  through  air;  that  it  radiates  in  the  same  place 
in  all  directions,  without  impediment  from  the  crossing  rays ; — that  it  be- 
comes instantly  sensible  on  the  condensation  of  any  material  mass,  as  if  then 
squeezed  out  from  the  mass  ;  as  when,  by  compressing  air  suddenly,  we  in- 
flame a  match  immersed  in  it ;  or  when,  on  reducing  the  bulk  of  iron  by 
hammering,  we  render  it  very  hot,  the  warming  being  greater  at  the  first 
blow  (which  most  changes  the  bulk)  than  afterwards, — that  when,  on  mix- 
ing bodies  which  combine  so  intimately  as  to  occupy  less  space  than  when 
separate,  there  is  a  disengagement  of  heat  proportioned  to  the  diminution  'of 
the  volume : — that  the  laws  of  the  spreading  of  heat  in  bodies  do  not 
resemble  those  of  the  spreading  of  sound,  or  of  any  other  motion  known 
to  us; — and  that,  as  to  the  great  and  sudden  extrication  of  heat  by  friction 
or  explosion,  it  may  be  as  truly  a  rush  of  the  fluid  to  the  part,  as  in  the 
case  of  an  electrical  accumulation  or  discharge  These  facts,  moreover, 
they  think,  square  well  with  their  assumption  that  the  phenomena  of  heat 
are  produced  by  an  exceedingly  subtle  fluid,  or  ether,  pervading  the  whole 
universe,  and  softening,  or  melting,  or  gasifying  bodies,  according  to  the 
quantity  present  in  each;  its  own  parts  being  strongly  repulsive  of  each 
other,  and  seeking,  therefore,  widest  and  most  equable  diffusion. 

"  The  change  of  its  quantity  in  bodies  is  most  conveniently  estimated  by  the 
concomitant  change,  of  their  bulk,  any  substance  so  circumstanced,  as  to 
allow  this  to  be  accurately  measured,  constituting  a  thermometer ."  (Read 
the  Analysis,  page  256.) 

If  we  heat  a  wire,  it  is  lengthened ;  if  we  heat  water  in  a  full  vessel,  a 
part  runs  over ;  if  we  heat  air  in  a  bladder,  the  bladder  is  distended ;  in  a 
word,  if  we  heat  any  substance,  its  volume  increases  in  some  proportion  to 
the  increase  of  temperature, — and  we  may  measure  the  increase  of  volume. 
The  reasons  why,  in  such  investigations,  a  contrivance  in  which  the  expan- 
sion of  .mercury  may  be  observed,  viz.,  the  mercurial  thermometer,  is  com- 
monly preferred  to  others,  can  only  be  fully  understood  by  the  mind  which 
has  considered  the  whole  subject  of  heat ;  and  we  touch  upon  the  matter 
here,  only  for  the  purpose  of  stating  that  a  mercurial  thermometer  is  a 
small  bulb,  or  bottle  of  glass  filled  with  mercury,  and  having  a  long  very 
narrow  stalk  or  neck,  in  which  the  mercury  rises  when  expanded  by  heat, 
or  falls  when  heat  is  withdrawn ;  the  stalk  between  the  points  at  which 
mercury  standings  in  freezing  and  boiling  water,  being  divided  into  an 
arbitrary  number  of  degrees,  which  division  appearing  on  a  scale  applied  to 
the  stalk,  is  continued  similarly  above  and  below  these  points. 

"  Heat  diffuses  itself  among  neighboring  bodies  until  all  have  acquired  the 
same  temperature  ;  that  is  to  say,  until  all  will  similarly  affect  a  thermo- 
meter."  (See  the  Analysis.) 

An  iron  bolt  thrust  in  among  burning  coals  soon  becomes  red  hot  like 


260  HEAT. 

them.  If  it  be  the  heater  of  a  tea-urn,  it  will,  when  afterwards  placed 
amidst  the  water,  part  with  its  lately  acquired  heat  to  the  water,  until  both 
are  of  the  same  temperature.  Boiling  water,  again  soon  imparts  heat  to  an  egg 
placed  in  it,  and  a  feverish  head  yields  its  heat  to  a  bladder  of  cold  water 
or  ice.  A. hundred  objects  enclosed  in  the  same  apartment,  if  tested,  after 
a  time,  by  the  thermometer,  will  all  indicate  the  same  temperature. 

"  TJie  inferior  degrees  of  heat  are  denoted  ~by  the  term  COLD." 

When  the  hand  touches  a  body  of  a  higher  temperature  than  itself,  it  re- 
ceives heat  according  to  the  law  now  explained,  and  it  experiences  a  pecu- 
liar sensation ;  when  it  touches  a  body  of  lower  temperature  than  itself,  it 
gives  out  heat  for  a  like  reason,  and  experiences  another  and  very  different 
sensation.  The  two  are  called  the  sensations  of  heat  and  of  cold.  Now 
heat  and  cold,  considered  as  existing  in  the  bodies  themselves,  although  thus 
appearing  opposities,  are  really  degrees  of  the  same  object,  temperature , 
contrasted  by  name,  for  convenience  sake,  in  reference  to  the  particular 
temperature  of  the  individuals  speaking  of  them — just  as  any  two  nearest 
mile-stones  on  a  road,  although  merely  marking  degrees  of  the  same  object, 
distance,  might  receive  from  persons  living  between  them  the  opposite  names 
of  east  and  west,  or  of  north  and  south.  It  is  to  be  remarked,  moreover, 
that  the  sensation  of  heat  is  producible  also  by  a  body  colder  than  the 
hand,  provided  it  be  less  cold  than  a  body  touched  immediately  before,  or 
than  the  usual  temperature ;  and  the  sensation  of  cold  is  produced  under 
the  opposite  circumstances  of  touching  a  comparatively  warm  body,  but 
which  is  less  warm  than  something  touched  just  before.  This  explains  the 
remarkable  fact  that  the  same  body  may  appear  at  the  same  time,  and  to 
the  same  person,  both  hot  and  cold.  If  a  person  transfer  one  hand  to  com- 
mon spring-water  from  touching  ice,  that  hand  will  deem  the  water  very 
warm :  while  the  other  hand,  transferred  to  it  from  a  warm  bath,  would 
deem  it  very  cold.  For  a  like  reason ;  a  person  from  India,  arriving  in 
England  in  the  spring,  deems  the  air  cold,  while  the  inhabitants  of  the 
country  are  diminishing  their  clothing,  because  the  heat  to  them  is  becom- 
ing oppressive.  Such  facts  show  how  necessary  it  was  for  men  to  discover 
more  correct  thermometers  than  their  bodily  sensations. 

"  Spreading  partly  through  their  structure,  or  by  conduction,  as  it  is  called, 
with  a  progress  proper  to  each  substance."  (Read  the  Analysis,  page 
256.) 

If  one  end  of  a  rod  of  iron  be  held  in  the  fire,  a  hand  grasping  the  other 
end  soon  feels  the  heat  coming  through  it.  Through  a  similar  rod  of  glass 
the  transmission  is  much  slower,  and  through  one  of  wood  it  is  slower  still. 
The  hand  would  be  burned  by  the  iron,  before  it  felt  warmth  in  the  wood, 
although  the  inner  end  were  blazing. 

On  the  fact  that  different  substances  are  permeable  to  heat,  or  have  the 
property  of  conducting  it,  in  different  degrees,  depend  many  interesting 
phenomena  in  nature  and  the  arts ;  hence  it  was  important  to  ascertain  the 
degrees  exactly,  and  to  classify  the  substances.  Various  methods  for  this 
purpose  have  been  adopted.  For  solids — similar  rods  of  the  different  sub- 
stances, after  being  thinly  coated  with  wax,  have  been  placed  with  their  in- 
ferior extremities  in  hot  oil,  and  then  the  comparative  distances  to  which,  in 
a  given  time,  the  wax  was  melted,  furnished  one  set  of  indications  of  the 
comparative  conducting  powers  : — or,  equal  lengths  of  the  different  bare  rods 


SPREADING    BY    CONDUCTION.  261 

being  left  above  the  oil  and  a  small  quantity  of  explosive  powder  being  placed 
on  the  top  of  each,  the  comparative  intervals  of  time  elapsing  before  the 
explosions  gave  another  kind  of  measure  :— or  equal  balls  of  different  sub- 
stances, with  a  central  cavity  in  each  to  receive  a  thermometer,  being  heated 
to  the  same  degree  and  then  suspended  in  the  air  to  cool,  until  the  thermo- 
meter fell  to  a  given  point,  gave  still  another  list.  A  modification  of  the  last 
method  was  adopted  by  Count  Runiford  to  ascertain  the  relative  degrees  in 
which  furs,  feathers,  and  other  materials  used  for  clothing,  conduct  heat,  or, 
which  is  the  same  thing,  resist  its  passage.  He  covered  the  ball  or  stem  of 
a  thermometer  with  a  certain  thickness  of  the  substance  to  be  tried,  by  placing 
the  thermometer  in  a  larger  bulb  and  stem  of  glass,  and  then  filling  the 
interval  between  them  with  tfie  substance  ;  and  after  heating  this  apparatus 
to  a  certain  degree,  by  dipping  it  in  liquid  of  the  desired  temperature,  he 
surrounded  it  by  ice,  and  marked  the  comparative  time  required  to  cool  the 
thermometer  a  certain  number  of  degrees.  The  figures  following  the  names 
of  some  of  the  substances  in  the  subjoined  list,  mark  the  number  of  seconds 
required  respectively  for  cooling  it  60°. 

These  experiments  have  shown,  as  a  general  rule,  that  density  in  a  body 
favours  the  passage  of  heat  through  it.  The  best  conductors  are  the  metals, 
and  then  follow  in  succession  diamond,  glass,  stones,  earths,  woods,  &c.,  as 
here  noted : 

Metals — silver,  copper,  gold,  iron,  lead. 

Diamond. 

Glass. 

Hard  stones. 

Porous  earths. 

Woods. 

Fats  or  thick  oils. 

Snow. 

Air 576 

Sewing  silk          -         -         -         917 

Wood  fishes  927 

Charcoal      ....         937        / 

Fine  lint    -        ...      1,032 

Cotton         ....      1,046 

Lamp  black         ...      1,117 

Wool          -        -        -        -      1,118- 

Raw  silk     ....      1,284 

Beavers'  fur  1,296 

Eiderdown          -         -         -      1,305 

Hares'  fur  -  -  1,315 

Air  appears  near  the  middle  of  the  preceding  list,  but  if  its  particles  are 
not  allowed  to  move  about  among  themselves,  so  as  to  carry  heat  from  one 
part  to  another,  it  conducts  (in  the  manner  of  solids)  so  slowly  that  Count 
Runiford  doubted  whether  it  conducted  at  all.  It  is  probably  the  worst  con- 
ductor known,  that  is,  the  substance  which  when  at  rest  impedes  the  passage 
of  heat  the  most.  To  this  fact  seem  to  be  owing,  in  a  considerable  degree, 
the  remarkable  non-conducting  quality  of  porous  or  spongy  substances,  as 
feathers,  loose  filamentuous  matter,  powders,  &c.,  which  have  much  .air  in 
their  structure,  often  adherent  with  the  force  of  attraction  which  immersion 
in  water,  or  even  being  placed  in  the  vacuum  of  an  air-pump,  is  insufficient 
to  overcome. 

While  contemplating  the  facts  recorded  in  the  above  table,  one  cannot  but 


262  *        HEAT. 

reflect  how  admirably  adapted  to  their  purposes  the  substances  are  which 
nature  has  provided  as  clothing  for  the  inferior  animals ; — and  which  man 
afterwards  accommodates  with  such  curious  art  to  his  peculiar  wants.  Ani- 
mals required  to  be  protected  against  the  chills  of  night  and  the  biting  blasts 
of  winter,  and  some  of  them  which  dwell  among  eternal  ice,  could  not  have 
lived  at  all,  but  for  a  garment  which  might  shut  up  within  it  nearly  all  the 
heat  which  their  vital  functions  produced.  Now  any  covering  of  a  metallic, 
or  earthy,  or  woody  nature,  would  have  been  far  from  sufficing ;  but  out  of  a 
wondrous  chemical  union  of  carbon  with  the  soft  ingredients  of  the  atmo- 
sphere, those  beautiful  textures  are  produced  called  fur  and  feather,  so  greatly 
adorning  while  they  completely  protect  the  wearers ; — textures,  moreover, 
which  grow  from  the  bodies  of  the  animals,  in*  the  exact  quantity  that  suits 
the  climate  and  season,  and  which  are  reproduced  when  by  any  accident  they 
are  partially  destroyed.  In  warm  climates  the  hairy  coat  of  quadrupeds  is 
comparatively  short  and  thin ;  as  in  the  elephant,  the  monkey,  the  tropical 
sheep,  &c.  It  is  seen  to  thicken  with  increasing  latitude,  furnishing  the  soft 
and  abundant  fleeces  of  the  temperate  zones ;  and  towards  the  poles  it  is 
externally  shaggy  and  coarse,  as  in  the  arctic  bear.  In  amphibious  animals, 
which  have  to  resist  the  cold  of  water  as  well  as  of  air,  the  furs  grow  particu- 
larly defensive,  as  in  the  otter  and  beaver.  Birds,  from  having  very  warm 
blood,  require  plenteous  clothing,  but  require  also  to  have  a  smooth  sur- 
face, that  they  may  pass  easily  through  the  air; — both  objects  are  secured 
by  the  beautiful  structure  of  feathers,  so  beautiful  and  wonderful  that  writers 
on  natural  theology  have  often  particularized  it  as  one  of  the  most  striking 
exemplifications'  of  creative  wisdom.  Feathers,  like  fur,  appear  in  kind  and 
quantity  suited  to  particular  climates  and  seasons.  The  birds  of  cold  regions 
have  covering  almost  as  bulky  as  their  bodies,  and  if  it  be  warm  in  those  of 
them  which  live  only  in  air,  in  the  water-fowl  it  is  warmer  still.  These 
last  have  the  interstices  of  the  ordinary  plumage  filled  up  by  the  still  more 
delicate  structure  called  down,  particularly  on  the  breast,  which  in  swim- 
ming first  meets  and  divides  the  cold  wave.  There  are  animals  with  warm 
blood  which  yet  live  very  constantly  immersed  in  water,  as  the  whale,  seal, 
walrus,  &c.  Now  neither  hair  nor  feathers,  however  oiled,  would  have 
been  a  fit  covering  for  them;  but  kind  nature  has  prepared  an  equal  protec- 
tion in  the  vast  mass  of  fat  or  thick  oil  which  surrounds  their  bodies — sub- 
stances which  are  scarcely  less  useful  to  man  than  the  furs  and  feathers  of 
land  animals. 

While  speaking  of  clothing  we  may  remark,  that  the  bark  of  trees  is  also 
a  structure  very  slowly  premeable  to  heat,  and  securing,  therefore,  the  tem- 
perature to  vegetable  life. 

And  while  we  admire  what  nature  has  thus  done  for  animals  and  vegetables, 
let  us  not  overlook  her  scarcely  less  remarkable  provision  of  ice  and  snow, 
as  winter  clothing  for  the  lakes  and  rivers,  for  our  fields  and  gardens.  Ice, 
as  a  protection  to  water  and  its  inhabitants,  was  considered  in  Sec.  I.,  in  the 
explanation  of  why,  although  solid,  it  swims  on  water.  We  have  now  to 
remark  that  snow,  which  becomes  as  a  pure  white  fleece  to  the  earth,  is  a 
structure  which  resists  the  passage  of  heat  nearly  as  much  as  feathers.  It, 
of  course,  can  defend  only  from  clouds  below  32°  or  the  freezing  point;  but 
it  does  so  most  effectually,  preserving  the  roots  and  seeds  and  tender  plants 
during  the  severity  of  winter.  When  the  green  blade  of  wheat  and  the  beau- 
tiful snow-drop  flower  appear  in  spring  rising  through  the  melting  snow,  they 
have  recently  owed  an  important  shelter  to  their  wintry  mantle.  Under  deep 
snow  while  the  thermometer  in  the  air  may  be  far  below  zero,  the  tempera- 


SPREADING    BY    CONDUCTION.  263 

ture  of  the  ground  is  rarely  below  the  freezing  point.  Now  this  temperature, 
to  persons  some  time  accustomed  to  it,  is  mild  and  even  agreeable.  It  is 
much  higher  than  what  often  prevails  for  long  periods  in  the  atmosphere  of. 
the  centre  and  north  of  Europe.  The  Laplander,  who  during  his  long  winter 
lives  under  ground  is  glad  to  have  additionally  over  head  a  thick  covering  of 
snow.  Ainorijfr  the  hills  of  the  west  and  norm  of  Britain,  during  the  storms 
of  winter,  a  house  or  covering  of  snow  frequently  preserves  the  lives  of  tra- 
vellers, and  even  of  whole  flocks  of  sheep,  when  the  keen  north  wind  catch- 
ing them  unprotected,  would  soon  stretch  them  lifeless  along  the  earth. 

It  is  because  earth  conducts  heat  slowly,  that  the  most  intense  frost  pene- 
trates but  a  few  inches  into  it,  and  that  the  temperature  of  the  ground  a  few 
feet  below  its  surface  is  nearly  the  same  all  the  world  over.  In  many  mines, 
even  although  open  to  the  air,  the  thermometer  does  not  vary  one  degree  in 
a  twelvemonth.  Thus  also  water  in  pipes  two  or  three  feet  under  ground 
does  not  freeze,  although  it  may  be  frozen  in  all  the  smaller  branches 
exposed  above.  Hence,  again,  springs  never  freeze,  and  therefore  become 
remarkable  features  in  a  snow  covered  country.  The  living  water  is  seen 
issuing  from  the  bowels  of  the  earth,  and  running  often  a  considerable  way 
through  fringes  of  green,  before  the  gripe  of  the  frosts  arrest  it;  while 
around  it,  as  is  well  known  to  the  sportsman,  the  snipes  and  wild  duck  and 
other  birds  are  wont  to  congregate.  A  spring  in  a  frozen  pond  or  lake  may 
cause  the  ice  to  be  so  thin  over  the  part  where  it  issues,  that  a  skater  arriving 
there  will  break  through  and  be  destroyed.  The  same  spring  water  which 
appears  warm  in  winter,  is  deemed  cold  in  summer,  because  although  always 
of  the  same  heat,  it  is  in  summer  surrounded  by  warm  atmosphere  and 
objects.  In  proportion  as  buildings  are  massive,  they  acquire  more  of  those 
qualities  which  have  now  been  noticed  of  our  mother  earth.  Many  of  the 
G-othic  halls  and  cathedrals  are  cool  in  summer  and  warm  in  winter — as  are 
also  old-fashioned  houses  or  castles  with  thick  walls  and  deep  cellars. 
Natural  caves  in  the  mountains  or  sea-shores  furnish  other 'examples  of  a 
similar  kind. 

When  in  the  arts  it  is  desired  to  prevent  the  passage  of  heat  out  of,  or.  into 
any  body  or  situation,  a  screen  or  covering  of  a  slow  conducting  substance 
is  employed.  Thus  to  prevent  the  heat  of  a  smelting  or  other  furnace  from, 
being  wasted,  it  is  lined  with  fire  bricks,  or  is  covered  with  clay  and  sand,  or 
sometimes  with  powdered  charcoal.  A  furnace  so  guarded  may  be  touched 
by  the  hand,  even  while  containing  within  the  melted  gold.  .  To  prevent  the 
freezing  of  water  in  pipes  during  the  winter,  by  which  occurrence  the  pipes 
would  be  burst,  it  is  common  to  cover  them,  with  straw  ropes,  or  coarse  flan- 
nel, or  enclose  them  in  a  larger  outer  pipe  with  dry  charcoal,  or  saw  dust, 
or  chaff,  filling  up  the  interval  between.  If  a  pipe  on  the  contrary,  be  for 
the  conveyance  of  steam  or  other  warm  fluid,  the  heat  is  retained,  and,  there- 
fore, saved  by  the  very  same  means.  Ice-houses  are  generally  made  with 
double  walls,  between  which,  dry  straw  placed,  or  saw  dust,  or  air,  prevents 
the  passage  of  heat.  Pails  for  carrying  ice  in  summer,  or  intended  to  serve 
as  wine  coolers,  are  made  on  the  same  principle — viz.,  double  vessels,  with 
air  or  charcoal,  filling  the  interval  between  them.  A  flannel  covering  keeps 
a  man  warm  in  winter — it  is  also  the  best  means  of  keeping  ice  from  melting 
in  summer.  Urns  for  hot  water,  tea  pots,  coffee  pots,  &c.,  are  made  with 
wooden  or  ivory  handles,  because  if  metal  were  used,  it  would  conduct  the 
heat  so  readily  that  the  hand  could  not  bear  to  touch  them. 

It  is  because  glass  and  earthenware  are  brittle,  and  do  not  allow  ready  pas- 
sage to  heat,  that  vessels  made  of  them  are  so  frequently  broken  by  sudden 


261  HEAT. 

change  of  temperature.  On  pouring  boiling  water  into  such  a  vessel,  the 
internal  part  is  much  heated  and  expanded  (as  will  be  explained  more  fully 
.in  a  subsequent  page  )  before  the  external  part  fcas  feit  the  influence,  and 
fiis  is  hence  riven  or  cracked  by  its  connection  with  the  internal.  A  chimney 
mirror  is  often  broken  by  a  lamp  or  candle  placed  on  the  marble  shelf  too 
near  it.  The  glass  cylinder  of  an  electrical  machine  will  sometimes  be 
broken  by  placing  it  near  the  fire,  so  that  one  side  is  heated  while  the  othor 
side  receives  a  cold  current  of  aii»  approaching  the  fire  from  a  door  or  window. 
A  red  hot  rod  of  iron  drawn  along  the  pane  of  glass  will  divide  it  almost 
like  a  diamond  knife.  Even  cast  iron,  or  the  backs  of  grates,  iron  pots,  &c., 
although  conducting  readily,  is  often,  owing  to  its  brittleness,  cracked  by 
unequal  heating  or  cooling,  as  from  pouring  water  on  it  when  hot.  Pouring 
cold  water  into  a  heated  glass  will  produce  a  similar  effect.  Hence  glass 
vessels  intended  to  be  exposed  to  strong  heats  and  sudden  changes,  as  retorts 
for  distillation,  flasks  for  boiling  liquids,  &c.,  are  made  very  thin;  that  the 
heat  may  pervade  them  almost  instantly  and  with  impunity. 

There  is  a  toy  called  a  Prince  Rupert's  Drop,  which  well  illustrates  our 
present  subject.  It  is  a  lump  of  glass  let  fall  while  fused  into  water,  and 
thereby  suddenly  cooled  and  solidified  on  the  outside  before  the  internal  part 
is  changed ;  then  as  this  at  last  hardens  and  would  contract,  it  is  kept 
extended  by  the  arch  of  external  crust,  to  which  it  coheres.  Now  if  a  por- 
tion of  the  neck  of  the  lump  be  broken  off,  or  if  other  violence  be  done, 
which  jars  its  substance,  the  cohesion  is  destroyed,  and  the  whole  crumbles 
to  dust  with  a  kind  of  explosion.  Any  glass  cooled  suddenly  when  first 
made,  remains  very  brittle,  for  the  reason  now  stated.  What  is  called  the 
Bologna  jar  is  a  very  thick  small  bottle,  thus  prepared,  which  bursts  by  a 
grain  of  sand  falling  into  it.  The  process  of  annealing,  to  render  glass  ware 
more  tough  and  durable,  is  merely  the  allowing  it  to  cool  very  slowly  by 
placing  it  in  an  oven,  where  the  temperature  is  caused  to  fall  gradually. 
The  tempering  of  metals  by  sudden  cooling  seems  to  be  a  process  having 
some  relation  to  that  of  rendering  glass  hard  and  brittle. 

It  is  the  difference  of  conducting  power  in  bodies  which  is  the  cause  of  a 
very  common  error  made  by  persons  in  estimating  the  temperature  of  bodies 
by  the  touch.  In  a  room  without  a  fire  all  the  articles  of  furniture  soon 
acquired  the  same  temperature;  but  if  in  winter,  a  person  with  bare  feet  were 
to  step  from  the  carpet  to  the  wooden  floor,  from  this  to  the  hearth-stone,  and 
from  the  stone  to  the  steel  fender,  his  sensation  would  deem  each  of  these 
in  succession  colder  than  the  preceding.  Now  the  truth  being  that  all  had 
the  same  temperature,  only  a  temperature  inferior  to  that  of  the  living  body, 
the  best  conductor,  when  in  contact  with  a  body,  would  carry  off  heat  the 
•fastest,  and  would,  therefore,  be  deemed  the  coldest.  Were  a  similar  expe- 
riment made  in  a  hot-house,  or  in  India,  while  the  temperature  of  every  thing 
around  were  98°,  viz.,  that  of  the  living  body,  then  not  the  slightest  difference 
would  be  left  in  any  of  the  substances  :  or  lastly,  were  the  experiment  made 
in  a  room  where  by  any  means  the  general  temperature  were  raised  consider- 
ably above  blood  heat,  then  the  carpet  would  be  desrned  considerably  the 
coolest  instead  of  the  warmest,  and  the  other  things  would  appear  hotter  in 
the  same  order  in  which  they  appeared  colder  in  the  winter  room.  Were  a 
bunch  of  wool  and  a  piece  of  iron  exposed  to  the  severest  cold  of  Siberia,  or 
of  an  artificial  frigorific  mixture,  a  man  might  touch  the  first  with  impunity, 
(it  would  merely  be  felt  as  rather  cold;)  but  if  he  grasped  the  second,  his 
hand  would  be  frost-bitten  and  possibly  destroyed ;  were  the  two  substances, 
on  the  contrary,  transferred  to  an  oven;  and  heated  as  far  as  the  wool  would 


SPREADING    BY    CONDUCTION.  265 

bear,  he  might  again  touch  the  wool  with  impunity  (it  would  then  be  felt  as 
a  little  hot,)  but  the  iron  would  burn  his  flesh.  The  author  has  entered  a 
room  where  there  was  no  fire,  but  where  the  temperature  from  hot  air 
admitted  was  sufficiently  high  to  boil  the  fish,  &c.,  of  which  he  afterwards 
partook  at  dinner ;  and  he  breathed  the  air  with  very  little  uneasiness. 
He  could  bear  to  touch  woolen  cloth  in  this  room,  but  no  body  more  solid. 

The  foregoing  considerations  make  manifest  the  error  of  supposing  that 
there  is  a  positive  warmth  in  the  materials  of  clothing.  The  thick  cloak 
which  guards  a  Spaniard  against  the  cold  of  winter,  is  also  in  summer  used 
by  him  as  a  protection  against  the  direct  rays  of  the  sun  : — and  while  in 
England,  flannel  is  our  warmest  article  of  dress,  yet  we  cannot  'more 
effectually  preserve  ice  than  by  wrapping  the  vessel  containing  it  in  many 
folds  of  softest  flannel. 

In  every  case  where  a  substance  of  different  temperature  from  the  living 
body  touches  it,  a  thin  surface  of  the  substance  immediately  shares  the  heat 
of  the  bodily  part  touched — the  hand  generally  ;  and  while  in  a  good  con- 
ductor, the  heat  so  received  quickly  passes  inwards,  or  away  from  the  surface, 
leaving  this  in  a  state  to  absorb  more,  in  the  tardy  conductor  the  heat  first 
received  tarries  at  the  surface,  which  consequently  soon  acquires  nearly  the 
same  temperature  as  the  hand,  and  therefore,  however  cold  the  interior  of  the 
substance  may  be,  it  does  not  cause  the  sensation  of  cold..  The  hand  on  a 
good  conductor  has  to  warm  it  deeply,  a  slow  conductor  it  warms  only  super- 
ficially. The  following  cases  further  illustrate  the  same  principle.  If  the 
ends  of  an  iron  poker,  and  of  a  piece  of  wood  of  the  same  size,  be  wrapped  in 
paper  and  then  thrust  into  a  fire,  the  paper  on  the  wood  will  begin  to  burn 
immediately,  while  that  on  the  metal  will  long  resist : — or  if  pieces  of  paper 
be  laid  on  a  wooden  plank  and  on  a  plate  of  steel,  and  then  a  burning  coal 
be  placed  on  each,  the  paper  on  the  wood  will  begin  to  burn  long  before  that 
on  the  plate.  The  explanation  is,  that  the  paper  in  contact  with  the  good 
conductor  loses  to  this  so  rapidly  the  heat  received  from  the  coal,  that  it 
remains  at  too  Iowa  temperature  to  inflame,  and  will  even  cooi  to  blackness 
the  touching  part  of  the  coal;  while  on  the  tardy  conductor  the  paper  becomes 
almost  immediately  as  hot  as  the  coal.  It  is  because  water  exposed  to  air 
cannot  be  heated  beyond  212°,  that  it  may  be  made  to  boil  in  an  egg-shell  or 
a  vessel  made  of  paper,  held  over  a  lamp,  without  the  containing  substance 
being  destroyed;  but  as  soon  as  it  is  dried  up,  the  paper  will  burn  and  the 
shell  will  be  calcined,  as  the  solder  of  a  common  tin  kettle  melts  under  the 
same  circumstances.  The  reason  why  the  hand  judges  a  cold  liquid  to  be  so 
much  colder  than  a  solid  of  the  same  temperature  is,  that  from  the  mobility 
of  the  liquid  particles  among  themselves,  those  in  contact  with  the  hand  are 
constantly  changing.  The  impression  produced  on  the  hand  by  very  cold 
meruairy  is  almost  insufferable,  because  mercury  is  both  a  ready  conductor  and 
a  liquid.  Again,  if  a  finger  held  motionless  in  water  feel  cold,  it  will  feel 
colder  still  when  moved  about;  and  a  man  in  the  air  o'f  a  calm  frosty  morning 
does  not  experience  a  sensation  nearly  so  sharp  as  if  with  the  same  tempera- 
ture there  be  wind.  A  finger  held  up  in  the  wind  discovers  the  direction  in 
which  the  wind  blows*by  the  greater  cold  felt  on  one  side,  the  effect  being 
still  more  remarkable  if  the  finger  is  wetted.  If  a  person  in  a  room  with  a 
thermometer,  were  with  a  fan  or  bellows  to  blow  the  air  against  it,  he  would  not 
thereby  lower  it,  because  it  had  already  the  same  temperature  as  the  air,  yet 
the  air  blown  against  his  own  body  would  appear  colder  than  when  at  rest, 
because  -being  colder  than  his  body,  the  motion  would  supply  heat-absorbing 
particles  more  quickly.  In  like  manner,  if  a  fan  or  bellows  were  used  against 


266  HEAT. 

a  thermometer  hanging  in  a  furnace  or  hot-house,  the  thermometer  would 
suffer  no  change,  but  the  air  moved  by  them  against  a  person,  would  be 
distressingly  hot,  like  the  blasting  sirocco  of  the  sandy  deserts  of  Africa.  If 
two  similar  pieces  of  ice  be  placed  in  a  room  somewhat  warmer  than  ice,  one 
of  them  may  be  made  to  melt  much  sooner  than  the  other,  by  blowing  on  it 
with  a  bellows.  The  reason  may  here  be  readily  comprehended  why  a  person 
suffering  what  is  called  a  cold  in  the  head,  or  catarrh  from  the  eyes  and  nose, 
experiences  so  much  more  relief  on  applying  to  the  face  a  handkerchief  of 
linen  or  cambric  than  one  of  cotton  : — it  is,  that  the  former,  by  conducting, 
readily  absorbs  the  heat  and  diminishes  the  inflammation,  while  the  latter, 
by  refusing  to  give  passage  to  the  heat,  increases  the  temperature  and  the 
distress.  Popular  prejudice  has  held  that  there  was  a  poison  in  cotton. 

"  Heats  spreading  in  fluids  chiefly  ly  the  motion  of  their  par  ticks." 
(Read  the  Analysis,  page  256.) 

Owing  to  the  mobility  among  themselves  of  fluid  particles,  heat  entering 
a  fluid  anywhere  below  the  surface,  by  dilating  and  rendering  specially  lighter 
the  portion  heated,  allows  the  denser  fluid  around  to  sink  down  and  force  up 
the  rarer;  and  the  continued  currents  so  established,  diffuse  the  heat  through 
the  mass  much  more  quickly  than  heat  spreads  by  conduction  in  any  solid. 

Count  Rumford's  experiments  led  him  at  first  to  conclude  that  liquids, 
but  for  this  carrying  process,  by  the  particles  changing  their  place,  were 
absolutely  impassable  to  heat.  A  piece  of  ice  will  lie  very  long  at  the  bot- 
tom of  water  which  is  made  to  boil  at  the  top  by  the  contact  of  any  hot 
body  y  and  when  it  at  last  melts,  Count  R.  believed  that  it  did  so  entirely 
from  the  heat  which  passed  downwards  through  the  sides  of  the  vessels 
containing  the  water.  But  an  ingenious  experiment  by  Dr.  Murray  decided 
the  question  differently.  He  made  a  vessel  of  ice,  which,  ef  course,  could 
Dot  carry  downwards  any  heat  greater  than  32°,  as  ice  melts  at  that  degree  ; 
and  having  put  into  the  vessel  a  quantity  of  oil  at  32°,  with  the  bulb  of  a 
thermometer  being  a  quarter  of  an  inch  under  the  surface  of  the  oil,  he 
placed  a  cup  of  boiling  water  in  contact  with "  the  surface  of  the  oil : — in  a 
minute  and  a  half-the  thermometer  rose  nearly  a  degree,  ancf  in  seven  minutes 
it  rose  five  degrees,  beyond  which  it  did  not  go.  The  heat  then  must  have 
passed  downwards  through  the  liquid,  proving  a  conducting  power ; — unless, 
indeed,  it  passed  by  radiation,  as  explained  in  a  subsequent  page. 

The  internal  currents  or  circulation  produced  by  heat  in  fluid  masses,  and 
of  which  there  are  so  many  important  instances  in  nature,  were  more  fitly 
explained  in  the  chapter  on  Hydrostatic*  and  Pneumatics ;  we  shall  here, 
therefore,  allude  to  them  very  shortly. 

Perhaps  the  best  experimental  illustration  of  the  subject  is  obtained  by 

lacing  a  tall  glass  jar,  filled  with  water,  in  which  small  pieces  of  amber  are 

iffused  to  show  its  movements,  first  in  a  warm   bath,  and  then  in  a  cold 

bath.     In  the  first  case,  the  water  and  amber  near  the  outside  of  the  jar 

where  they  are  heated,  will  exhibit  a  rapid  upward  current,  while  in  the 

centre  of  the  jar  they  will  form  an  opposite  and  downward  current.     In  the 

second  case,  or  when  the  jar  is  placed  in  a  cold  bath,  the  direction  of  the 

currents  will  be  reversed. 

Consideration  of  these  currents  led  the  author  of  this  work,  some  years 
ago,  to  propose  what  he  deemed  a  great  improvement  on  the  construction  and 
management  of  boilers  and  evaporating  pans  generally;  namely,  to  convert 
the  upward  and  downward  current  in  the  mass  of  boiling  liquid  into  a  lateral 


SPREADING    BY    CONDUCTION.  267 

current  below,  constantly  and  rapidly  sweeping  the  bottom  of  tlie  vessel. 
In  ordinary  boilers,  when  a  portion  of  liquid  is  converted  into  steam  in  con- 
tact with  the  horizontal  bottom,  it  does  not  separate  from  the  bottom  im- 
mediately, but  remains  until  a  steam-bubble  of  considerable  size  be  formed, 
and  in  the  mean  time  the  part  of  the  boiler  defended  by  it  from  the  con- 
tact of  the  liquid,  becomes  overheated,  and  the  following  evil  consequences 
ensue: — 1.  Rapid  destruction  of  the  boiler,  and  on  that  account  a  rapid 
expense;  2.  Necessity  for  having  originally  much  thicker,  and  therefore 
dearer  boilers ;  3.  The  thickness  being  an  impediment  to  the  passage  of 
heat,  there  is  a  proportionate  waste  of  fuel;  4.  and  last,  When  the  liquid  is 
a  vegetable  juice  or  extract,  as  sugar-cane  juice,  of  a  nature  to  be  carbonized 
and  blackened  when  overheated,  the  quality  of  the  product  is  often  exceed- 
ingly deteriorated.  I'he  patentees  of  an  apparatus,  described  at  184,  for 
boiling  sugar  in  vacuo,  and  therefore  at  a  low  and  steady  temperature, 
gained,  it  was  said,  more  than  $40,000  a  year  by  preventing  the  injury  now 
spoken  of.  And  when  the  liquid  is  a  saline  solution,  like  the  sea-water 
used  in  steam  ships,  the  salt  soon  encrusts  the  bottom  of  the  boiler;  and 
powerfully  both  prevents  the  passage  of  heat  and  destroys  the  boiler.  Now 
a  current  sweeping  the  bottom  prevents  all  these  consequences,  and  may  be 
easily  obtained.  The  most  obvious  method  is,  to  place  in  the  liquid  some 
upright  tubes  with  open  tops  at  a  certain  distance  under  the  surface  of  the 
liquid,  and  with  the  bottoms  also  open,  but  latterly,  and  all  in  one  direc- 
tion ;  the  consequence  will  be,  that  as  soon  as  the  liquid  begins  to  boil,  the 
general  mass,  consisting  of  liquid  mixed  with  bubbles  of  steam,  becomes  of 
considerably  less  specific,  gravity  than  the  liquid  in  the  tubes,  remaining 
unmixed,  because  steam  will  not  enter  the  lateral  mouths  of  the  tubes,  and 
the  columns  of  heavier  liquid  will  therefore  descend  rapidly  and  issuing  by 
the  lateral  openings  of  the  tubes  all  in  one  direction  along  the  bottom  of  the 
boiler,  will  powerfully  and  uninterruptedly  sweep  it.  In  a  long  wagon- 
shaped  boiler  the  tubes,  instead  of  being  round,  should  be  made  flat  and 
broad  enough  to  reach  from  side  to  side  ;  and  if  a  very  rapid  current  be  de- 
sired, they  must  be  made  larger  than  the  spaces  between  them  in  which  the 
steam  has  to  rise,  for  thus  the  steam  bubbles,  being  driven  closer  together, 
will  make  the  rising  column  so  much  the  lighter,  and  its  ascent  consequently 
the  more  rapid ;%  and  there  will  be  a  corresponding  rapidity  of  issue  of  the 
sweepmg  current.  In  a  moderate  sized  pan  or  boiler  of  the  usual  basin  or 
half-globe  shape,  the  simplest  method  of  producing  the  current  is  to  have  a 
smaller  vessel  of  similar  shape  made  of  thin  metal,  and  placed  within  the 
other  so  as  to  have  about  an  inch  space  all  round  between  them,  and  having 
one  large  opening  at  its  bottom, — then  all  the  steam  mixed  with  fluid  will 
rise  between  the  outer  and  inner  vessel,  while  the  unmixed  liquid  will 
descend  through  the  open  bottom  of  the  inner  vessel,  and  spread  in  every 
direction,  sweeping  the  bottom  of  the  outer.  The  sweeping  of  the  bottom 
of  a  boiler  might  also  be  effected  by  a  wheel  kept  turning,  to  cause  the 
liquid  to  resolve  horizontally  (  as  was  done  with  another  view  in  the  large 
Scotch  whiskey  stills,)  or  by  a  frame  made  somewhat  like  a  rake  or  gridiron, 
kept  moving  backwards  and  forwards  upon  the  bottom. 

As  stated  in  a  previous  section,  it  is  the  heating  and  dilatation  of  the  fluid 
air  over  a  tropical  island  while  acted  upon  during  the  middle  of  the  day  by' 
the  powerful  rays  of  the  sun,  which  allow  the  colder  and  heavier  air  from 
the  face  of  the  ocean  around  to  press  inwards  upon  it  and  force  it  upwards 
in  the  atmosphere — the  cold  current  forming  the  delightful  sea-breeze  of 
the  climate.  And  it  is  the  general  heating  of  the  air  over  the  whole  equa- 


268  HEAT. 

torial  belt  of  the  earth,  which,  rendering  it  specifically  lighter  than  the  air 
nearer  the  poles,  allows  this  to  assume  the  form  of  cool  trade-winds,  con- 
stantly-blowing towards  the  sun's  path,  and  pressing  upwards  the  hot  air, 
which  then  spreads  away  on  the  top  of  the  atmosphere  towards  the  poles,  to 
mitigate  the  severity  of  the  northern  and  southern  cold.  In  the  watery 
ocean  also  there  is  a  circulatory  motion  of  the  same  kind,  although  less  in 
degree,  tending  to  distribute  heat  and  equalize  temperature,  and  contributing 
to  produce  some  of  the  great  sea  currents  known  to  mariners. 

The  vertical  currents  produced  by  heat,  in  the  ocean  and  in-great  masses 
of  water  generally,  preserve  in  and  over  them  a  comparatively  uniform  tem- 
perate freshness,  while  the  rocks  and  soil  on  the  shores  around  may  be 
either  parched  under  a  burning  sun,  or  bound  up  in  frost.  A  keen  frost 
chills,  and  soon  hardens  in  its  icy  grasp  the  surface  of  the  ground  j  but  of 
water  similarly  exposed,  the  part  first  cooled  descends  to  the  bottom  by  its 
increased  density,  and  forces  up  a  warmer  water  to  take  its  place ;  this  in  its 
turn  is  cooled  and  descends,  and  a  continued  circulation  is  established,  so 
that  the  surface  cannot  become  ice  until  the  whole  mass,  of  whatever  depth, 
has  been  cooled  down  to  its  greatest  density.  Hence  the  very  deep  sea  is 
not  frozen  even  in  the  coldest  climates,  and  in  temperate  climates,  the 
severest  winter  does  not  freeze  even  the  ordinary  lakes.  During  this  intes- 
tine movement  in  the  water,  that  which  ascends  to  the  surface  to  be  cooled, 
by  losing  one  degree  of  its  heat,  warms  more  than  500  times  its  bulk  of  air 
one  degree,  and  thus  tempers  remarkably  the  air  passing  over  it.  Hence 
places  in  the  vicinity  of  the  sea  and  of  lakes  are  warmer  in  winter  than 
places  further  inland,  although  nearer  to  the  equator.  England  is  much 
warmer  in  winter  than  central  Germany,  which  lies  south  of  England  ;  and 
the  coast  of  Scotland  and  of  the  north  of  Ireland  are  warmer  than  Lon- 
don : — snow  never  lies  long  upon  these  coasts.  As  continental  or  inland 
countries  have  thus  in  winter  an  extreme  of  cold,  so  they  have  in  summer 
an  extreme  of  heat.  Water  admits  the  rays  of  the  sun,  and  absorbs  the 
heat  into  the  whole  thickness  of  its  mass,  and  therefore  is  warmed  very 
slowly ;  but  the  dry  earth  retains  all  the  heat  near  its  surface,  and  is  there- 
fore soon  heated  to  excess. 

The  ventilation  of  our  dwellings  and  halls  of  assembly  (as  explained  pre- 
viously) is  owing  to  the  motion  produced  by  the  changed  specific  gravity  of 
air  when  heated.  The  air  which  is  within  the  house  becomes  warmer  than 
the  external  air,  and  the  latter  then  presses  in  at  every  opening  or  crevice 
to  displace  the  other.  The  ventilation  of  the  person  by  the  slow  passage  of 
air  through  the  texture  of  our  clothing  is  a  phenomenon  of  the  same  kind; 
and  thicker  clothing  acts  chiefly  by  diminishing  the  rapidity  of  this  passage. 
Hence  an  oiled-silk  or  other  air-tight  covering  laid  on  a  bed  has  greater  in- 
fluence in  preserving  warmth  than  one  or  two  additional  blankets,  and  is 
not  generally  used,  only  because  it  prevents  ventilation,  and  by  shutting  in 
the  insensible  perspiration,  soon  produces  dampness.  From  the  part  of  bed- 
clothes immediately  over  the  person  there  is  a  constant  outward  oozing  of 
warm  air,  and  there  is  an  oozing  inward  of  cold  air  in  lower  situations 
around.  In  many  persons  the  circulation  of  the  blood  is  so  feeble  that  in 
.winter,  they  have  great  difficulty  in  keeping  their  feet  warm,  even  in  bed, 
unless  with  the  assistance  of  a  bottle  of  hot  water  or  some  such  means,  and 
in  consequence  they  often  pass  sleepless  nights,  and  suffer  in  their  general 
health.  In  such  cases,  at  the  suggestion  of  the  author,  a  long  flexible  tube 
has  been  used, — as  of  spiral  wire,  covered  with  leather  or  varnished  cloth, 
by  which  a  person  can  send  down  to  his  feet  his  hot  breath,  and  thus  apply 


SPREADING    BY    CONDUCTION.  269 

them  effectually  a  natural  animal  warmth,  as  in  a  cold  day  he  does  to  his 
hands  by  blowing  upon  them  through  his  gloves. 

The  power  of  fluids  to  diffuse  heat  being  due  to  their  power  of  carrying r, 
and  not  of  conducting  it,  the  consequence  should  follow,  that  any  circum- 
stance which  impedes  the  internal  motion  of  the  fluid  particles,  should 
diminish  the  suffusing  power.  Accordingly  we  find,  that  fluids  in  general 
transfer  heat  less  readily  in  proportion  as  they  are  more  viscid.  Water,  for 
instance,  transfers  less  quickly  than  spirits  ;  oil  than  water;  molasses  or  syrup 
than  oil ;  and  water  thickened  by  starch  dissolved  in  it,  or  which  has  its 
internal  motion  impeded  by  feathers  or  thread  immersed  in  it,  less  quickly 
than  where  it  is  pure  and  at  liberty.  Cooling  being  merely  a  motion  the 
reverse  of  heating,  it  is  influenced  by  the  same  law.  Hence  the  reason  why 
thick  soups,  pies,  puddings,  and  all  semifluid  masses,  retain  their  heat  so 
long — so  much  longer  than  equal  bulks  of  mere  fluid.  The  same  law  affords 
explanation  of  the  facts,  that  very  porus  masses  and  powders,  as  charcoal, 
metal  filings,  saw  dust,  sand,  &c.,  conduct  heat  more  slowly  than  denser 
masses, — their  interstices  being  filled  with  air,  which  scarcely  conducts  heat, 
and  which,  by  the  structure  of  the  substance,  has  no  freedom  of  motion  or 
circulation  by  which  it  might  carry  the  heat. 

"Heat  spreads,  also,  partly  ~by  being  radiated  or  shot  light  from  one  Ioc7y 
to  another,  through  transparent  media  or  space  with  readiness  affected  by 
the  material  and  the  state  of  the  giving  and  receiving  surfaces.''  (Read 
the  Analysis,  page  256.) 

If  a  heated  ball  of  metal  be  suspended  in  the  air,  a  hand  brought  in  any- 
direct  ion  near  to  it,  will  experience  the  sensation  of  heat ;  and  beneath  it  the 
sensation  will  be  as  strong  as  on  the  sides,  although  the  heat  has  to  shoot 
down  through  an  opposing  current  of  air  approaching  the  heated  ball,  to  rise 
from  it,  as  explained  in  a  previous  section.  A  delicate  thermometer  substi- 
tuted for  the  hand,  will  equally  detect  the  spreading  heat,  and  if  held  at 
different  distances,  will  prove  it  to  diminish  in  the  same  ratio  as  light  di- 
minishesjn  spreading  from  any  luminious  centre,  viz.,  to  be  only  a  fourth 
part  as  intense  at  a  double  distance,  in  a  corresponding  proportion  for  other 
distances.  If  the  heated  body  be  enclosed  in  a  vacuum,  a  thermometer 
placed  near  it  will  still  be  affected  in  the  same  manner.  If  a  screen  be  inter- 
posed between  the  body  and  the  thermometer,  the  latter  will  not  be  affected 
at  all,  proving  the  heat  to  spread  in  straight  lines.  Heat,  when  diffusing 
itself  in  this  way,  to  distinguish  it  from  heat  passing  by  contact  and  com- 
munication, as  described  in  the  last  section,  is  called  radiant  heat;  that  is 
to  say,  spreading  in  radii  or  rays  all  around  its  source  as  light  spreads. 

Radiant  heat  resembles  light  yet  in  other  respects.  It  as  rapidly  perme- 
ates certain  transparent  substances,  and  its  course  suffers  in  them  a  degree 
of  the  bending,  termed  refraction  by '  opticians.  It  is  reflected  from  many 
kinds  of  polished  surfaces,  just*  as  light  is  reflected  from  a  common  mirror; 
and  many  such  surfaces  directed  to  one  point  or  centre  (as  when  Archimedes 
made  the  sun  his  assistant  to  burn  the  Roman  ships)  or  a  single  concave 
surface,  having  its  own  centre  or  focus,  will  concentrate  heat  just  as  light. 
Its  motion  in  the  sun-beam  is  so  rapid,  as  for  any  distance  at  which  men  can 
try  the  experiment,  to  appear  instantaneous;  and  the  rays  of  heat  from  hot 
iron  or  burning  charcoal  concentrated  at  great  distances  by  suitable  mirrors, 
affect  a  thermometer  as  quickly  as  the  heat  of  the  sun  similarly  reflected. 
Although  light  and  heat  are  united  in  the  sun's  ray,  they  are  still  separable 


270  HEAT. 

by  our  glass  prisims  or  lenses;  and  the  focus  of  heat  behind  a  burning  glass 
is  not  precisely  the  focus  of  light.  Heat,  in  radiating  through  air,  does  not 
warm  the  air,  and  its  passage  is  not  sensibly  affected  by  winds  or  any  other 
motion  of  the  air. — These  resemblances  in  the  phenomena  of  light  and  heat 
have  by  some  inquirers  been  held  to  prove  that  the  two  classes  of  appear- 
ances are  only  different  modifications  of  action  in  the  same  subtile  substance 
or  either, 

The  diffusion  of  heat  by  radiation,  as  it  takes  place  in  an  instant  to  any 
distance,  and  begins  whenever  there  is  any  inequality  of  temperature  between 
bodies  exposed  to  each  other,  would  produce  instant  balance  of  temperature 
throughout  nature,  but  that  heat  leaves  and  enters  bodies  with  readiness, 
depending  on  the  condition  of  their  surfaces,  and  on  their  internal  conduct- 
ing powers.  A  black  stone-ware  ter-pot,  for  instance,  will  radiate  away  100 
degrees  of  its  heat  in  the  same  time  that  a  pot  of  polished  metal  will  radiate 
only  12  degrees. 

Professor  Leslie  was  the  first  to  see  the  importance  of  investigating  this 
subject,  and  he  had  the  merit  of  contriving  well-adapted  means,  and  of  de- 
tecting many  of  the  important  facts.  As  common  thermometers  are  not  suffi- 
ciently delicate  to  determine  very  sudden  changes  of  temperature,  where  the 
influence  is  so  light  as  in  many  cases  of  radiant  heat,  he  used  the  beautiful 
differential  thermometer  contrived  by  himself,  in  conjunction  with  concave 
mirrors,  (as  represented  on  next  page,)  to  concentrate  the  heat,  and  accumu- 
late its  energy.  Then  taking  as  his  heated  body  a  cubical  tin  vessel  filled 
with  boiling  water,  and  covering  it  successively  with  plates  or  layers  of  dif- 
ferent substances  and  with  different  colors,  and  exposing  the  thermometer 
to  it  for  a  given  time  under  all  the  changes,  he  noted  the  number  of  degrees 
which  the  thermometer  rose,  (as  seen  in  the  table  which  here  follows)  and 
thus  ascertained  the  radiating  power  of  each  sort  of  covering. 

Lamp  black  .         .         .         .         .         .         .         100° 

Writing  paper  .         .         .         .         .         .         .         .98 

Crown  glass  .         .         .         .         .         .         .  90 

Ice .87 

Isinglass        ........  75 

Tarnished  lead 45 

Clean  lead 19 

Iron  polished    .         .         .         .         .         .         .         .15 

Tin  plate 12 

Gold,  silver  and  copper 12 

He  next  reversed  the  experiments  by  using  his  hot-water  vessel  always  in 
the  same  state,  and  covering  the  thermometer  bulb  with  the  different  sub- 
stances and  colours,  and  thus  he  ascertained  that  the  comparative  absorbing 
powers  of  the  substances  and  colours  were  very  nearly  proportioned  to  their 
radiating  powers :  lamp-black,  for  instance,-  absorbed  or  was  heated  100°, 
while  the  polished  metals  absorbed  or  were  heated  only  12°,  and  so  for  the 
others.  And,  lastly,  the  absorbing  powers  being  an  indication  of  the  oppo- 
site or  reflecting  powers  (for  a  body  absorbing  only  a  given  proportion  of 
the  heat  which  falls  on  it,  must  reflect  the  remainder,)  he,  by  the  same 
experiments,  ascertained  the  radiating,  absorbing,  and  reflective  or  mirror 
powers  of  the  bodies,  and  therefore  all  the  important  points  respecting  radiant 
heat  in  its  relation  to  them. 

It  seems  paradoxical  that  the  putting  a  clothing  of  a  thin  cotton  or  woolen 
fabric  upon  the  polished  tin  vessel,  should  cause  the  heat  to  be  received  by 


SPREADING    BY    RADIATION.  271 

it  or  dissipated  from  it  much  sooner  than  if  the  vessel  were  naked,  but  such 
is  the  fact.  And  metal  with  a  scratched  or  roughened  surface  radiates  or 
receives  much  more  rapidly  than  polished  metal. 

The  property  of  absorbing  heat  depends  much  upon  the  colour  of  the 
substance,  and,  as  a  general  rule,  the  dark  colours,  viz,,  those  which  absorb 
most  light,  absorb  also  most  heat.  Dr.  Franklin  proved  this  by  laying 
pieces  of  cloth  of  different  colours  on  snow,  and  exposing  them  during  a 
given  period  to  the  sun's  rays  :  while  he  noted  the  different  depths  to  which 
by  the  melting  of  the  snow  under  them,  the  pieces  sank.  Hence  comes  the 
importance  of  having  a  white  dress  in  summer,  that  by  it,  with  the  sun's 
light,  the  heat  also  may  be  repelled.  And  a  white  dress  in  winter  is  good 
because  it  radiates  little.  Polar  animals  have  generally  white  furs.  White 
horses  ars  both  less  heated  in  the  sun,  and  less  chilled  in  winter,  than  those 
of  darker  hues. 

The  rate  of  cooling  in  heated  bodies  must  be  influenced  by  all  the  particu- 
lars noted  above  viz.,  substance,  surface,  colour,  and  by  the  excess  of  heat 
in  the  cooling  body  as  compared  with  those  around  it. 

The  concentrating  apparatus  used  for  experiments  on  the  radiation  of 
heat  consists  of  two  concave  tin  mirrors,  here  represented  at  a  and  b,  so 
formed  and  placed  in  relation  to  each  other  that  all  the  rays  of  light  or  heat 
issuing  from  the  focus  of  one,  as  at  c,  shall,  after  a  double  reflection,  be 
collected  in  the  focus  of  the  other,  d.  A  stand  under  one  focus  c  is  intended 

Fig.  124. 

£ 

^ 


el 

R'ff^iUJV) 

IT,  x  M — LM 


to  support  the  body  giving  out  or  receiving  heat,  and  a  stand  under  the 
other  d  is  meant  to  support  the  thermometer.  For  farther  explanation  of 
the  action  of  such  mirrors,  we  may  refer  to  what  was  said  of  the  concen- 
tration of  sound  in  the  section  on  Acoustics^  or  to  what  follows  in  the  sec- 
tion on  Optics,  on  the  concentration  of  light.  The  general  rationale  of  such 
facts  is,  that  heat,  light,  sound,  and  elastic  ball,  &c.,  when  reflected  from  any 
point  of  a  surface,  returns,  if  it  fall  perpendicularly  to  that  point,  in  the 
same  line  by  which  it  approached ;  but  if  it  fall  obliquely,  or  from  one  side 
of  the  perpenpicular,  it  returns  in  a  line  deviating  as  much  on  the  other  side. 
Now  the  surfaces  of  concave  mirrors  are  so  formed,  that  every  ray  issuing 
from  the  focus  shall,  when  reflected,  become  parallel  to  every  other  ray — as 
represented  by  the  dotted  lines  in  the  figure ;  and  it  is  the  property  of  a 
similar  mirror  receiving  parrallel  rays  to  make  them  all  meet  in  its  focus  : — 
thus,  any  influence  radiating  from  c  towards  the  mirror  d,  will  again,  after 
two  reflections,  be  collected  at  d.  The  purpose  and  effect  of  such  mirrors 
in  experiments  on  heat,  are  merely  to  concentrate  feeble  influences,  so  that 
they  may  be  more  accurately  estimated.  To  show  the  effect  and  mode  of 
action  of  such  mirrors,  they  may  be  placed  exactly  facing  each  other  at  any 
convenient  distance,  and  then  a  hot  body  of  any  kind,  as  a  metallic  ball  or 
a  canister  of  boiling  water,  being  placed  in  one  focus  while  a  thermometer 


272  HEAT. 

stands  in  the  other,  the  thermometer  will  instantly  rise ;  although  if  left  in 
any  intermediate  situation  nearer  to  the  hot  body,  and  therefore  not  in  the 
focus,  it  will  not  be  affected.  If  burning  charcoal  be  placed  in  one  focus, 
and  a  readily  combustible  substance  in  the  other,  the  latter  may  be  set  fire 
to,  at  the  distance  of  thirty  feet  or  more. 

If,  in  one  focus  of  the  mirror  apparatus  described  above,  there  be  placed, 
instead  of  the  canister  of  hot  water,  a  piece  of  ice,  the  thermometer  in  the 
.other  focus  immediately  falls.  This  has  been  called  the  radiation  of  cold, 
and  persons  were  at  one  time  disposed  to  think  that  it  proved  cold  to  have 
a  positive  existence  distinct  from  heat.  The  case,  however,  is  merely  that 
the  thermometer  happens  then  to  be  the  hotter  body  in  one  focus  of  the 
mirrors,  placed  in  close  relation  with  a  colder  body,  the  ice,  in  the  other, 
and  consequently  by  the  law  of  equal  diffusion,  it  must  share  its  heat  with 
the  ice,  and  will  fall.  The  mirrors  in  any  case  have  merely  the  effect,  by 
preventing  the  spreading  and  dissipation  of  the  radiant  heat  from  either 
focus  except  towards  the  other  of  making  two  distant  bodies  act  upon  each 
other  as  if  they  were  very  near.  All  the  heat  that  seeks  to  radiate  from 
the  thermometer  d  in  the  direction  of  the  surface  of  the  mirror  b,  if  not 
met  by  an  equal  tension  or  force  of  temperature  in  the  other  mirror  or 
focus,  to  which  they  are  directed  at  a  and  c,  will  radiate  away  to  c,  and 
become  deficient  at  d.  Some  inquirers  have  believed  that  heat  was  constantly 
radiating  in  exchange  from  substance  to  substance  (as  light  radiates 
between  opposed  bodies)  only  more  copiously  from  the  side  where  the 
temperature  was  highest :  others  have  held  that  motion  took  place  only 
where  there  was  excess  of  heat ;  that  is,  when  the  balances  of  temperature 
was  destroyed ;  and  this  is  the  simplest  view. 

There  is  a  remarkable  difference  in  one  respect  between  the  heat  of  the 
sun  and  that  radiated  from  any  other  source,  namely  that  the  first  passes 
through  air,  glass,  water,  and  transparent  bodies  generally,  very  readily, 
while  the  latter,  although  not  obstructed  by  air,  is  almost  totally  intercepted 
or  absorbed,  in  passing  through  any  of  the  other  substances  named.  In  our 
drawing-rooms  it  is  common  to  have  plate  glass  fire-screens,  which,  while 
they  allow  the  light  to  pass,  defend  the  face  from  the  heat;  but  all  persons 
know  that  the  heat  of  the  sunbeams,  as  well  as  their  light,  enters  our  green- 
houses through  the  glass  which  covers  them.  A  glass  screen  interposed  be- 
tween the  concave  mirrors  in  the  apparatus  above  described,  destroys  almost 
entirely  the  effect  of  the  heated  body  placed  in  one  focus,  on  the  thermo- 
meter in  the  other,  and  the  trifling  effect  really  produced  has  appeared  to 
some  to  be  owing  to  the  heat  that  is  absorved  by  the  screen  on  one  of  its 
sides,  and  then  after  passng  through *it  by  conduction,  is  radiated  from  the 
other.  This  conclusion  seemed  to  be  supported  by  the  fact  that  screens  of 
metal  or  of  glass,  covered  with  lamp  black,  paper,  &c.,  allow  transmission 
nearly  in  proportion  to  their  several  absorbent  and  radiant  powers.  More 
careful  experiments,  however,  have  seemed  to  prove  that,  even  at  a  low 
temperature,  a  certain  portion  of  the  heat  is  suddenly  radiated  through  the 
glass,  and  at  a  high  temperature,  a  much  larger  portion.  A  glass  mirror 
reflects  the  light  of  a  fire,  but  at  first  retains  nearly  all  the  heat,  and  only 
radiates  it  afterwards  as  a  hot  body. 

The -doctrines  of  radiant  heat  make  us  aware  of  the  importance  of  having 
vessels  of  polished  metal  for  containing  liquids  or  other  things  which  we  de- 
sire to  keep  warm;  hence,  tea  and  coffee-pots,  dishes  for  soup,  &c.,  should 
be 'polished.  As  a  black  earthen  tea-pot  loses  heat  by  radiation  nearly  in 
proportion  to  the  number  100,  while  one  of  silver  or  other  polished  metaJ 


SPREADING    BY    RADIATION.  273 

loses  only  as  12,  there  will  be  a  corresponding  difference  in  their  aptitude 
for  extracting  the  virtues  of  any  substance  infused  in  them.  Pipes  for  the 
conveyance  of  steam  or  hot  air,  if  left  naked,  should  be  of  polished  metal ; 
but  after  arriving  at  a  place  where  they  have  to  give  out  their  heat,  their 
surface  should  be  blackened  and  rough.  A  (?oat  of  polished  mail  is  not  a 
cold  covering.  A  mirror  intended  to  reflect  "heat  should  be  of  highly 
polished  metal,  and  such,  for  an  obvious  reason,  the  interior  of  a  screen  behind 
roasting  meat  is  attempted  to  be  made.  A  fireman's  mark  is  usually  covered 
externally  with  smooth  tin  foil.  It  is  of  advantage  that  the  bottom  of  a  tea- 
kettle or  other  cooking  vessel  be  externally  black,  because  the  bottom  has 
to  absorb  heat,  but  the  top  should  be  polished  because  it  has  to  confine. 

The  interesting  phenomenon  of  dew  was  not  at  all  understood  until  lately, 
since  the  laws  of  radiant  heat  have  been  investigated.  At  sun-rise,  in  par- 
ticular states  of  the  sky,  every  blade  of  grass  and  leaflet  is  found  not  wet- 
ted, as  if  by  a  shower,  but  studded  with  a  row  of  distinct  globules  most 
transparent  and  beautiful,  bending  it  down  by  their  weight,  and  falling  like 
pearls  when  the  blade  is  shaken.  These  are  formed  in  the  course  of  the 
night  by  a  gradual  deposition  on  bodies  rendered  by  radiation  colder  than 
the  air  around  them,  of  part  of  the  moisture  which  rises  invisibly  from 
water  surfaces  into  the  air  during  the  heat  of  the  day.  In  a  clear  night  the 
objects  on  the  surface  of  the  earth  radiate  heat  to  the  sky  through  the  air 
which  impedes  not,  while  there  is  nothing  nearer  than  the  stars  to  return 
the  radiation ;  they  consequently  soon  become  colder,  and  if  the  air  around 
has  its  usual  moisture,  part  of  this  will  be  deposited  on  them,  in  the  form 
of  dew,  exactly  as  the  invisible  moisture  in  the  air  of  a  room  is  deposited 
on  a  cold  bottle  of  wine  when  first  brought  from  the  cellar.  Clouds,  by 
obstructing  the  radiation  spoken  of,  obstruct  the  formation  of  dew.  Air 
itself  seems  not  to  lose  heat  by  radiation.  A  thermometer  placed  upon  the 
earth  any  time  between  sunset  and  sunrise,  generally  stands  considerably 
lower  than  another  suspended  in  the  air  a  few  feet  above  it ;  owing  to  the 
radiation  of  heat  upwards  from  it  and  from  the  earth,  while  the  surrounding 
air  remains  nearly  in  the  same  state.  During  the  day,  while  the  sun  shines, 
the  earth  is  warmer  than  the  air.  The  reason  why  the  dew  falls,  or  is 
formed  so  much  more  copiously  upon  the  soft  spongy  surface  of  leaves  and 
flowers,  where  it  is  wanted,  than  on  the  hard  surface  of  stones  and  sand 
where  it  would  be  of  no  use,  is  the  difference  of  their  radiating  powers, 
There  is  no  state  of  the  atmosphere  in  which  artificial  dew  may  not  be  made 
to  form  on  a  body,  by  sufficiently  cooling  it  and  the  degree  of  heat  at  which 
the  dew  begins  to  appear  is  called  the  dew-point  being  an  important 
particular  in  the  meteorological  report  of  the  day.  In  cloudy  nights  heat 
is  radiated  back  from  the  clouds,  and  the  earth  below  not  being  so  much 
cooled,  the  dew  is  scanty  or  deficient.  And  it  is,  when  uninformed  persons 
would  least  expect  the  dew,  viz. :  in  very  warm  clear  nights,  and  perhaps 
when  the  beautiful  moon  invites  to  walking,  as  in  some  of  the  evenings  of 
autumn  with  the  harvest  moon  and  harvest  occupations — that  the  dew  is 
more  abundant,  and  the  danger  greater  to  delicate  persons  of  taking  harm 
by  walking  among  the  grass. 

18 


274  HEAT. 

"  Heat  by  entering  bodies  expands  them,  and  through  a  range  which  includes 
as  three  successive  stages  tne  forms  of  solid,  liquid,  and  air  or  gas  ;  be- 
coming thus,  in  nature,  the  grand  antagonist  and  modifier  of  the  effects 
of  that  attraction  which  holds  corporeal  atoms  together,  and  which,  if  act- 
ing alone,  would  reduce  the  whole  material  universe  to  one  solid,  lifeless 
mass.  (Read  Analysis,  page  256.) 

If  an  experimenter  take  a  body  which  is  as  free  from  heat  as  human  art 
can  obtain  it — a  bar  of  solid  mercury,  for  instance,  as  it  exists  in  a  polar 
winter — and  if  he  then  gradually  heat  such  body,  it  will  acquire  an  in- 
crease of  bulk  with  every  increase  of  temperature ;  first,  for  a  time,  there 
will  be  simple  enlargement  or  expansion  in  every  direction ;  then  the  mass 
will  in  addition  be  softened  ;  then  it  will  be  melted  or  fused ;  that  is  to  say, 
in  the  case  supposed,  the  solid  bar  will  be  reduced  to  the  state  of  liquid  mer- 
cury, with  the  cohesive  attraction  of  the  atoms  nearly  overcome :  if  the 
mass  be  still  farther  heated,  it  will  gain  bulk  until  at  a  certain  point,  the 
atoms  will  be  repelled  from  one  another  to  much  greater  distances,  constitu- 
ting then  a  very  elastic  fluid  called  an  air  or  gas,  many  hundred  times  more 
bulky  than  the  same  matter  in  the  solid  or  liquid  state,  and  forcibly  distend- 
ing an  appropriate  vessel  as  common  air  distends  a  bladder ;  susceptible, 
moreover,  of  dilating  indefinitely  farther,  by  farther  additions  of  heat,  or  by 
diminution  of  the  atmospheric,  or  other  pressure,  against  which  it  had  to 
rise  during  its  formation.  A  subsequent  removal  of  the  heat  from  the  gas- 
eous mercury,  will  cause  a  progress  of  contraction  corresponding  to  the  pre- 
vious progress  of  expansion,  and  the  various  conditions  or  forms  of  the  sub- 
stance above  enumerated,  will  be  reproduced  in  a  reversed  order,  until  the 
solid  mass  re-appear,  as  at  first. 

What  is  thus  true  of  mercury  is  proved,  by  modern  chemical  art,  to  be 
true  also  of  all  the  ponderable  elements  of  our  globe,  and  of  many  of  the 
combinations  of  these  elements, — as  water,  for  instance,  familiarly  known 
in  its  three  forms  of  ice,  water  and  steam  ;  although  compound  substances 
generally,  by  great  changes  of  temperature,  are  decomposed  into  their 
elements. 

A  student  might  at  first  have  difficulty  in  believing  that  the  beautiful 
variety  of  soil,  liquid,  and  air,  found  among  natural  bodies  could  depend 
upon  the  quantities  of  heat  in  them,  because  these  forms  are  all  seen  exist- 
ing at  the  same  common  temperature  ;  but  he  afterwards  learns  that  each 
substance  has  its  peculiar  relation  or  affinity  to  heat,  and  that  hence,  while 
at  the  medium  temperature  of  the  earth,  some  bodies  contain  so  little  as  to 
be  solids — like  the  metals,  stones,  earths,  &c. ;  others  have  enough  to  be 
liquids — as  mercury,  water,  oils,  &c. ,  and  others  have  enough  to  be  airs—- 
as oxygen,  nitrogen,  hydrogen,  &c.  Men,  until  better  informed,  are  prone 
to  deem  the  tastes  in  which  bodies  are  most  frequently  observed  by  them, 
the  natural  or  essential  states  of  such  bodies ;  and  the  Indian  king  reasoned 
but  in  the  usual  way  when  he  held  the  Dutch  navigators,  newly  arrived  on 
his  shores,  to  be  gross  impostors,  because  they  said  that  in  their  country,  at 
one  time  of  the  year,  water  became  so  hard  that  they  could  walk  upon  it, 
and  drive  -their  carriages  upon  it,  and  shape  it  into  solid  blocks.  All  per- 
sons err  like  this  king,  who  in  thinking  of  the  different  substances  familiarly 
known  to  them,  regard  their  accidental  state  of  solid,  liquid  or  gas,  which 
state  is  really  dependent  on  the  temperature  of  the  bodies,  and  therefore 
on  the  particular  climate  or  situation  on  earth  where  they  are  found,  to  be  in 
them  an  essential  natural  character.  As  well  might  a  person  who  had  never 


CAPACITY    FOR    HEAT.  275 

seen  silk,  but  as  a  delicate  gauze  or  satin  enveloping  some  lovely  human  form, 
refuse  to  recognize  it  in  the  unsightly  coil  of  the  worm  which  produces  it. 

The  degree  in  a  general  state  of  temperature  at  which  the  substances  most 
important  to  man  change  their  states  from  solid  to  liquid,  or  from  liquid  to 
air,  will  be  noticed  in  a  future  page.  Here  we  have  only  to  remark,  that  the 
differences  are  very  great.  Mercury  melts  at  about  80  degrees  below  the 
melting  point  of  ice,  and  porcelain  at  about  30,000  degrees  above.  There 
are  some  substances  which  require  so  high  a  temperature  for  their  fusion  or 
for  their  conversion  into  gas,  that  human  art  has  difficulty,  or  even  finds  it 
impossible,  to  produce  the  changes  by  simple  concentration  of  heat ;  but  all 
such  substances  are  quickly  reducible  to  the  liquid  form  when  placed  in  con- 
tact with  others  for  which  they  have  a  chemical  affinity,  and  which  possess 
already  the  form  of  liquid  or  air;  as  when  gold  or  platinum  are  dissloved  in 
mtro-muriatic  acid — flint  in  the  fluoric  acid — carbon  in  hydrogen  gas.  Now 
many  persons  may  not  have  reflected  that  the  dissolving  a  solid  in  any  fluid 
menstruum  is  merely  another  mode  of  melting  it  by  heat;  yet  this  is  the 
truth,  for  the  menstruum  is  itself  fluid,  only  because  of  the  much  heat  which 
it  contains,  and  in  dissolving  the  more  obdurate  substances,  it  does  so  merely 
because  its  atraction  for  the  substance  brings  the  particles  into  .union  with 
the  heat  which  already  exists  in  itself.  Heat,  then,  is  the  one  universal 
solvent,  or  cause  of  fluidity.  Its  influence  in  this  view  is  interestingly  seen 
in  the  fact,  that  a  fluid  when  heated  can  dissolve  much  more  of  solid  than 
when  cold.  Water,  while  hot,  keeps  dissolved  twice  as  much  of  many  salts 
as  it  can  when  its  temperature  has  fallen,  as  is  proved  by  the  crystals  of  salt 
formed  in  any  saturated  solution  as  it  cools. — rThere  are,  again,  in  nature 
many  substances  having  such  an  affinity  for  heat,  that,  until  lately,  they 
have  been  known  only  as  airs ;  and  even  in  the  present  advanced  state  of 
art,  they  cannot,  by  any  degree  of  mere  cooling,  be  reduced  to  the  liquid  or 
solid  form  ;  yet  all  such,  when  pressure  is  added  to  the  cooling,  or  when  the 
chemical  attraction  for  them  of  some  other  substances  which  already  exist 
in  the  liquid  or  solid  state  is  made  to  co-operate,  may  be  reduced.  An  instance 
is  afforded  when  oxygen  is  made  part  of  a  liquid  acid,  or  of  a  solid  ore. 

Of  solids,  some  on  receiving  heat  become  very  soft  before  they  are  lique- 
fied, as  pitch,  glue,  iron,  &c. ;  others  change  completely  at  once,  as  ice  in 
becoming  water  :  and  some  pass  at  once  to  the  state  of  air,  without,  therefore, 
having  assumed  at  all  the  intermediate  state  of  liquid — they  are  sublimed, 
as  it  is  called,  and  on  cooling  again  may  be  caught  in  a  powdery  state,  as 
seen  in  that  form  of  sulphur  or  of  benzoin,  termed  the  flower  of  the  sub- 
stance. Of  the  latter  class,  also,  are  camphor,  arsenic,  corrosive  sublimate, 
and  the  substance  called  iodine,  which  last,  from  the  state  of  rich  ruby 
crystals,  becomes  at  once,  on  being  heated,  a  dense  transparent  gas  of  the 
same  hue,  and  in  cooling  resumes  its  crystalline  form. 

The  reader  having  arrived  at  this  place,  may  peruse  again  with  advantage 
the  five  pages  near  the  beginning  of  this  work,  which  treat  of  the  influence 
of  heat  on  the  constitution  of  masses. 

((  Each  particular  substance,  according  to  the  nature,  proximity,  &c.,  of  its 
ultimate  particles,  takes  a  certain  quantity  of  heat,  (said  to  mark  its  CAPA- 
CITY) to  produce  in  it  a  given  change  of  temperature  or  caloric  tension.'9 
(Read  the  Analysis,  page  256.) 

A  pound  of  water,  for  instance,  that  its  temperature  may  be  raised  one 
degree,  takes  thirty  times  as  much  heat  as  a  pound  of  mercury.  This  may 


276  HEAT. 

be  proved  in  various  ways.  First,  if  the  heat  be  derived  from  any  uniform 
source,  the  water  must  remain  exposed  to  it  thirty  times  as  long  as  the  mer- 
cury. Second,  if  both  substances,  after  being  equally  heated,  be  placed  in 
ice  until  cooled  to  the  freezing  point,  the  heat  which  escapes  from  the  water 
will  melt  thirty  times  as  much  ice  as  that  which  escapes  from  the  mercury, 
instead  of  the  two  becoming  of  a  middle  temperature,  as  in  the  case  when 
equal  quantities  of  hot  and  cold  water  are  mixed,  and  every  degree  of  heat 
lost  by  the  one  quantity  becomes  just  a  degree  gained  by  the  other — the  pound 
of  hot  water,  by  giving  up  one  degree  to  the  pound  of  cold  mercury,  raises 
the  temperature  of  the  latter  thirty  degrees ;  and  in  the  same  proportion  for 
other  differences  : — or  on  reversing  the  experiment,  a  pound  of  hot  mercury 
will  be  cooled  thirty  degress  by  warming  a  pound  of  water  one  degree. 

Now  each  particular  substance  in  nature,  just  as  water  or  mercury,  has 
its  peculiar  capacity  for  heat ;  and  experiments  made  by  the  modes  of  mix- 
ture  and  of  melting  ice  above  described  have  led  to  the  construction  of  tables 
which  exhibit  the  relations.  The  following  short  table  is  an  abstract,  show- 
ing the  comparative  capacities  of  equal  weights  of  some  common  substances. 
Water,  for  reasons  of  convenience,  has  been  chosen  as  the  standard  of  com- 
parison. It  appears  that  a  pound  of  hydrogen  gas  takes  about  twenty  times 
more  heat  "to  produce  it  in  a  given  change  of  temperature,  than  a  pound  of 
water,  while  a  pound  of  gold  takes  about  twenty  times  less,  and,  therefore, 
four  hundred  times  less  than  the  hydrogen.  The  figures  in  the  table,  by 
marking  the  comparative  capacities  for  heat  of  various  substances  necessarily 
indicate,  also,  the  comparative  quantities  of  ice  melted  by  equal  weights  of 
the  substances  in  cooling  through  an  equal  number  of  degrees. — A  pound 
of  water,  the  standard,  must  cool  140  degrees,  that  is;  must  give  up  140 
degrees  of  its  heat,  to  melt  one  pound  of  ice. 

Gases. 

Hydrogen  -  -  2H 

Atmospheric  air  -  -  -  1| 

Carbonic  acid  gas  -  -  -  -  1| 

Common  steam       -  -  li 

Liquids. 

Solution  of  carbonate  of  ammonia  2 

Alcohol       -  -  -  -  -  lj!0 

Water          .....  1 

Milk 1 

Olive  oil     -  -  -  -  -  | 

Linseed  oil-  ....  \ 

Sulphuric  acid        -  -  -  i 

Quicksilver  ....  ^ 

Solids. 

,  Ice ft 

Wheat  -             -  * 

Charcoal  .....  £ 

Chalk i 

Glass  -  J 

Iron  ...  £ 

Zinc  ...  T'ff 

Gold  - 


CAPACITY    FOR    HEAT.  277 

We  may  remark  here  that  some  late  researches,  by  another  mode  of  trial, 
make  the  capacity  of  air  to  be  only  a  quarter  that  of  water,  although  in  the 
preceding  table  it  appears  to  be  one  and  three-quarters.  Now  as  the  other 
aeriform  fluids  have  been  compared  with  water  through  the  medium  of  atmo- 
spheric air,  if  there  be  an  error  with  respect  to  this,  it  must  run  through  all 
the  figures  noting  the  capacity  of  other  aeriform  substances. 

If  we  seek  a  reason  or  reasons  why  there  should  be  among  bodies  the  dif- 
ferences of  capacity  here  stated,  the  circumstances  chiefly  attracting  attention 
are  the  following.  1st.  Equal  weights  of  the  various  substances  have  very 
different  bulks  or  volumes,  and  therefore  have  different  room  in  which  the 
heat  may  lodge ; — as  a  pound  of  mercury,  for  instance,  is  only  one-fourteenth 
part  as  bulky  as  a  pound  of  water.  That  the  bulk,  however,  is  not  the  only 
influencing  circumstance  appears  in  the  fact,  that  mercury  only  has  one 
thirtieth  of  the  capacity  of  water.  2d.  In  equal  bulks  of  different  sub- 
stances, the  space  may  be  more  completely  occupied  by  the  particles  of  one 
than  of  another — as  is  probably  true  of  the  particles  of  mercury  compared 
with  those  of  water.  3d.  But  as  the  facts  are  not  fully  accounted  for  by 
these  two  circumstances,  we  must  infer  that  there  is  some  difference  in  the 
ultimate  particles  of  bodies  affecting  their  relations  to  heat.  We  shall  now 
review  more  particularly  the  various  circumstances  mentioned. 

First.  The  influence  of  bulk  or  volume,  in  determining  the  capacity  for 
heat,  is  proved  by  the  facts  stated  in  the  preceding  table,  and  by  many  others. 
In  the  table,  for  instance,  it  is  seen  that  hydrogen  and  the  gases  generally, 
with  their  great  comparative  bulk,  have  also  great  capacity  ;  that  liquids 
have  less  capacity  than  gases ;  that  solids  have  less  than  liquids — but  the 
capacity,  as  already  stated,  is  not  in  strict  proportion  to  bulk;  for  hydrogen 
which  is  many  thousand  times  more  bulky  than  an  equal  weight  of  water, 
has  only  twenty-one  times  the  capacity.  Again,  if  any  body  whatever  be 
suddenly  compressed  into  less  bulk,  heat  will  issue  from  it  as  if  squeezed 
out.  Thus  iron  or  other  metal,  suddenly  condensed  by  the  heavy  blow  of 
a  hammer,  is  thereby  rendered  hotter,  that  is  expelled  heat  gradually  spreads 
from  it.  Because  water  and  spirit,  on  being  mixed,  occupy  less  space  than 
when  separate,  there  is  from  the"  mixture  a  corresponding  discharge  of  heat. 
But  the  truth  is  most  remarkably  exemplified  in  airs  or  gases,  owing  to  their 
great  range  of  elasticity.  They  may  be  condensed  or  dilated  a  hundred-fold 
or  more,  and  there  will  be  a  simultaneous  concentration  or  diffusion  of  their 
heat,  that  is  to  say,  the  production,  in  the  space  occupied  by  them  of  intense 
heat  or  cold.  The  heat  of  air  just  condensed,  or  the  cold  of  that  which  has 
just  expanded,  is  much  greater  than  even  the  most  delicate  thermometer  can 
indicate,  for  there  is  so  little  heat  altogether  even  in  a  considerable  volume  of 
air,  that  the  mass  of  a  mercurial  thermometer,  although  absorbing  a  great 
part  of  it,  would  be  little  affected.  The  extent,  however,  of  the  change  of 
temperature  is  seen  in  the  facts,  that,  by  the  sudden  condensation  of  air  we 
may  inflame  tinder  immersed  in  it,  and  by  allowing  air  suddenly  to  expand, 
we  may  convert  any  watery  vapour  diffused  through  it  into  ice  or  snow. 
Nay,  air,  containing  carbon  in  perfect  solution,  as  is  true  of  the  common  coal 
gas,  if  first  condensed  to  expel  heat,  and  then  allowed  suddenly  to  expand, 
will  be  so  cooled  that  the  carbon  will  be  separated  like  a  black  cloud,  as  snow 
is  separated  in  the  case  before  described.  The  cold  which  separates  or 
freezes  carbon  from  a  gas  holding  it  in  solution,  must  be  very  intense.  It 
might  be  expected  that  air  suddenly  compressed  into  half  its  previous  volume, 
should  become  just  twice  as  hot  as  before,  or  if  suddenly  dilated  to  double 
volume,  should  be  only  half  as  hot,  thus  enabling  us  to  ascertain  the  whole 


278  HEAT. 

quantity  of  heat  contained  in  it ;  but  the  facts  are  not  so ;  the  temperature 
changes,  near  the  middle  degrees  of  the  scale  at  least,  much  less  than  the 
density.  Air  in  doubling  its  volume  from  a  common  density,  becomes  colder 
only  by  about  50°  of  Farenheit's  thermometer. 

The  different  capacity  for  heat  of  air  in  different  states  of  dilation,  pro- 
duces effects  of  great  importance  in  nature  as  well  as  in  the  arts.  Thus, 

On  the  surface  of  the  earth  near  the  sea-shore,  the  air  of  the  atmosphere 
has  a  certain  density  (a  cubic  foot  weighs  about  one  ounce  and  a  quarter)  de- 
pendent on  the  weight  and  pressure  of  the  superincumbent  mass;  but  on  a 
mountain  top  15,000  feet -high,  as  half  the  mass  of  the  atmosphere  is  below 
that  level  (see  "  Pneumatics,")  the  air  is  bearing  but  half  the  pressure,  and/ 
consequently  any  quantity  of  it  has  twice  the  volume  of  an  equal  quantity 
at  the  sea-side,  with  a  temperature  consequently  many  degrees  inferior.  The 
air  which  is  at  any  time  on  a  mountain-top,  may,  a  little  while  before,  have 
been  on  an  adjoining  plane  or  shore,  and  in  gradually  climbing  the  mountain 
side  as  a  wind,  it  must  have  been  gradually  expanding  and  becoming  cooler 
in  proportion  to  the  diminishing  pressure.  It  is  found  that  air,  on  rising 
from  the  sea-shore,  becomes  one  degree  colder  nearly,  for  the  first  200  feet 
of  perpendicular  ascent,  and  that  air  becomes  altogether  about  50°  colder  in 
rising  15,000  feet;  so  that  at  this  latter  elevation,  water  exposed  to  the  air 
is  frozen  even  near  the  equator,  where  the  temperature  of  low  plains  is  at 
least  80°.  It  thus  appears  that  if  a  man  could  travel  with  the  wind  so  as  to 
remain  always  surrounded  by  the  same  air,  he  might  begin  his  journey 
with  it  from  the  summer  vineyard  of  the  Rhine,  might  soon  after  find  it 
the  piercing  blast  of  the  Alpine  summits  ;  and  again,  a  little  after,  without 
any  change  having  occurred  in  the  absolute  quantity  of  its  heat,  might  feel 
it  as  the  warm  breath  of  the  flowers  on  the  plains  of  Italy. 

The  explanation  is  thus  given  of  why  very  elevated  mountains  in  all  parts 
of  the  earth  are  hooded  in  perpetual  snows.  We  have  just  said  that  even  at 
the  equator,  where  the  average  temperature  hear  the  sea  is  84°,  water  will 
be  frozen  when  carried  to  an  elevation  of  15,000  feet.  A  line,  therefore, 
traced  round  a  mountain  at  this  level  would  divide  the  portion  of  it  destined 
to  sleep  under  lasting  ice  and  snow  from  the  portion  below  covered  with 
green  herbage.  This  line,  wherever  found,  is  called  the  snow  line,  or  line 
of  perpetual  congelation.  At  the  equator  it  is  high  in  the  atmosphere,  be- 
cause there  is  a  difference  of  about  40°  between  the  average  temperature  of 
the  country  and  the  freezing  point  of  water,  viz.,  the  difference  between  84° 
and  32°,  and  an  elevation  of  15,000  feet  corresponds  to  this  difference;  but  in 
a  progress  towards  the  poles,  the  line  is  met  with  gradually  nearer  to  the  earth, 
as  the  difference  between  the  average  temperature  and  the  freezing  point  is 
less.  In  Switzerland,  the  snow  line  is  at  6,500  feet  above  the  sea ;  in  Norway, 
it  is  below  5,000.  With  respect  to  the  line  of  congelation,  it  is  farther  to  be 
remarked,  that  in  tropical  countries,  because  the  temperature  of  the  air  is  nearly 
uniform  during  the  whole  year,  the  line  or  limit  of  frost  and  snow  is  distinct 
and  unvarying,  that  is  to  say,  is  narrow,  particularly  where  the  acclivity  is 
considerable ;  but  in  countries  to  the  north  and  south,  which  have  strong  con- 
trast of  summer  and  winter,  the  line  rises  in  summer  and  falls  in  winter,  and 
thus  becomes  broad  and  less  evident ;  in  the  hot  season  much  snow  is  melted 
or  half  melted  above  the  middle  of  the  line  or  belt,  while  in  winter  much  snow 
and  ice  is  accumulated  below  that,  to  be  melted  again  when  summer  returns. 

In  the  breadth  of  the  line  of  congelation  for  changeable  climates,  we  have 
the  reason  of  the  formation  of  what  are  called  glaciers  around  snow-capped 
mountains  situated  in  such  climates,  and  around  such  only.  The  snow  near 


CAPACITY    FOR    HEAT.  279    . 

the  upper  part  of  ths  broad  line  having  been  only  softened  or  half  thawed  in 
the  preceding  summer,  becomes  in  winter  almost  as  solid  as  ice,  and  in  the 
succeeding  summer  vast  masses  of  itr  detached  by  the  action  of  the  sun  and 
of  the  internal  heat  of  the  earth,  and  loaded  with  more  recently  deposited  snow, 
are  constantly  falling  down  into  the  neighbouring  valleys  within  the  broad 
line  of  congelation ;  where, beingaccuraulated,  and  the  crevices  filled  up  with 
snow  or  with  water  which  hardens  to  ice,  they  form  at  last  the  huge  glaciers 
or  seas  of  ice,  mers  de  glace,  which  renders  certain  regions  so  remarkable. 
The  falling  of  the  masses  above  described  (called  in  Switzerland  avalanches,} 
is  what  renders  the  ascent  to  snow-clad  mountains  so  terrific  and  dangerous. 
Around  Mount  Blanc,  in  the  awful  solitudes  of  the  elevated  valleys,  the 
avalanches  are  thundering  down  almost  without  interruption  during  the  whole 
summer, — in  which  season  only  the  attempt  to  ascend  the  mountain  can  be 
made  :  and  a  pistol  shot,  or  any  considerable  agitation  of  the  air,  may  suffice 
to  set  loose  masses  that  would  sweep  away  a  whole  convoy.  Beneath  gla- 
ciers there  is  always  going  on  a  melting  of  that  part  of  the  ice  which  is  in 
contact  with  the  earth,  and  hence  a  stream  of  water  constantly  issues  from 
the  bed  of  every  glacier.  These  streams  in  Switzerland  are  the  beginnings 
of  the  magnificent  rivers  the  Rhine  and  Rhone  — Like  the  avalanches  break- 
ing loose  in  summer  among  the  mountains,  there  are  in  polar  seas  vast  masses 
of  ice  detached  from  the  shores,  and  which  afterwards  drift  into  warmer  seas 
to  be  melted.  These  often  become  as  rafts  to  the  arctic  bear,  and  to  his  sur- 
prise, carry  him  to  new  latitudes,  and  leave  him  at  last  to  perish  in  the  midst 
of  the  wide  ocean,  when  his  support  has  vanished  from  beneath  him. 

Although  the  proofs  are  not  so  immediately  apparent,  the  line  of  congela- 
tion exsits  as  truly  every  where  in  the  open  sky,  over  sea  and  plane,  as 
where  there  are  mountain  heights  to  wear  its  livery;  and  considerably  below 
the  line,  the  cold,  aided  by  electrical  agency,  is  sufficient  to  produce  in  the 
form  of  mist  or  clouds,  a  deposition  from  the  air  of  the  watery  vapour  con- 
tained in  it.  There  is  thus  in  nature  an  admirable  provision  to  shade  the 
earth  at  proper  times  from  the  too  powerful  rays  of  the  sun,  or  to  supply  rain 
as  wanted,  without  the  transparency  of  the  inferior  regions  of  the  atmosphere 
being  much  affected.  As  the  watery  vapour  rising  from  sea  or  lake,  and  in- 
visibly diffused  in  the  atmosphere,  can  only  reach  to  the  height  where  the  cold 
is  great  enough  to  condense  it,  the  clouds  may  in  general  be  regarded  as  the 
top  of  that  atmosphere  of  watery  vapour  or  aeriform  water,  which  is  always 
mixed  more  or  less  with  the  atmosphere  of  mere  air;  and  as  the  quantity  of 
watery  vapour  which  can  exist  invisibly  in  a  given  space,  depends  altogether 
on  the  intensity  of  heat  present,  the  clouds  in  a  cold  or  humid  atmosphere 
will  be  low,  and  in  a  warm  or  a  dry  atmosphere  will  be  high,  or  there  may 
be  none.  An  aeronaut  mounting  in  bis  balloon  through  a  clear  sky,  often 
enters  a  dense  cloudy  stratum,  and  for  a  time  is  surrounded  by  the  gloom 
almost  of  night,  the  face  of  earth  being  hidden  from  him  below,  while  the 
heavenly  bodies  are  equally  veiled  from  him  above  ;  but  rising  still  higher,  he 
again  emerges  to  brightness,  and  looks  down  upon  the  fleecy  ocean  rolling 
beneath  him,  as  the  climber  to  a  lofty  peak  looks  down  from  the  ever  pure 
atmosphere  around  on  the  inferior  region  of  clouds  and  storms. 

The  diminished  temperature  of  air  in  the  higher  regions  of  the  atmosphere, 
often  enables  the  natives  of  temperate  climates,  when  forced  to  reside  in  hot 
tropical  countries,  inimical  to  their  health,  to  find  near  at  hand,  on  some 
mountain  height,  the  congenial  temperature  of  their  early  home.".  The 
author  of  this  work,  during  a  visit  to  the  then  not  long  inhabited  Island  of 
Penang  in  the  strait  of  Malacca,  examined  this  fact  with  pleasure  not  readily 


280  HEAT. 

forgotten.  The  centre  of  the  island  is  occupied  by  a  lofty  mountain  ridge 
thickly  wooded,  on  the  northern  summit  of  which  a  few  residences  visible 
from  the  sea-shore  like  eagles'fnests  on  a  cliff,  had  just  been  constructed. 
Towards  these  one  morning  at  sunrise,  on  an  active  little  horse  of  the 
country,  and  along  a  tolerable  road,  he  began  to  climb  from  the  hot  plain 
below.  At  first  there  were  around  him  purely  tropical  objects,  inspiring 
tropical  feelings, — the  latter  modified  indeed  by  the  reflection  that  his  track 
lay  through  a  forest,  in  which  until  lately  the  foot  of  man  never  penetrated, 
and  where  the  trees  nursed  through  ages  to  their  greatest  growth,  and  the 
stupendous  precipices  and  the  sublime  waterfall  had  so  recently  been  exposed 
to  human  observation ;  but  as  he  gradually  ascended,  he  perceived  the 
character  of  the  vegetation  to  be  changing  and  the  air  to  be  becoming  so  light 
and  cool  as  strongly  to  awaken  in  him  thoughts  of  distant  England — nay, 
almost  the  illusion  that  he  was  there.  When  he  had  reached  the  summit, 
however,  'and  a  clear  space  opened  to  view  the  whole  country  around,  his 
attention  was  soon  recalled  to  the  fervid  land  of  the  sun.  At  first,  from  the 
elevation  being  so  great,  the  eye  took  account  only  of  the  grander  features  of 
the  scene,  and  which  were  such  nearly  as  might  be  met  with  on  a  Grecian 
or  Italian  shore :  the  expanse  of  sunny  water  in  that  beautiful  strait,  the 
opposite  continent  with  its  river  winding  seaward  across' the  plain,  the  town 
and  the  roadstead  near  it  crowded  with  ships,  which  appeared  only  as  specks 
on  a  wide-spread  map ;  but,  on  closer  inspection,  and  particularly  with  the 
aid  of  the  telescope,  were  described  the  rich  groves  or  cocoa-nut  and  banana, 
the  plantations  of  spice,  and  cotton,  and  sugar-cane,  the  tawny  labourers,  the 
bamboo  dwellings,  the  fanciful  canoes,  or  prows/  and  other  objects  of  the 
like  character.  And  such  was  the  scene  which  even  under  the  equator,  a 
person  could  place  under  his  eye,  while  the  thermometer  near  him  stood  as 
in  an  English  month  of  May. 

The  interiors  of  the  islands  of  Jamacia  and  Hayti  have  many  situations 
of  great  extent,  which  combine,  as  above  described,  the  advantages  of  tropi- 
cal situation  and  temperate  climate,  and  which  might  well  be  inhabited  by 
English  labouring  colonists.  The  vast  plain  of  Mexico,  and  much  of  the 
central  land  of  South  America,  are  similarly  circumstanced :  and  it  is  not 
uncommon,  where  the  ascent  to  the  gigantic  Andes  is  gradual,  to  find  at  the 
bottom  of  the  ridge  a  town,  whose  markets  are  stored  only  with  the  produc- 
tions of  the  equator,  while  in  a  town  higher  up  will  be  seen  only  what 
belongs  to  the  temperate  skies  of  Europe  : — climates  of  the  earth  naturally 
distant,  thus  meeting,  as  it  were,  in  amicable  vicinity,  on  the  same  rising 
plain. 

The  facts  detailed  in  the  preceding  paragraphs  are  intended  to  illustrate 
the  subject,  of  the  relation  of  volume  in  a  body  to  the  capacity  for  heat.  We 
now  proceed  to  speak  of  Density  in  the  same  respect. 

Second.  It  might  be  anticipated  that  a  dense  body,  or  one  in  which  the 
constituent  particles  may  be  supposed  to  fill  more  completely  the  space  occu- 
pied by  it  than  the  particles  do  in  a  rarer  body,  would  have  smaller  capacity 
for  heat,  in  proportion  to  the  smaller  space  left  vacant  in  its  mass  :  and  in  a 
general  comparison  of  the  capacities  of  equal  bulks  of  different  substances, 
such  anticipation  is  partly  verified, — as  when  a  pint  of  dense  mercury  is  found 
to  have  only  about  half  the  capacity  which  a  pint  of  lighter  water  has.  The 
relation,  however,  is  by  no  means  universal,  nor  at  all  in  proportion  to  the 
differences  of  density.  Water,  which  is  denser  than  oil,  and  according  to 
the  hypothesis  should  have  less  capacity,  yet  has  nearly  double  the  capacity  ; 
and  mercury,  which  being  nearly  fourteen  times  denser  than  water,  might 


EXPANSION    OF    BODIES.  281 

be  expected  to  have  only  a  fourteenth  of  the  capacity,  has  really  for  equal 
volumes  a  half,  or,  as  formerly  stated,  for  equal  weights,  a  thirtieth. 

Third.  We  are  at  last,  therefore -compelled  to  admit  that  the  relation  be- 
tween various  substances  and  heat)  which  we  call  capacity  for  heat,  depends 
much  more  on  the  nature  of  the  ultimate  particles  of  the  substances  than 
either  on  the  absolute  bulk  or  comparative  density  of  the  masses.  Throwing 
much  light  on  this  subject,  it  has  been  ascertained  in  late  times,  that  all 
material  substances  are  composed  of  extremely  minute  unchangeable  atoms, 
of  which,  in  different  substances,  the  comparative  weights  have  been  deter- 
mined, although  not  the  absolute  weights ;  that  is  to  say,  for  example,  the 
atom  of  gold  is  known  to  weigh  four  times  as  much  as  the  atom  of  iron, 
although  we  do  not  know  how  many  thousands  or  millions  of  atoms  are 
required  to  form  a  grain  of  either.  Now  very  recent  researches  seem  to 
prove  that  for  each  ultimate  atom,  no  matter  of  what  substance,  nearly  the 
same  quantity  of  heat  is  required  to  produce  in  a  mass  of  the  atoms  a  given 
change  of  temperature.  Thus  an  ounce  of  iron,  which  has  four  times  as 
many  atoms  as  an  ounce  of  gold,  has  four  times  the  capacity  for  heat.  This 
law  seems  to  hold  for  all  simple  substances ;  but  for  compounds  there  seems 
to  be  another  law  not  yet  ascertained. 

Instead  of  the  term  capacity  for  heat  used  in  the  preceding  pages,  with 
respect  to  particular  substances,  that  of  specific  heat  has  by  some  authors 
been  preferred ;  but  as  the  latter  gives  to  a  commencing  student  the  idea 
rather  of  kinds  of  heat  than  of  quantities,  the  term  capacity  has  been 
retained. 

' '  Each  substance  in  nature,  for  a  given  change  of  temperature,  undergoes 
expansion  in  a  degree  proper  to  itself,  the  expansion  generally  increasing 
more  rapidly  than  the  temperature,  as  the  cohesion  of  the  particles  be- 
comes weaker  from  increased  distance  j  being  remcwkably  greater,  there- 
fore, in  liquids  than  in  solids,  and  in  airs  than  in  liquids;  the  rate  being 
quickened,  moreover,  near  the  points  of  change."  (See  the  Analysis, 
page  256.) 

The  following  table,  containing  the  names  of  some  common  substances, 
solid,  liquid,  and  aeriform,  shows,  by  the  figures  following  each  name,  how 
much  the  substance  increases  in  bulk,  by  having  its  temperature  raised  from 
that  of  freezing  to  that  of  boiling  water.  A  lump  of  glass,  for  instance, 
would  gain  one  cubic  inch  for  every  416  cubic  inches  contained  in  it;  while 
a  mass  of  water  would  gain  one  inch  for  twenty-three,  dilating  thus  for  the 
same  range  of  temperature  eighteen  times  more  than  glass. 

Solids, 

Glass  gains  one  part  in                          -  416 

Deal 416 

Steel           ....                 -  283 

Iron -  271 

Brass 177 

Silver         -        -        -        -'       -         -  375 

Lead 117 

Liquids. 

Mercury  gains  one  part  in    -  55 

Water         ...                           -  23 

Fixed  oils            -----  12 

Alcohol      ......  9 


282  HEAT. 

Airs. 

Common  air,") 

all  gases  and  I  gain  one  part  in  -         -         3 

vapours          J 

We  have  to  warn  readers  here  not  to  confound  the  increase  by  heat  of 
the  general  bulk  of  a  solid  body  with  the  increase  of  its  length.  The  latter 
is  only  one-third  as  great  as  the  former.  This  will  be  understood  by  consi- 
dering that  the  increase  of  bulk  is  divided  between  the  length,  breadth,  and 
depth  (or  thickness.  If  the  substance  of  a  metalic  square  rod  or  wire,  be 
dilated  by  heat  a  one-hundreth  part  of  its  bulk,  it  does  not  gain  all  that 
hundreth  at  its  end,  becoming  101  inches  (or  other  measure)  long,  instead 
of  100 ;  but  every  part  becomes  deeper  and  broader  in  the  same  proportion 
as  it  becomes  longer  (we  may  suppose  it  divided  into  a  row  of  equal  little 
cubes,)  and  the  rod  gains  in  length  only  the  third  part  of  an  inch.  A  fluid 
enclosed  in  a  tube  unchangeable  by  heat  (if  such  tube  there  were)  would 
show  its  whole  dilatation  in  an  increase  of  length,  because  there  could  be  no 
swelling  laterally,  and'  its  extremity,  therefore,  from  any  variation  of  tempe- 
rature, would  have  a  triple  extent  of  motion.  A  degree  of  this  consequence 
is  obtained  in  our  common  thermometers,  because  the  containing  glass,  al- 
though dilatable  by  heat,  is  so  much  less  dilatable  than  the  fluid  within. 
As  regards  solids,  we  have  to  inspire  so  much  more  frequently  respecting  the 
dilatation  in  length,  breadth,  &c.,  that  is'to  say,  the  linear  dilatation,  in  one 
direction,  than  the  increase  of  general  bulk,  that  tables  are  frequently  made 
stating  only  the  linear  dilatation.  It  may  be  found  at  once  from  the  above 
table,  by  recollecting  that  it  is  one-third  of  the  increase  of  bulk:— thus,  as 
glass,  in  passing  from  the  freezing  to  boiling  heat  of  water,  dilates  one  part 
in  416  of  its  bulk,  it  will  dilate  only  one-third  of  a  part  in  length,  or  a  whole 
part  in  an  extent  of  three  times  416  or  1,248. 

The  expansion  of  solids  by  heat  has  been  ascertained  by  bringing  micro- 
scope instruments  to  bear  on  rods  of  the  different  substances  heated  to  various 
degrees,  in  troughs  of  oil  or  water.  The  expansion  of  fluids,  again,  is  found 
by  filling  a  glass  vessel  with  a  known  weight  of  any  fluid,  and  then  ascer- 
taining how  much  is  made  to  run  over  or  escape  by  a  given  increase  of  heat ; 
or  how  much  the  fluid  rises  into  a  long  tubular  neck  like  the  stock  of  a  ther- 
mometer. This  quantity,  added  to  what  is  required  to  fill  the  increased  di- 
mensions of  the  heated  glass  vessel,  (which  from  the  ascertained  expansion 
of  glass  is  known,)  forms  the  whole  of  the  increase 

The  general  and  comparative  expansion  of  solids  by  heat  is  exemplified  in 
the  following  cases : 

An  iron  bullet,  when  heated,  cannot  be  made  to  enter  an  opening,  through 
which  when  cold  it  passes  readily. 

A  glass  stopper  sticking  in  the  neck  of  the  bottle,  often  may  be  released 
by  surrounding  the  neck  with  a  cloth  taken  out  of  warm  water ;  or  by  im- 
mersing the  bottle  in  the  water  up  to  the  neck ;  or  by  making  strong  friction 
on  the  neck  by  a  tape  or  any  soft  rope  put  around  it,  and  then  pulled  back- 
wards and  forwards.  By  any  one  of  these  means  the  binding  ring  of  the 
neck  is  heated  and  expanded  sooner  than  the  stopper,  and  so  becomes  for  a 
short  time  slack  or  loose  upon  it. 

Pipes  of  cast-iron  for  conveying  hot  water,  steam,  &c.,  if  of  considerable 
length,  must  have  joinings  which  allow  a  degree  of  shortening  and 


EXPANSION    OF    BODIES.  283 

lengthening,  otherwise  a  change  of  temperature  may  destroy  them.  An  in- 
competent person  undertook  to  warm  a  large  manufactory  by  steam  from  one 
boiler.  He  laid  a  rigid  main  pipe,  along  a  passage,  with  lateral  branches 
passing  through  holes  into  the  several  apartments ;  but  on  his  first  admitting 
steam,  the  expansion  of  the  main  pipe  tore  it  away  from  all  its  branches. 

In  an  iron  railing,  a  gate  which  during  a  cold  day  may  be  loose  and  easily 
shut  or  opened,  in  a  warm  day  may  stick,  owing  to  their  being  greater  expan- 
sion of  it  and  of  the  neighbouring  railing  than  of  the  earth  on  which  they 
are  placed.  Thus  also  the  centre  of  the  arch  of  an  iron  bridge  is  higher  in 
warm  than  in  cold  weather;  while  on  the  contrary,  in  a  suspension  or  chain 
bridge,  the  centre  is  lowered. 

The  iron  pillars  now  so  commonly  used  to  support  the  front  walls  of  those 
houses  of  which  the  ground  stories  meant  to  serve  as  shops  have  spacious 
windows,  in  warm  weather  really  lift  up  the  wall  which  rests  upon  them, 
and  in  cold  weather  allow  it  again  to  sink  or  subside  considerably  more  than 
if  the  wall  were  brick  from  top  to  bottom. 

In  some  situations  (as  lately  was  seen  in  the  beautiful  steeple  of  Bow- 
church,  in  London)  where  the  stones  of  a  building  are  held  together  by 
clamps  or  bars  of  iron  driven  into  the  stones,  the  expansion  in  summer  of 
these  clamps  will  force  the  stones  apart  sufficiently  for  dust  or  sandy  partt- 
cles  to  lodge  between  them  :  and  then,  on  the  return  of  winter,  the  stones 
not  being  at  liberty  to  close  as  before,  will  cause  the  ends  of  the  shortened 
clamps  to  be  drawn  out,  and  the  effect  increasing  with  each  revolving  year, 
the  structure  will  at  last  be  loosened  and  may  fall. 

The  pitch  of  a  piano-forte  or  harp  is  lowered  in  a  warm  day  or  in  a  warm 
room,  owing  to  the  expansion  of  the  strings  being  greater  than  of  the  wooden 
frame-work  j  and  in  cold  the  reverse  will  happen.  A  harp  or  piano  which 
is  well  tuned  in  a  morning  drawing-room  cannot  be  perfectly  in  tune  when 
the  crowded  evening  party  has  heated  the  room. 

Bell-wires  too  slack  in  summer,  may  be  of  the  proper  length  in  winter. 

One  admirable  contrivance  for  keeping  the  pendulum  of  a  clock  always 
of  the  same  length,  by  making  the  greater  expansion  by  heat  of  a  middle 
bar  of  brass  counteract  the  smaller  expansion  of  two  side-rods  of  steel,  was 
explained  under  the  head  of  "  Pendulum,"  as  was  also  the  construction  of  a 
balance-wheel  having  a  corresponding  property.  A  difference  of  a  100th  of 
an  inch  in  the  length  of  a  common  pendulum,  causes  a  clock  to  err  ten 
seconds  in  twenty-four  hours,  and  a  rise  or  fall  of  25°  of  Farenheit's  ther- 
mometer produces  this  difference.  Another  kind  of  compensation  pendulum, 
not  less  admirable,  distinguished  by  the  name  of  its  inventor  Graham,  is 
obtained  by  substituting  for  the  solid  bob  or  ball  at  the  bottom,  a  glass  ves- 
sel containing  mercury.  The  mercury  on  expanding  by  heat,  has  its  centre 
of  gravity  raised  just  enough  to  compensate  for  the  lengthening  of  the  rod 
of  the  pendulum. 

Crystals  when  heated,  do  not  expand  quite  equally  in  breadth  and  in 
length.  The  same  is  true  of  fibrous  substances,  as  wood  which  expands 
and  contracts  more  in  breadth  than  in  length.  This  is  instanced  in  the 
leaking,  during  cold  weather,  of  a  ship's  deck  which,  in  warm  weather,  is 
tight; — an  occurrence  which  the  author,  in  rounding  the  Cape  of  Good 
Hope,  had  to  regret  as  the  cause  of  destruction  to  some  valuable  specimens 
of  natural  history  which  he  had  collected  among  the  Eastern  Islands. 

Bodies  expanded  by  heat,  unless  when  their  intimate  composition  is 
changed  by  it,  regain  exactly  their  former  dimensions  on  being  cooled. 


284  HEAT. 

As  is  seen  in  the  preceding  table,  the  expansion  of  liquids  by  heat  is  much 
greater  than  of  solids. 

A  cask  quite  filled  with  liquid  in  the  winter,  must,  in  summer,  force  its 
plug  or  burst ;  and  a  vessel  which  has  been  filled  to  the  lip  with  warm  liquid, 
will  not  be  full  when  the  liquid  has  cooled.  Hence  a  cunning  dealer  in 
liquids  has  tried  to  make  his  chief  purchases  in  very  cold  weather,  and  his 
chief  sales  in  warm  weather. 

There  exists  in  the  case  of  water,  an  extraordinary  exception,  already  men- 
tioned to  the  law  of  expansion  by  heat  and  contraction  by  cold,  producing 
unspeakable  benefits  in  nature.  Water  contracts  only  down  to  the  tempera- 
ture of  40°,  while,  from  that  to  82°,  which  is  its  freezing  point,  it  again 
dilates.  One  curious  consequence  of  this  peculiarity  is  exhibited  when  a 
pool  or  well  happens  to  be  formed  on  the  upper  surface  of  a  mass  of  ice,  as 
on  one  of  the  glaciers  of  Switzerland  and  elsewhere,  namely,  that  the  well 
goes  on  quickly  deepening  itself,  until  it  penetrates  to  the  earth  beneath. 
Supposing  the  surface  of  the  water  originally  to  have  nearly  the  temperature 
of  the  melting  ice,  or  32°,  but  to  be  afterwards  heated  by  the  air  and  sun, 
instead  of  the  water  being  thereby  dilated  or  rendered  specifically  lighter, 
and  detained  at  the  surface,  it  becomes  heavier  the  more  nearly  it  is  heated 
to  40°,  and  therefore  sinks,  down  to  the  bottom  of  the  pit  or  well;  but  there, 
by  dissolving  some  of  the  ice,  and  being  consequently  cooled,  it  is  again 
rendered  lighter,  and  rises  to  be  heated  as  before,  again  to  descend ;  and  this 
circulation  and  digging  cease  only  when  the  water  has  bored  its  way  quite 
through. 

Airs  are  expanded  by  heat  still  more  than  liquids. 

The  expansion  of  aeriform  bodies  by  heat  produces  many  important  effects 
in  nature.  Some  of  these  have  ah-eady  been  considered  in  the  preceding 
parts  of  this  work,  as  the  rising  of  heated  air  in  the  atmosphere,  causing  the 
winds  all  feover  the  earth;  the  same  in  our  fires  and  chimneys  supporting 
combustion  and  ventilating  and  purifying  our  houses;  the  same  again  from 
around  animal  bodies,  removing  the  poisonous  or  contaminated  air  which 
issues  from  the  lungs,  and  insuring  a  constant  supply  of  fresh  air  for  the 
support  of  life,  &c.  % 

It  is  remarkable  with  respect  to  aeriform  bodies,  that,  unlike  solids  and 
liquids,  they  are  all  equally  dilated  by  the  same  change  of  temperature,  re- 
ceiving an  increase  of  about  a  third  part  of  their  bulk  (37£  parts  in  100)  on 
being  heated  from  the  freezing  to  the  boiling  point  of  water,  viz.,  180°, — 
their  bulk  being  therefore  doubled  from  the  same  standard  point  by  about 
500°.  This  general  truth  holds,  not  only  with  respect  to  the  more  perma- 
nent airs  or  gases,  but  also  with  respect  to  all  steams  or  vapours  in  the  dry 
state,  that  is,  when  not  in  contact  with  the  liquid  producing  them.  The 
probable  reason  of  this  uniformity  is,  that  cohesive  attraction  which  varies 
so  much  in  different  solids  and  liquids,  modifying  the  effects  of  heat  upon 
them  in  aeriform  fluids  does  not  exist  at  all. 

The  extent  of  this  dilatation  for  airs  is  so  much  greater  than  for  liquids 
or  solids,  that  it  forces  itself  much  more  strikingly  upon  the  common  atten- 
tion. Thus,  a  bladder  containing  considerably  less  than  its  fill  of  air,  be- 
comes tense  immediately  on  being  held  to  the  fire.  The  air  in  a  ballooa  just 
escaping  from  a  cloud,  has  been  so  suddenly  expanded  by  the  direct  rays  of 
the  sun,  as  to  injure  the  texture  of  the  balloon;  and  probably  some  of  the 
fatal  accidents  among  aeronauts  have  been  owing  to  this  occurrence.  Burn- 
ing fuel  conveyed  into  a  vessel  or  case  which  can  be  suddenly  and  strongly 


EXPANSION    OF    AIR.  285 

closed,  will  produce  an  expansion  of  the  air  confined  with  it,  capable  of  burst- 
ing any  vessel  of  ordinary  strength — in  short,  will  produce  an  explosion. 

Now,  if  not  before,  at  any  rate  soon  after  steam-engines  began  to  be  used, 
and  had  so  strikingly  shown  to  what  important  purposes  the  force  of  an 
expanding  aeriform  fluid  might  be  applied  the  thought  would  naturally 
occurr  that  the  force  of  com'mon  air  dilating  by  heat  might  also  be  rendered 
useful.  Accordingly  a  variety  of  air-expansion  engines  have  been  proposed, 
but  as  yet  no  one  has  been  reduced  to  profitable  practice.  Had  the  truth  been 
generally  known,  which  very  recent  investigations  have  proved,  that  a  given 
quantity  of  heat,  when  used  to  dilate  air,  produces  several  times  as  mifch 
expansive  power  as  when  used  to  form  steam,  the  attempts  to  bring  such  an 
application  of  heat  under  control  would  probably  have  been  more  numerous, 
and  possibly,  by  this  time,  more  successful.  The  subject  is  so  interesting 
that  we  shall  subjoin  a  few  remarks  upon  it. 

To  produce  a  cubic  foot  of  common  steam  from  water  originally  cold, 
about  1,160  degrees  of  heat  are  required,  as  will  be  explained  a  few  pages 
hence.  The  same  quantity  of  heat  would  double  the  volume  of  about  five 
cubic  feet  of  atmospheric  air, — as  is  known  from  the  comparative  capacities 
for  heat  of  the  two  substances,  and  the  rate  of  dilatation  of  air  when  heated. 
Now  the  value  for  work  of  the  foot  of  steam  passing  from  the  boiler  into  a 
working  cylinder  would  be,  to  press  up  the  piston  of  the  steam-engine 
through  a  foot,  as  from  c  d  to  a  b,  with  a  force  all  the  way  of  15  Ibs.  per 
inch  of  the  piston  surface ;  while  the  working  valve  of  the  five  feet  of  air  in 
dilating  to  double  bulk,  would  be  to  lift  up  the  piston  five  times  as  far  as 
the  steam,  viz.,  from  g  h  to  e  f,  but  with  a  force 
gradually  diminishing  (represented  here  by  the  Fig.  125. 

shaded  part  of  the  figure)  as  the  expansion  went 
on,  from  15  Ibs  per  inch  at  the  begining  until  the 
air  had  dilated  to  its  destined  volume,  when  the 
force  would  altogether  cease ,  its  whole  effect, 
therefore,  would  be  five  feet  impulsion  of  the  pis- 
ton with  a  pressure  averaging  between  15  Ibs.  and 
nothing,  viz.,  7£  Ibs.  per  inch  ',  and  the  friction  in 
the  two  cases  and  the  varying  intensity  of  the  lat- 
ter pressure  being  neglected,  the  force  of  the  air 
would  be  2£  times  as  great  as  that  of  the  steam. 
But  it  is  farther  to  be  considered,  that  only  about 
half  the  heat  of  a  fire  is  applied  to  use  in  a  steam- 
engine,  viz.,  that  part  which  enters  the  boiler, 
while  the  remainder  passed  up  the  chimney ;  and 

in  an  air-engine  probably  the  whole  might  be  applied.  In  an  air-engine, 
moreover,  there  might  be  a  great  increase  of  power  from  the  combustion,  or 
semi-explosion  of  the  inflammable  gas  evolved  from  the  fuel.  We  see  from 
this  of  what  importance  the  discovery  would  be  of  a  means  enabling  us 
effectually  to  apply  the  force  of  expanding  air. 

If  we  suppose  a  fire  a  to  be  placed  on  a  grate  near  the  bottom  of  a  close 
cylinder,  d  a,  and  the  cylinder  to  be  full  of  fresh  air  recently  admitted,  and 
if  we  farther  suppose  the  loose  piston  g  d  to  be  pulled  upwards,  it  is  evi- 
dent that  all  the  air  in  the  cylinder  above  d  will  be  made  to  pass  by  the 
tube  e  through  the  fire,  and  will  receive  an  increased  elasticity  tending  to  the 
expansion  or  increase  of  volume,  which  the  fire  is  capable  of  giving  it.  If 
there  were  only  the  single  close  vessel  da,  the  expansion  might  be  so  strong 
as  to  burst  it;  but  if  another  vessel  b  c  of  equal  size  were  provided,  com- 


286 


HEAT. 


Fig.  126. 


d\ 


Fig.    127 


municating  with  the  first  through  the  passage  b, 
and  containing  a  cZose-fitting  piston  e  /,  like  that  of 
a  steam-engine,  the  expansion  of  the  air  in  the 
first  cylinder  would  act  to  lift  the  said  piston,  and 
so  might  work  water-pumps,  or  do  any  other  ser- 
vice which  a  steam-engine  can  perform,  At  the 
end  of  the  lifting  stroke  of  the  piston  fc,  it  might 
be  made  to  open  an  escape- valve  for  the  hot  air, 
placed  in  any  convenient  part  of  the  apparatus, 
and  to  cause  the  descent  of  the  blowing  piston  d 
to  expel  that  air,  while  a  new  supply  of  fresh  air 
would  enter  by  another  valve  into  the  cylinder 
above  d.  The  engine  would  then  be  ready  to 
repeat  its  stroke  as  before,  and  the  working 
would  be  continued  as  in  a  steam-engine. 

The  preceding  simple  conception  of  an  air-engine  occurred  to  the  author's 
thoughts  while  considering  the  application  of  a  condensed  air-furnace  to  some 
chemical  purposes.  It  appeared  to  him  that,  in  applying  any  such  engine 
to  use,  the  chief  difficulties  to  be  surmounted  would  be,  to  prevent  the  very 
heated  air  and  dust  from  injuring  the  valves  and  other  working  parts  of  the 
engine,  and  to  obviate  the  inconvenience  of  the  inequality  of  power  at  differ- 
ent parts  of  the  stroke.  Various  expedients  occurred  to  him.  The  over- 
heating might  be  prevented  by  surrounding  the  cylinder, 
&c.,  with  water;  and  both  cylinder  and  piston  would 
suffer  less  from  dust,  if,  instead  of  the  common  piston  c. 
represented  above,  a  great  hollow  plunger  a  were  used, 
(such  as  is  here  represented,  and  is  now  common  in 
water-pumps  for  mines)  embraced  by  an  air-tight  neck  or 
collar  at  b  c,  which  neck  would  be  the  only  part  of  the 
cylinder  requiring  to  be  made  with  nicety.  But  a  more 
complete  security  would  be  obtained  by  interposing  water 
between  the  hot  air  and  the  piston,  as*  represented  in  this 
other  sketch,  where  the  working  cylinder  d  has  a  water- 
vessel  b  connected  with  it,  and  the  heated  air  is  admitted 
to  b  to  press  upon  a  float  on  the  water-surface,  to  lift  the 
working  piston  d  e.  This  construction,  too,  if  desired, 
would,  allow  the  fire  chamber  a  to  be  made  larger  than 
the  cylinder,  and  to  be  kept  constantly  filled  with  highly 
expansive  air,  each  discharge  of  which  into  the  space  b  would  be  replaced  by 

cold  air  either  from  the  space 
above  the  piston  d,  driven  in 
through  a  tube  as  the  piston 
ascended,  or  from  a  distinct 
blowing  cylinder  worked  by 
the  beam.  And  if  it  were 
wished  to  apply  the  same 
principle  to  an  engine  work- 
ing with  double  strokes,  that 
is,  forcing  the  piston  alter- 
nately up  and  down,  as  in  the 
double  stroke  steam-engine, 
the  object  might  be  attained, 
by  having  a  second  water-ves- 
sel f  communicating  with  the 


Fig.  128. 


EXPANSION     OF    AIR. 


287 


Fig.  129. 


part  of  the  working  cylinder  above  the  piston  d;  and  the  air  would  pass 
alternately  to  the  one  or  the  other  vessel  b  or  /,  by  the  operation  of  the  cock 
c,  as  steam  passes  in  a  steam-engine ;,  the  supply  of  fresh  air  to  the  chamber 
a  would  be  given  by  a  blowing  cylinder  worked  through  a  connection  with 
the  engine,  as  the  air-pump  of  a  steam-engine  is  worked: 

The  sketch  of  an  air-engine,  as  here  given,  was  include^  in  the  specifica- 
tion of  a  patent  for  another  object  engaged  some  years  ago  by  a  friend  of 
the  author's ;  but  that  friend  being  almost  immediately  called  to  other  busi- 
ness, and  the  author's  professional  engagements  forbidding  his  attention  to  the 
subject,  it  was  not  prosecuted.  In  the  specification,  drawn  up  by  an  engi- 
neer in  the  town,  some  minor  adaptations  were  described.  One  experiment 
has  lately  been  made  by  a  Swedish  engineer  with  the  simple  form  of  dry 
apparatus  described  at  page  285,  for  the  purpose  of  ascertaining  its  power, 
and  the  effect  was  found  to  be  several  fold  greater  than  of  steam  from  the 
same  quantity  of  fuel ;  but  the  apparatus  was  rude,  and  only  calculated  to 
prove  in  a  short  trial,  the  existence  of  the  power,  but  not  •  the  fitness  of  the 
machine  to  endure  uninjured,  or  to  be  rendered  easily  obedient  to  control;  a 
complete  experiment,  therefore,  remains  still  to  be  made.  Could  an  obedi- 
ent and  durable  engine  be  contrived,  at  all  approaching  in  simplicity  to  the 
plan  given  above,  its  advantages  over  the  steam-engine  would  be  very  con- 
siderable. First,  its  original  cost  would  be  much  less,  by  reason  of  its  small 
comparative  size,  its  simplicity,  and  the  little  nicety  of  workmanship  re- 
quired. Secondly,  it  would  require  much  less  room,  and  would  be  very 
light;  hence  its  peculiar  fitness  for  purposes  of  propelling  ships  and  wheel- 
carriages.  Thirdly)  the  quantity  of  fuel  required  being 
so  much  less,  would  not  load  the  ship  or  carriage  leav- 
ing little  room  for  anything  else.  Fourthly,  the  ex- 
pense of  fuel  and  repairing  would  be  little.  Fifthly, 
the  engine  could  be  set  to  work  in  a  few  minutes, 
where  a  steam-engine  might  require  hours.  Sixthly, 
little  or  no  water  would  be  required  for  it. 

Another  modification  of  air-engine,  called  a  gas  va- 
cuum engine,  has  lately  been  proposed,  and  many 
expensive  trials  have  been  made  of  it;  but  it  is  in  its 
nature  a  most  wasteful  machine,  evidently  throwing 
away  at  least  nine-tenths  of  the  power  which  the 
combustion  generates,  It  was  of  this  nature  in  an  ex- 
periment which  the  author  witnessed.  A  little  of  the 
common  coal-gas  was  admitted  by  the  cock  b  at  the 
bottom  of  the  cylinder  a,  and  was  there  inflamed,  the 
lid  e  being  at  the  time  raised.  The  combustion  rari- 
fied  the  lower  stratum  of  air,  so  that  the  air  above  was 
expelled,  and  about  one-fifth  of  the  original  contents 
of  the  cylinder  was  caused  to  occupy  the  whole".  The 
lid  was  shut  down,  as  nearly  as  could  be  judged,  at 
the  moment  of  greatest  expansion,  so  that  when  the 
small  portion  of  air  and  vapour  remaining  within  was 
again  cooled,  the  interior  of  the  cylinder  approached 
nearly  to  the  state  of  vacuum.  It,  in  fact,  retained 
only  a  fifth  of  the  air.  A  communication  being  then 
opened  from  the  vacuous  cylinder  by  the  tube  e  to  a 
water  reservoir  ten  feet  below,  the  water  was  driven 
up  by  the  atmospheric  pressure,  until  it  filled  more 


288 


HEAT. 


than  half  of  the  cylinder.  The  water  so  raised  was  then  made  to  turn  a 
common  water-wheel,  and  to  do  work.  A  much  larger  quantity  of  water, 
however,  could  be  raised  to  the  same  height  at  less  expense  by  a  steam- 
engine.  The  proposer  also  hoped  that  he  would  be  able  to  make  the  atmos- 
phere pressing  into  its  imperfect  vacuum,  act  directly  upon  a  piston  as  steam 
does,  and  with  power  cheaper  than  that  of  steam ;  but  in  this  anticipation 
too  he  was  completely  in  error.  To  produce  his  imperfect  vacuum  cost  him 
very  nearly  at  the  same  rate  as  it  costs  to  produce  the  perfect  vacuum  in 
a  steam-engine,  and  his  vacuum  for  equal  bulks  was  worth 
as  a  working  power,  only  about  one-fourth  as  much  as  the 
steam  vacuum.  This  may  be  understood  by  considering, 
that  in  a  perfect  vacuum  a  piston  rises  all  the  way  with  the 
same  force,  which  if  common  steam  be  used,  is  15  Ibs. 
per  inch,  the  piston  may  be  supposed  to  rise  from  c  d  to 
a  b,  but  if  the  vacuum  were  only  three-fourths  towards 
being  complete,  the  pressure  on  the  piston  would  be  only 
three-fourths  of  15  Ibs.  at  the  commencement  of  the  stroke, 
and  then  rapidly  diminishing,  would  have  ceased  altogether 
when  the  piston  had  made  three-quarters  of  its  journey,  or 
to/.  The  force  in  the  first  case  would  be  represented  by 
the  whole  line  c  d  and  the  whole  space  c  d  b  a,  and  in  the  second  by  the 
shortening  lines  and  the  triangular  space  c  ef. 

On  considering  the  foregoing  diagrams,  we  may  perceive  that  in  the  vacuum 
engine,  by  far  the  greater  part  of  the  force  produced  by  the  combustion  of  the 
gas  is  absolutely  wasted,  or  put  to  no  use,  namely,  the  whole  expansive  force 
during  the  sudden  combustion  or  explosion.  It  is  evident  that  if  a  tenth 
part  of  the  aeriform  contents  of  a  cylinder  acquire  elasticity  enough,  (a  four- 
teenth part  of  a  nice  experiment  does  so)  to  be  able  afterwards  to  occupy  the 
whole  cylinder,  that  tenth  must  begin  its  expansion  with  the  force  of  a  ten- 
fold atmospheric  condensation,  or  pressure  of  150  Ibs.  on  the  square  .inch  of 
a  piston  withstanding  it,  which  pressure  will  then  gradually  diminish  as  the 
piston  rises,  but  will  amount. to  an  average  of  five  times  the  atmospheric 
pressure,  or  75  Ibs.  per  inch  all  the  way;  being  therefore  quadruple,  or  more, 
that  of  steam  against  a  perfect  vacuum,  and,  therefore,  again,  by  our  former 
calculation,  more  than  twelve  times  greater  than  the  force  obtained  from  the 
imperfect  vacuum  of  the  engine  under  consideration. 

It  is  a  question  which  the  author  thinks  will  one  day  be  answered  in  the 
affirmative,  whether  nearly  the  whole  force  of  exploding  gas  may  not  be  con- 
verted into  a  calmly  working  power,  producing  from  a 
Fig.  131.  given  expenditure,  ten  times  or  more  the  effect  obtained 

in  the  vacuum-engine  described  above,  and,  therefore,  an 
effect  more  than  equal  to  that  of  a  steam-engine  incur- 
ring the  same  expense.  There  are  probably  various  ways 
in  which  the  object  may  be  obtained.  The  following 
sketch  is  offered  merely  to  give  the  reader  an  idea  of  a 
machine  for  such  a  purpose. 

Suppose  b  to  be  a  very  heavy  close-fitting  piston  sliding 
in  the  cylinder  containing  it;  and  suppose  the  space  d 
\  open  to  the  cylinder,  to  be  filled  with  atmospheric  air  of 

double  or  greater  density;  then  if  a  mixture  of  explosive 
Lil.          '       gases  admitted  by  a  cock  to  the  chamber  a  (formed  be- 
lle tween  the  piston  and  end  of  the  cylinder)  be  inflamed, 
the  heavy  piston  will  be  shot  forward  Lke  a  cannon-ball, 


EXPANSION    OF    AIR.  289 

against  the  condensed  air  in  d  ',  and  owing  to  the  momentum  acquired  in  the 
first  instance,  it  will  advance  much  beyond  the  point  where  the  exploded  gas 
and  air  in  d  would  balance  each  other  at  rest.  The  quantity  of  gases  admitted 
would  be  just  such  as  to  carry  it  to  the  end  of  the  cylinder.  The  piston  rod 
e  would  then  by  a  catch  or  racket,  be  connected  with  the  work  to  be  done, 
and  after  the  condensation  of  the  exploded  gases  in  a  cylinder,  would  be 
pressed  back  again,  with  the  greater  than  atmospheric  force  in  d,  as  if  urged 
by  high  pressure  steam.  Figure  127  at  page  286  represents  a  form  of  cylin- 
der which  might  also  answer  for  this  purpose,  the  heavy  plunger  being 
thrown  up,  to  work  by  its  weight  in  descending. 

It  is  to  be  remarked  that  the  first  modification  of  air-engine  described  at 
page  285,  is  partly  an  explosive  engine  such  as  contemplated  above,  for  the 
gas  separated  from  the  coal  during  the  moment  of  slackened  combustion  while 
the  lately  used  air  is  passing  out,  becomes  an  explosive  accumulation  for  the 
fresh  air  about  to  enter,  The  trial  alluded  to  above  proved  this  to  be  the  fact. 

"  The  expansion  of  bodies  by  heat  increases  more  rapidly  than  the  tempera- 
ture, and  particularly  near  the  melting  and  boiling  points,  that  is,  their 
points  of  changing  into  liquid  or  air  being,  however,  exactly  proportioned 
to  the  temperature  after  the  change  into  air.  (See  Analysis,  page  256.) 

If  a  given  quantity  of  heat,  that,  for  instance,  contained  in  some  measure  of 
boiling  water  or  of  common  steam,  be  added  to  a  mass  of  cool  water,  it  will 
produce  in  this  a  certain  increment  of  bulk ;  and  if  other  equal  quantities  of 
heat  be  afterwards  successively  added  under  the  nice  management  which 
such  an  experiment  requires,  each  new  addition  will  produce  a  greater  incre- 
ment of  bulk  than  the  preceding,  particularly  when  the  water  approaches  to 
boiling ;  but  after  the  water  is  converted  into  steam,  any  farther  increase  of 
bulk  will  be  exactly  proportioned  to  the  increase  of  temperature.  The  same 
truths  may  be  proved  by  the  converse  experiment  of  abstracting  successively 
equal  quantities  of  heat  from  steam  or  water  (as  by  making  it  melt  equal 
quantities  of  ice,)  and  noting  the  rate  of  contraction.  What  is  thus  true  of 
water  in  relation  to  heat,  is  true  also  of  bodies  generally,  each,  however, 
having  a  rate  of  expansion  and  temperature  for  melting  and  boiling  proper 
to  itself.  The  quickened  rate  of  expansion  in  solids  and  liquids  might  have 
been  anticipated  from  reflecting,  that  each  successive  quantity  of  heat  added 
to  a  mass,  meets  with  less  resistance  to  its  expanding  power  than  the  pre- 
ceding quantity,  owing  to  the  diminishing  force  of  cohesion  of  the  particles 
as  the  mass  enlarges ;  while  in  an  air  or  gas,  again,  as  cohesion  has  alto- 
gether ceased,  each  addition  of  heat  is  at  liberty  to  produce  its  full  and  equal 
effect. — If  the  capacity  of  substances  for  heat  did  not  increase  with  their 
bulk,  the  terms  "increase  of  heat'7  and  "increase  of  temperature"  would 
have  the  same  meaning,  and  the  subject  would  be  more  simple. 

The  reflection  will  naturally  occur  here,  that  as  in  the  common  thermo- 
meter, the  mercury  must  rise  or  expand  more  for  a  given  quantity  of  heat 
added  at  a  high  than  at  a  low  temperature,  the  scale  should  be  divided  to 
correspond  with  the  inequality.  Now  this  reasoning  is  good,  but  the  diffi- 
culty of  complying  with  it  in  practice  is  such,  that  the  inconvenience  of  the 
slight  error  arising  from  an  equal  division  is  commonly  submitted  to.  An 
air  thermometer  with  equal  divisions  is  very  correct,  but  from  wanting  many 
of  the  advantages  of  the  mercurial  thermometer,  is  little  employed ;  and 
fortunately  in  the  mecurial  thermometer  there  is  such  a  counterbalancing 
relation  between  the  expansion  of  the  mercury  and  of  the  containing  glass, 

19 


290  HEAT. 

as  to  render  the  error  alluded  to,  at  least  for  any  middle  range  of  temperature, 
very  trifling.  The  subject  of  unequal  thermometric  dilatation  in  the  same 
liquid,  and  of  the  differences  in  that  respect  in  different  liquids,  depending 
on  the  proximity  to  their  boiling  points,  &c.,  is  well  illustrated  by  Du  Luc's 
experiment  of  filling  various  thermometer-glasses  with  different  liquids,  and 
while  they  are  being  heated  through  the  same  range  of^temperature,  noting 
their  comparative  indications.  He  marked  on  each  tube  the  points  at  which 
the  liquid  in  it  stood  when  the  bulb  was  placed,  first  in  freezing  and  after- 
wards in  boiling  water,  and  he  then  divided  the  intervening  space  into  eighty 
parts  or  degrees.  The  discordance  of  the  dilatations  in  the  different  tubes 
when  the  instruments  were  afterwards  placed  together,  and  heated  from  the 
freezing  to  the  boiling  degrees  of  water,  was  as  here  detailed. 

Mercury.  Olive  Oil.  Alcohol.  Water. 

0  -  -     0            0  -  0 

10  9.5  -       7.9  -  -  0.2 

20  -  -  19.3  -  -  16.5  -  -  4.1 

30  -  -  29.3  -  -  25.6  -  -  11.2 

40  -  -  39.2  -  -  35.1  -  -  20.5 

50  -  -  49.2  -  -  45.3  -  -  32 

60  -  -  59.3  -  -  56.2  -  -  45.8 

70  -  -  69.4  -  -  678  -  -  62 

80  -  -  80  -  80  80 

The  singular  discrepancy  in  the  case  of  water  is  owing  to  the  peculiarity 
described  in  former  pages,  of  its  contracting  by  cold  only  down  to  40°  of 
Fahrenheit,  and  then  dilating  again  until  it  freezes. 

Laborious  investigations  have  been  made  by  the  French  chemists  to  dis- 
cover a  comprehensive  law  determining  the  rate  of  expansion  in  all  bodies, 
but  the  object  is  not  yet  satisfactorily  accomplished. 

"  To  melt  a  solid  body,  or  to  vaporize  a  liquid,  a  large  quantity  of  heat 
enters  it,  but  in  the  new  arrangement  of  the  particles  and  generally  in- 
creased volume  of  the  mass,  the  heat  becomes  hidden  from  the  thermometer 
and  is  called  LATENT  HEAT.  It  reappears  during  the  contrary  changes, 
after  ichatever  interval."  (See  the  Analysis,  page  256.) 

The  expansion  of  bodies  by  heat,  instead  of  proceeding  throughout  in 
some  nearly  uniform  or  gradual  manner,  exhibits  in  its  course  two  singular 
transformations  of  the  body :  the  first,  when  the  solid  breaks  down  into  a 
liquid ;  the  second,  when  the  liquid  swells  out  into  an  air  or  gas ;  so  that 
there  are,  in  all,  three  very  distinct  modifications  or  states  of  existence  for 
the  body  dependent  on  the  agency  of  heat.  The  substance  of  water,  for 
instance  when  at  a  low  temperature,  exists  in  the  solid  form  called  ice  ;  but 
at  32°  of  Fahrenheit,  on  receiving  more  heat  it  gradually  becomes  liquid  or 
water,  and  on  receiving  more  at  215°,  even  under  the  resisting  pressure  of 
the  atmosphere,  it  acquires  a  bulk  nearly  2,000  times  greater  than  it  had  as 
a  liquid,  (gradually  as  regards  the  whole,  but  suddenly  as  regards  each  sepa- 
rate portion,)  being  then  called  steam  or  aeriform  water.  And  other  bodies 
under  analogous  circumstances,  undergo  similar  changes.  »It  is  farther 
remarkable,  that  although  during  the  changes  a  large  quantity  of  heat  enters 
the  mass,  producing  in  one  case  liquidity,  in  the  other  the  form  of  air,  the 
temperature  is  the  very  same,  immediately  after,  as  immediately  before  the 


LATENT-HEAT.  291 

change,  the  last  received  heat  becoming  hidden  or  latent  in  the  mass  : — thus 
water  running  from  melting  ice  affects  the  thermometer  but  as  the  ice  does, 
and  steam  over  boiling  water  appears  no  hotter  than  the  water.  The  glory  of 
originally  discovering  the  facts,  to  recall  which  the  terms  latent  heat 
or  caloric  of  fluidity,  have  since  been  used,  belongs  to  the  illustrious  Dr. 
Black.  The  construction  of  the  modern  steam-engine  was  an  early  result  of 
kindred  investigations  made  by  his  friend,  James  Watt. 

We  select  the  following  instances  as  serving  to  display  the  subject  of  latent 
heat  in  its  various  bearings. 

A  mass  of  ice  brought  into  a  warm  room,  and  there  receiving  heat  frojji 
every  object  around  it,  will  soon  reach  the  temperature  of  melting  or  32° 
but  afterwards  both  the  ice  and  the  water  formed  from  it  will  continue  at 
that  temperature  until  all  be  melted ; — the  heat  which  continues  to  enter, 
effecting  a  change  only  in  the  form  of  the  mass.  And  in  the  case  supposed, 
whatever  time  was  required  for  heating  the  mass  of  ice  one  degree,  just  one 
hundred  and  forty  times  as  much  will  be  required  for  melting  it ;  proving 
that  140°  is  the  latent  heat  of  water. 

If  two  similar  flasks,  one  filled  with  ice  at  32°,  and  the  other  with  water 
at  32°  be  placed  in  the  same  oven  or  over  the  same  flame,  the  water  will 
gain  140  degrees  of  heat  while  the  ice  is  nearly  being  melted  into  water  at 
32  :  and  in  the  course  of  the  experiment,  .a  correspondence  will  always  exist 
between  the  phenomena;  for  instance,  when  the  water  has  gained  14°  of 
heat,  it  will  be  found  that  just  a  tenth  part  of  the  ice  is  melted. 

If  equal  quantities  of  hot.  and  cold  water- be  mixed  together,  the  whole 
acquires  a  middle  temperature,  each  degree  lost  by  the  hot  water  becoming 
a  degree  gained  by  the  cold ;  but  if  a  pound  of  ice  at  §2°,  and  a  pound  of 
water  140°  hotter  be  mixed  together,  the  140°  of  heat  will  go  merely  to  melt 
the  ice,  for  there  will  result  two  pounds  of  water  at  32°. 

If  a  flask  of  water  at  32°,  or  its  freezing  point,  and  a  similar  flask  of  strong 
brine  (which  does  not  freeze  until  cooled  to  near  zero)  also  at  32°,  be 
exposed  together  in  the  same  cold  place,  it  will  be  found  that  when  the  brine 
has  lost  10°  of  its  heat  the  water  flask  will  still  exhibit  an  undiminished 
temperature,  but  a  fourteenth  part  of  its  contents  will  be  converted  into  ice. 
Now  as  in  such  a  case  the  water  flask  must  continue  to  radiate  away  heat  just 
as  much  as  the  other,  it  can  maintain  its  temperature  by  absorbing  into  its 
general  mass  the  heat  which  was  latent  in  the  portion  of  water  frozen. 

It  is  possible,  by  slowly  cooling  water  which  is  kept  in  perfect  repose,  to 
lower  its  temperature,  while  yet  liquid,  ten  degrees  below  its  ordinary  freez- 
ing point;  but  then  on  the  slightest  agitation,  ice  will  be  formed.  It  might 
be  expected,  in  such  a  case,  that  the  whole  water  will  instantly  freeze, 
because  all  is  colder  than  common  ice ;  but  no,  only  a  fourteenth  part  freezes  ; 
and  singularly,  both  that  fourteenth  and  the  remaining  liquid  are  rendered 
in  the  moment  ten  degrees  warmer — rising  to  32°.  Here  the  140°  of  latent 
heat  escaping  from  the  fourteenth  part  of  the  water  which  freezes,  become 
10°  of  sensible  heat  for  the  whole  mass,  so  that  the  remaining  water  has  the 
temperature  at  which  it  only  begins  to  freeze. 

Strong  solutions  in  hot  water  of  various  neutral  salts,  if  allowed  to  cool 
while  exposed  to  atmospheric  pressure,  soon  deposit  crystals  of  the  salts ;  but 
in  a  close  vessel,  which  protects  them  from  such  pressure,  they  will  remain 
liquid  even  when  cold.  Now  at  the  moment  of  opening  such  a  vessel  to 
admit  the  pressure,  the  salt  immediately  crystallizes,  and  the  latent  heat 


292  HEAT. 

given  out  by  the  solidifying  particles  warms  very  sensibly  the  remaining 
liquid  and  the  vessel. 

From  the  preceding  facts  it  may  be  perceived,  that  the  quantity  of  ice 
formed  or  melted  in  any  case,  becomes  a  correct  measure  of  the  quantity  of 
heat  transferred.  From  this  consideration,  the  illustrious  Lavoisier  con- 
structed his  calorimeter,  or  heat  measure.  It  is  a  case  or  Vessel  lined  with 
ice,  and  the  quantity  of  heat  given  out  by  any  body  placed  in  it  is  indicated 
by  the  quantity  of  water  collected  from  the  melted  ice. 

Had  the  latent  heat  of  water  been  only  1°  or  2°  instead  of  140°,  the  earth, 
except  in  its  tropical  regions,  would  have  been  scarcely  habitable.  The  cold 
of  a  single  night  might  have  frozen  an  ocean,  and  the  heat  of  a  single  day 
ijight  have  converted  the  accumulated  snows  of  a  winter  into  one  sudden 
and  frightful  inundation.  As  the  fact  is,  however,  both  changes  are  beauti- 
fully gradual,  and  easily  controlled  or  prepared  for. 

The  fact  of  latent  heat  in  other  liquids  than  water  is  familiarly  exhibited 
in  the  slow  melting  of  various  substances  as — of  the  metals  j  lead  or  pig-iron 
for  instance — of  butter  or  oils — of  glass,  &c. ;  and  on  the  other  hand,  in  the 
slow  solidification  of  any  melted  masses  when  heat  is  again  abstracted. 

The  substances  below  enumerated,  while  passing  from  the  solid  to  the 
liquid  state,  absorb  and  render  latent  the  quantities  of  heat  here  noted ; 
which  quantities  are  therefore  called  the  latent  heats  of  the  liquids. 

Ice             -  ...        140° 

Mercury  -                          -               142 

Bees'  wax  -            -                     170 

Tin  -            -              442 

Zinc          -  -            -            -        492 

If  a  piece  of  frozen  mercury  (the  temperature  of  which  is  at  least  40° 
below  zero)  be  thrown  into  a  little  water  at  32°,  the  latent  heat  of  the  water 
immediately  passes  into  the  mercury  and  melts  it ;  but,  singularly,  the  water 
in  the  act  of  melting  the  mercury,  is  itself  frozen. 

"  Latent  Heat  of  aeriform  fluids" 

Water  in  a  vessel  placed  over  a  fire  gradually  attains  the  boiling  temper- 
ature or  212°,  but  afterwards  its  temperature  rises  no  more,  for  the  farther 
addition  of  heat  becomes  latent  in  the  steam  escaping  during  the  ebulition. 
One  way  of  determining  the  quantity  of  heat  which  becomes  latent  in  steam 
is  to  note  how  much  more  time  is  required  for  boiling  a  quantity  of  water  to 
dryness,  than  for  merely  heating  it  to  the  boiling  point,  or  through  any  cer- 
tain number  of  degrees.  The  experiment  indicates  about  1,000° ;  that  is  to 
say,  1,000  times  as  much  heat  is  latent  in  any  quantity  of  water  formed 
into  steam,  as  would  raise  the  temperatnre  of  the  liquid  water  one  degree. 
Watt  had  found  that  water  in  a  vessel  placed  over  a  lamp  was  about  six  times 
as  long  in  being  completely  evaporated,  as  in  being  originally  heated  from 
an  ordinary  temperature  to  that  of  boiling. 

If  we  place  in  the  same  oven,  or  over  similar  flames,  two  like  vessels  con- 
taining water,  one  of  which  is  open  at  the  end  and  the  other  is  strongly  closed, 
the  two  will  gain  heat  equally  up  to  the  boiling  point,  but  afterwards  the 
open  vessel  from  giving  out  steam  will  remain  at  the  same  temperature,  while 
the  other,  by  confining  the  heat  which  enters,  will  show  the  temperature 
continuing  to  rise  as  before,  until  the  increasing  tendency  of  the  water  to 


LATENT    HEAT.  293 

dilate  forces  the  vessel  open.  Supposing  the  water  in  the  latter  vessel,  be- 
fore vent  is  given,  to  have  become  100°  hotter  than  common  boiling  water, 
instead  of  the  whole,  when  at  liberty,  being  immediately  converted  into 
steam,  as  might  be  expected,  only  a  tenth  part  will  be  so  changed  (the  same 
quantity  as  will  be  found  to  have  already  escaped  from  the  other  vessel,)  for 
the  tenth  part  requiring  in  the  form  of  steam  1000°  of  latent  heat,  will  take 
the  excess  of  100°  from  the  other  nine  parts,  and  will  leave  them  as  common 
boiling  water.  If,  however,  water  heated  considerably  beyond  the  boiling 
point  be  allowed  to  expand  very  suddenly,  the  whole  is  blown  out  of  the 
vessel  as  a  mist,  by  the  steam  formed  at  the  same  instant  through  every  part 
of  the  mass;  but  the  whole  mass  in  such  a  case  is* no  more  converted  into 
true  steam  than  the  whole  of  very  brisk  soda-water  is  converted  into  gas 
when  similarly  thrown  out  by  the  sudden  extrication  of  the  carbonic  acid 
gas,  on  uncorking  the  bottle.  Misconception  of  this  matter  has  led  to  most 
wasteful  experiments  on  steam-engines  of  very  high  pressure.  Mr.  Perkins, 
for  instance,  thought  he  truly  described  what  was  accomplished  by  saying  of 
the  water  that  it  had  "  flashed  into  steam." 

The  same  indication  of  the  latent  heat  of  steam  is  obtained  by  the  converse 
experiment  of  first  converting  a  quantity  of  water  into  steam,  and  then  admit- 
ting it  to  cold  water  or  to  ice.  A  pound  of  steam  will  raise  the  temperature 
of  ten  pounds  of  cold  water  100  degrees,  or  will  melt  about  8£  pounds  of  ice. 

In  the  great  quantity  of  heat  which  becomes  latent  in  steam,  we  perceive 
the  reason  why  water  projected  upon  a  raging  fire  so  powerfully  represses  it, 
and  hence,  again,  why^re  and  water  are  so  often  adduced  proverbially  as 
exemplifying  a  fierce  antagonism.  t 

It  was  when  Watt  had  discovered  how  much  heat  was  lost  when  steam 
was  lost,  that  he  contrived  the  separate  condenser  for  his  steam-engine,  by 
which  he  at  once  saved  three-fourths  of  the  fuel  formerly  used 

Substances  diifer  among  themselves  in  regard  to  the  latent  heat  of  their 
vapours  as  much  as  in  their  other  relations  to  heat.  Thus  the  latent  heat 
of  the  vapor  or  steam  of — 

Water .     is  1,000° 

Vinegar 900 

Alcohol       .......         442 

Ether 300 

Oil  of  turpentine 177 

From  the  less  latent  heat  in  these  last-mentioned  vapours  than  in  that  of 
water,  we  might  at  first  suppose  that  there  would  be  great  advantage  from 
using  them  in  steam-engines.  Accordingly  numerous  experiments  have  been 
made,  and  patents  secured  under  this  idea ;  but  the  fact  is,  that  in  the  same 
proportion  as  the  heat  is  less,  the  volume  of  the  vapour  is  less,  and  therefore 
no  mechanical  advantage  is  obtainable. 

The  influence  of  external  pressure  in  keeping  the  particles  of  liquids  together, 
in  opposition  to  the  repulsion  of  heat  seeking  to  render  their  mass  aeriform, 
was  considered  in  the  chapter  on  "  Pneumatics  ;"  but  to  make  the  present 
section  complete,  the  subject  must  be  shortly  resumed. 

Because  water  or  any  liquid,  under  the  pressure  of  the  atmosphere,  while 
receiving  heat,  remains  tranquil,  and  apparently  unchanged,  until  it  reaches 
what  is  called  its  boiling  point,  at  which  a  bubbling  or  conversion  into  vapour 
takes  place,  we  might  suppose  its  ordinary  boiling  temperature  necessary  to 


291 


HEAT. 


Fig.  132. 


enable  it  under  any  circumstances,  to  assume  or  to  maintain  the  form  of  air. 
33ut  this  is  no  more  true  than  that  a  common  spring  compressed  against  any 
obstacle  or  force,  has  no  tendency  to  expand  or  recover  itself  till  the  moment 
•when  at  last  it  overcomes  the  obstacle.  Liquid  water  with  its  heat  is  really 
a  spring  compressed  by  the  powerful  weight  of  the  atmosphere,  and  seeking 
to  expand  itself  into  steam  with  force  proportionate  to  its  temperature.  Even 
at  32°,  or  its  freezing  point,  as. is  found  by  placing  it  in  a  vacuum,  it  seeks 
to  assume  the  form  of  air,  with  a  force  of  pressure  1 J  ounce  on  each  square 
inch  of  its  surface,  and  can  be  restrained  only  by  a  counter-pressure  of  that 
amount;  and  at  any  higher  temperature,  to  correspond  with  the  greater 
dilating  tendency,  the  restraining  force  must  also  be  greater:  at  100°,  for 
instance,  it  must  be  13  ounces;  at  150°,  14  Ibs.;  at212°,151bs.;  at  250°,  30 
Ibs.,  and  so  on  as  stated  in  the  preceding  part  of  this  work : — and  whenever 
the  restraining  force  is  much  weaker  than  the  expansive  tendency  the  forma- 
tion of  steam  takes  place  rapidly  and  far  below  the  surface  of  the  liquid,  so 
as  to  produce  the  bubbling  and  agitation  called  boiling.  Now  it  is  because 
the  atmosphere  or  ocean  of  air  which  surrounds  the  earth  happens  to  have  in 
it  15  Ibs.  weight  of  air  over  every  square  inch  of  the  earth's  surface,  and 
presses  on  all  things  there  accordingly,  that  212°  happens  to  be  called  the 
boiling  point  of  water.  An  atmosphere  less  heavy 
would  have  allowed  liquid  to  burst  into  vapour  at 
lower  temperatures,  and  one  more  heavy  would 
have  had  a  contrary  effect. — The  exact  degree  of 
expansive  force  for  every  degree  of  temperature  in 
water  and  other  liquids,  has  been  ascertained  by 
heating  them  in  vessels  furnished  either  with  pro- 
perly loaded  valves,  as  at /in  this  figure,  or  with  a 
tall  upright  tube,  as  d  6,  into  which  the  liquid  c  may 
force  a  column  of  mercury  to  an  elevation  marking 
the  expansive  tendency ;  the  valve  and  mercury 
being,  of  course,  protected  from  the  external  atmo- 
spheric pressure,  or  the  necessary  allowance  being 
made  for  that  pressure.  Boiling  at  the  bottom  of  a 
deep  vessel  is  resisted  by  the  weight  of  the  liquid 
in  addition  to  that  of  the  atmosphere,  as  already  ex- 
plained, and  consequently  the  temperature  at  which 
it  occurs  there,  is  higher  than  near  the  surface  of  the 
vessel.  Boiling  heat  is  greater,  also, — in  a  deep 
mine,  where  of  course  there  is  additional  depth  and 
weight  of  atmosphere  over  any  exposed  liquid, — 

at  times  when  the  barometer  is  unusually  high,  that  is  to  say,  when  the 
atmosphere  is  unusually  heavy — in  cases  where  air  or  steam  is  confined  over 
the  boiling  surface  so  as  to  press  more  upon  it,  as  when  brewers  for  a  time 
shut  the  lid  or  valve  of  their  great  boilers,  &c.  Water  placed  on  the  fire  in 
a  strong  vessel,  from  which  steam  cannot  at  all  escape,  may  be  rendered  even 
red-hot,  without  a  bubble  forming  or  one  particle  being  dissipated ;  but  the 
tendency  to  expand  into  steam  is  then  great  enough  to  burst  any  known 
material  of  moderate  thickness.  The  Marquis  of  Worcester  exploded  a 
cannon  by  shutting  up  water  in  it  and  then  surrounding  it  with  fire. — Boiling 
temperature  is  lower  again  when  the  experiment  is  made  on  mountains  or  in 
other  situations  above  the  level  of  the  sea,  where  there  is  less  height  of  air 
resting  over  the  boiler.  In  the  city  of  Mexico,  which  is  7,000  feet  above  the 
sea,  water  boils  before  it  reaches  the  heat  of  200°,  instead  of,  as  in  places 


BOILING.  295 

near  the  sea-level,  at  212°.  Wollaston's  thermometer,  beautifully  adapted 
for  determining  the  height  of  mountains,  balloon  ascents,  &c.,  by  merely 
indicating  the  heat  of  boiling  water  in  any  situation,  is  a  fine  illustration  of 
this  truth.  If  in  any  place  we  take  off  the  atmospheric  pressure  from  a  liquid, 
as  by  placing  it  in  the  receiver  of  an  air-pump,  it  will  boil  at  very  low  tempera- 
tures indeed.  Water  thus  treated  boils  at  a  temperature  many  degrees 
below  the  heat  of  English  summer  days  ;  and  ether  boils  when  colder  than 
common  ice. — Generally,  in  a  vacuum,  substances  boil  at  a  temperature 
124°  lower  than  while  restrained  by  the  atmospheric  pressure. 

Consequences  of  these  truths  respecting  the  boiling  temperature  are  the 
following. 

As  water  at  any  temperature  is  tending  to  dilate  itself  into  steam,  with 
force  proportioned  to  the  temperature,  the  steam  rising  from  any  mass  of 
water  presses  on  the  surface  of  the  vessel  containing  it  with  that  force;  and 
in  a  steam-engine,  therefore,  the  temperature  of  the  water  in  the  boiler  tells 
the  degree  of  force  with  which  the  steam  is  acting  on  the  piston. 

Because  in  the  case  of  steam  the  same  law  holds  as  for  aeriform  fluids 
generally,  viz.,  that  the  outward  elasticity  or  spring  increases  in  proportion 
as  the  fluid  is  more  condensed — high  pressure  steam  is  merely  condensed 
steam,  just  as  high-pressure  air  is  condensed  air;  and  to  obtain  a  double 
or  triple  pressure,  we  must  have  twice  or  thrice  the  quantity  of  steam  under 
the  same  volume. 

The  reason  that  high  pressure  steam  issuing  from  a  boiler  heated  to  300° 
or  more  is  not  hotter  than  low-pressure  steam  from  a  boiler  at  212°,  is,-that 
in  the  instant  when  the  high-pressure  or  condensed  steam  escapes  into  the 
air,  it  expands  until  balanced  by  the  pressure  of  the  atmosphere,  that  is,  until 
it  becomes  low-pressure  steam,  and  it  is  cooled  by  the  expansion,  as  air  is 
cooled  on  escaping  from  any  condensation. 

The  vessel  called  Papin's  Digester,  is  merely  a  metallic  pot  or  boiler, 
which  can  be  kept  closed  in  spite  of  the  force  of  the  steam  formed  within 
it ;  and  in  such  a  vessel,  water  can  be  heated  far  beyond  the  ordinary  boiling 
point, — sufficiently  for  instance,  to  dissolve  and  extract  all  the  gluten  or 
jelly  of  bones,  and  to  form  from  them  a  rich  soup  where  common  boiling 
would  procure  nothing  •— .-or  even  to  melt  lead  lying  in  water. 

The  person  who  increases  the  fire  under  a  boiling  pot  with  the  hope  of 
making  the  water  hotter,  is  foolishly  wasting  the  fuel,  for  the  water  can  only 
boil,  and  it  does  boil  at  212°  of  the  thermometer. 

As  different  substances  under  any  given  pressure,  become  aeriform  at  dif- 
ferent temperatures,  mixtures  of  such  may  be  decomposed  by  heat.  If  a 
mixture  of  spirit  and  water,  for  instance,  be  placed  over  a  fire,  the  spirit  will 
boil  off  long  before  the  water.  If  the  spirituous  vapour  be  caught  apart  and 
condensed,  the  operation  is  called  distillation.  All  distillations  are  of  the 
same  nature. 

The  instrument  here  represented  consists  of  a  glass  tube  blown  into  bulbs 
at  the  two  ends  a  and  6,  and  hermetically  sealed 
after  receiving  into  it  some  water,  but  no  air.  Fig.  133. 

There  will  always  be  in  the  apparently  empty 

part  a  stream  or  aeriform  water  of  density  pro-        J--5tf  & 

portioned  to  the  temperature.  If  one  of  the  bulbs      C    S^ 
be  heated  more  than  the  other,  the  steam  or  va-      ^ — < 
pour  in  that  one  will,  for  the  reasons  stated  above 
be  denser  and  stronger  than  in  the  other,  and  will 


296  HEAT. 

therefore  be  forcing  its  way  into  the  other;  where,  owing  to  the  lower  tem- 
perature, a  part  of  it  will  be  relapsing  into  the  state  6f  water,  making  room 
for  more.  Hence,  if  the  difference  of  temperature  between  the  bulbs  be  long 
maintained,  the  whole  water  will,  by  a  sort  of  distillation,  gradually  pass 
into  the  colder  bulb.  If  the  difference  of  temperature  become  at  any  time 
considerable,  the  liquid  will  boil  in  the  warmer  bulb,  even  although  the 
source  of  heat  be  only  the  living  hand  grasping  it. 

To  the  author  of  this  work  it  appears  that  by  a  larger  apparatus  made  on 
this  principle,  fresh  water  might  be  conveniently  obtained  from  salt  water  on 
board  ship,  or  on  islands  deficient  in  fresh  springs.  Suppose  any  two 
air-tight  vessels  like  a  and  b,  of  large  size,  communicating  by  a  tube  furnished 
with  a  stop-cock  near  b :  then  if  the  vessel  a  were  filled  with  salt  water,  and 
were  heated  by  being  exposed  to  the  sun,  (its  surface  being  blackened  to  ab- 
sorb heat,  and  protected  by  glass  from  the  cooling  effect  of  the  air,)  and  if 
the  other  vessel  b  were  made  a  vacuum  by  pumping  out  from  its  bottom  the 
water  with  which  it  had  been  previously  filled,  and  were  then  kept  as  cold 
as  possible  by  wetted  coverings  and  a  current  of  air, — on  opening  the  cock 
at  6,  vapour  would  pass  over  from  the  warmer  vessel  to  be  condensed  in  the 
colder,  and  there  would  be  a  distillation  from  sea-water  by  the  natural  action 
of  the  sun  alone,  of  a  water  naturally  fresh  and  pure.  Cases  have  .occurred 
where  a  knowledge  of  this  fact  would  have  saved  shipwrecked  crews  from 
perishing  by  thirst;  and  there  are  rocky  islands  in  the  ocean  where  there  is 
no  supply  of  fresh  water  but  from  precarious  rains  or  importation  from  abroad, 
but  which  might  be  rendered  pleasantly  habitable  by  the  adoption  of  such  a 
means. 

When  a  substance  has  reached  the  temperature  at  which  it  boils,  that  is  to 
say,  at  which  its  power  of  emitting  vapour  becomes  rather  more  than  a  balance 
to  the"  atmospheric  pressure,  its  dilating  force  is  strong  indeed.  Persons  may 
not  reflect  that  1 5  Ibs.  on  a  square  inch  is  about  a  ton  on  a  square  foot. — 
and  such  is  the  power  with  which  the  vapour  of  all  boiling  substances  rises 
from  them — sufficient  in  a  single  Cornish  steam-engine  to  urge  the  piston 
with  the  force  of  600  horses !  But  even  at  temperature  much  below  boil- 
ing, the  tendency  to  expand,  as  already  stated,  is  still  very  great,  and  although 
not  attracting  common  attention,  is  silently  working  many  beautiful  and  im- 
portant ends  in  the  economy  of  nature. — As  in  a  perfect  vacuum,  freezing 
water  gives  out  a  steam  or  vapour  that  would  lift  an  opposing  weight  with 
force  of  1 J  ounce  per  inch,  or  16  Ibs.  on  a  square  foot:  and  even  solid  ice 
gives  out  its  vapour  of  nearly  equal  strength, — so  also  do  many  other  liquids 
and  solids  give  out  their  vapours.  Thus  in  the  apparently  empty  space  called 
the  Terricellian  vacuum,  over  the  mercury  in  a  barometer  tube,  there  is 
always  an  aeriform  mercury,  dense  in  proportion  to  the  temperature ;  and 
around  camphor,  and  the  essential  or  volatile  oils,  &cv  there  is  similarly  an 
atmosphere  of  the  substance  in  the  form  of  air. 

It  had  for  a  considerable  time  been  known  that  into  a  perfect  vacuum 
many  bodies  emitted  almost  instantly  in  the  form  of  air,  a  quantity  of  their 
substances  proportioned  to  their  temperature ;  but  it  was  reserved  for  Mr. 
Dalton  to  make  the  admirable  discovery,  that  even  into  any  space  filled  with 
air,  these  vapours  arise  in  quantity  and  density  the  same  as  if  air  were  not  pre- 
sent— the  two  fluids  seeming  to  be  independent  of  each  other,  with  the  ex- 
ception that  in  a  vacuum  the  equal  diffusion  of  vapour  takes  place  at  once, 
while  in  a  situation  already  occupied  by  air,  it  proceeds  more  slowly  as  the 
vapour  can  force  its  way  through  the  particles  of  the  air,  and  in  general  takes 
place  by  a  tranquil  evaporation  from  the  surf  ice  instead  of  the  agitation  of 


EVAPORATION.  297 

ebullition.  In  an  apartment  with  an  open  vessel  of  water  in  it,  there  is  soon, 
although  invisible,  a  steam  of  watery  vapour  mingled  with  the  air;  as  dense 
as  if  the  room  were  a  vacuum  at  the  same  temperature. 

Consequences  of  this  important  truth  are  the  following : 

That  it  is  only  an  atmosphere  of  the  substance  of  each  body,  which,  by 
pressing  on  the  body,  can  prevent  its  farther  dissipation  by  heat.  Thus  we 
can  save  camphor,  musk,  smelling  oils,  spirits,  water,  &c.,  only  by  placing 
them  in  closed  bottles  or  vessels,  in  which,  additionally  to  the  air  present,  an 
atmosphere  of  their  own  substance  is  formed,  involving  the  remaining  masses 
with  pressure  proportioned  to  their  temperature  and  its  density. 

The  important  process  of  drying  things  is  merely  the  placing  them  under 
an  elevated  temperature  if  attainable,  and  in  an  atmosphere  not  containing  so 
much  of  the  liquid  as  to  be  saturated, at  the  temperature.  The  effect  of 
wind  or  motion  of  the  air  in  quickening  evaporation,  is  owing  to  its  removing 
air  saturated  with  the  moisture,  and  substituting  air  which  is  not— thus 
producing  nearly  the  case  of  the  substance  placed  in  a  vacuum. 

If  air  at  a  certain  temperature,  contain  mixed  with  it  as  much  water  as  can 
be  sustained  in  the  form  of  invisible  vapour  at  that  temperature,  and  if  then, 
by  any  cause,  as  by  rising  in  the  atmosphere,  the  air  be  cooled,  it  will  abstract 
heat  from  the  vapour,  and  cause  a  portion  to  be  precipitated  or  visibly  con- 
densed into  a  fog  or  rain.  Water  rising  as  invisible  vapour  from  the  surface 
of  a  lake  or  river,  often,  when  it  has  reached  a  certain  height,  is  condensed 
into  the  stratum  of  clouds  which  there  appears,  and  which  for  a  time  may 
remain  usefully  protecting  the  fields  from  the  intense  meridian  sun,  or  may 
fall  again  as  refreshing  showers  over  the  country. 

It  is  the  tranquil  and  invisible  evaporation  of  which  we  are  now  speaking, 
which  lifts  from  the  surface  of  the  wide  ocean  all  the  water  which,  after 
condensation,  returns  to  the  ocean  in  the  form  of  the  myriads  of  river  streams 
which  give  life  and  beauty  to  the  face  of  nature. 

In  warm  climates  there  are  inlets  of  the  sea,  occasionally  shut  off  from  the 
parent  ocean,  and  where,  after  the  sun's  rays  have  drunk  up  all  the  water, 
the  deposited  salt  remains  to  be  carried  away  in  loads  for  the  uses  of  man,  as 
sand  is  carried  away  from  any  ordinary  shore.  There  are  in  the  bowels  of  the 
earth  prodigious  accumulations  of  salt,  some  of  which  may  have  been  formed 
in  this  way,  during  the  revolutions  of  the  world  in  remote  past  time,  and 
which  are  now  turned  to  man's  account  as  salt-mines.  When  the  Nile  over- 
flows its  banks  with  water  holding  in  solution,  although  in  almost  imper- 
ceptible proportion,  mineral  substances  brought  from  the  interior  of  Africa, 
some  of  that  water  admitted  into  reservoirs,  and  afterwards  dried  up  by  the 
sun's  heat,  leaves  a  rich  store  chiefly  of  chrystallized  natron  or  soda. 

The  following  are  other  instances  of  vapour  which  is  invisible  while  at  a 
higher  temperature,  but  is  thickly  precipitated  when  air,  with  which  it  is 
mixed,  is  cooled,  or  when  it  touches  a  colder  solid  body  : — the  steam  observed 
at  night  and  morning  hovering  over  brooks  and  marshes  heated  by  the  sun 
during  the  day  : — the  frost-smoke,  as  it  is  called,  which  lies  on  the  whole  face 
of  the  greenland  seas  in  the  beginning  of  winter,  where  the  water,  warmed 
by  the  long  day  of  the  polar  summer,  continues  to  emit  its  vapour  for  a  con- 
siderable time  after  summer  is  past,  into  an  atmosphere,  become  too  cold  to 
preserve  it  invisible : — the  breath  or  perspiration  of  animals,  of  horses  in 
particular,  after  strong  exertion,  becoming  so  strikingly  visible  in  cold  and 
damp  weather,  or  even  in  warm  weather,  when  the  air  is  already  charged  with 


298  HEAT. 

'moisture: — in  cities  where  there  are  deep  drains  communicating  with 
kitchens,  manufactories,  &c.,  and  constantly  filled  with  moist  warm  air/the 
vapour-loaded  air,  although  clear  or  transparent  in  the  drain,  immediately 
on  escaping  into  a  frosty  atmosphere  lets  go  its  moisture,  with  the  appearance 
of  steam  issuing  from  a  great  subterranean  caldron.  Steam  over  water  in 
any  boiler,  is  transparent  or  perfectly  aeriform — as  may  be  seen  when  water 
is  made  to  boil  in  a  vessel  of  glass,  but  as  soon  as  it  is  cooled  by  contact  or 
admixture  of  colder  air,  it  ceases  to  be  true  steam,  and  is  condensed  into 
small  particles  of  water  suspended  in  the  air.  Many  persons  while  thinking 
of  steam,  figure  it  only  in  this  latter  state,  as  particles  of  water  mixed  with 
air  nearly  as  a  subtile  powder  might  be  mixed,  and  its  substance  occupying 
really  no  more  space  than  the  original  water  did.  Now  uiAil  steam  is  cool 
and  condensed,  it  is  of  a  nature  to  fill  alone  any  appropriate  vessel  and 
powerfully  distend  it,  just  as  air  fills  and  distends  a  bladder.  Steam  issuing 
from  the  spout  of  a  kettle  is  hardly  seen  near  the  mouth,  but  as  its  distance 
from  the  spout  increases,  it  is  cooled  into  a  thick  cloud  or  vapour. 

In  a  vessel  from  which  air  and  atmospheric  pressure  are  excluded,  even 
the  temperature  of  freezing  air  being  sufficient  to  maintain  permanently  in 
the  state  of  gas  or  air,  many  substances  which  exist  as  liquid  under  the 
atmospheric  pressure — and  the  whole  mass  of  such  a  substance,  when  placed 
in  a  vacuum,  not  being  instantly  converted  into  gas,  because  the  portion 
which  first  rises  becomes  an  atmosphere  weighing  upon  the  remaining  mass, 
and  because,  moreover,  that  portion,  by  absorbing  from  the  mass  much  heat 
into  the  latent  state,  cools  the  mass  much  below  the  freezing  point;  we  see 
why  the  liquids  now  spoken  of  are  so  rapidly  cooled  to  at  least  the  freezing 
point  if  placed  where  a  vacuum  can  be  maintained,  that  is  to  say,  where, 
•after  common  air  has  been  removed,  the  aeriform  matter  arising  from  them, 
and  absorbing  their  heat,  is  also  promptly,  and  in  a  continued  manner, 
abstracted.  It  is  thus  that  water  placed  in  an  exhausted  receiver  of  an  air- 
pump  is  so  rapidly  cooled,  and  that  when  there  is  beside  it  a  vessel  of  con- 
centrated sulphuric  acid,  or  other  substance  capable  of  absorbing  the  watery 
vapour  as  formed,  it  is  soon  reduced  to  the  state  of  ice';  or  again,  that 
water,  or  even  mercury,  surrounded  by  ether  evaporating  in  a  vacuum,  is 
so  quickly  frozen.  It  is  thus,  also,  that  if  one  bulb  of  the  instrument 
described  at  page  296,  be  immersed  in  a  freezing  mixture,  the  water  in  the 
other  and  distant  bulb  will  soon  become  ice ;  for  the  vapor  arising  from 
that  water  in  the  vacuum  maintained  throughout  the  apparatus  by  the 
freezing  mixture,  is  immediately  condensed  again  in  the  immersed  bulb, 
and  leaves  the  vacuum  still  free  for  the  ascent  of  more  vapour,  to  carry 
away  more  heat  from  the  water  as  latent  heat,  and  to  make  it  freeze. 

As  we  have  explained,  also,  that  in  a  liquid  there  is  the  same  tendency 
to  evaporation,  whether  it  be  or  be  not  exposed  to  the  air,  we  see  the  reason 
why  all  evaporation  is  a  very  cooling  process.  The  effect,  however,  in  air 
is  neither  so  rapid  nor  so  great  as  in  a  vacuum  :  first,  because  the  presence 
of  the  air  impedes  the  spreading  of  the  newly-formed  vapour  from  the  liquid 
surface,  and  keeps  it  where  its  pressure  resists  the  formation  of  more  vapour ; 
and,  secondly,  because  the  air  in  contact  with  the  liquid,  shares  its  higher 
temperature  with  the  liquid.  Still,  in  India,  flat  dishes  of  water,  placed 
during  the  night  on  beds  of  twigs  and  straw  kept  wet  and  in  a  current  of 
air,  soon  exhibit  thin  cakes  of  ice — and  thus,  ice  is  procured  in  India  for 
purposes  of  luxury. 

The  absorption  of  latent  heat  in  the  evaporation  which  goes  on  from  the 
sea  and  earth  in  all  warm  climates,  greatly  tempers  the  heat  of  these  climates, 


EVAPORATION.  299 

and  the  vapour  afterwards  spreading  to  the  poles,  as  explained  in  "Pneuma- 
tics" under  the  head  of  Winds,  carries  warmth  thither  to  be  given  out 
when  it  is  recondensed  into  the  form  tof  rain,  or  is  solidified  as  snow.  The 
formation  any  where  of  mist  or  rain  warms  the  air  most  sensibly,  by  the 
liberation  of  the  latent  heat  from  the  precipitated  vapour.  Again,  the  liquid 
water,  which,  during  winter,  is  converted  into  snow  or  ice,  had  been  a  re- 
servoir of  latent  heat  stored  to  temper  the  frosty  air  of  the  commencing  cold 
season ;  and  in  the  following  spring,  such  ice  and  snow  serve  as  empty  re- 
ceptacles, in  which  the  first  violence  of  the  returning  sun  hides  or  expends 
itself;  allowing  the  temperature  to  change  more  gradually,  and  for  many 
living  beings,  therefore,  more  safely.  The  vast  stores  of  ice  and  snow 
among  high  mountains,  as  among  the  Alps  and  Pyrenees,  are  often,  during 
the  summer,  stores  of  mild  temperature  to  regions  around  :  for  besides  cool- 
ing the  air  near  them,  they  are  the  never-failing  sources  of  the  rivers  which 
run  from  them  during  the  whole  of  summer,  carrying  freshness  through  dis- 
tant lands  : — from  the  Alps,  for  instance,  proceed  the  Rhine  and  the  Rhone 
— most  romantic  and  beautiful  of  European  streams ;  and  from  the  Pyrenees, 
the  rapid  Gave,  &c.,  which,  while  channels  around  from  lower  regions  are 
almost  dried  up  by  the  summer  Jieat,  flows  only  the  more  freshly  as  the  heat 
is  greater,  and  the  feeding  snows  are  more  abundantly  dissolved. 

Men  in  artificially  raising  temperature,  are  generally  causing  the  liberation 
of  heat  which  had  been  previously  latent ;  and  in  lowering  temperature 
or  producing  cold,  they  affect  their  purpose  almost  solely  by  rendering  a 
quantity  of  heat  latent. 

Lavoisier  thought  that  the  heat  of  all  combustion  was  merely  the  latent 
heat  of  the  oxygen  gas  concerned  in  the  combustion,  given  out  during  its 
combination  with  the  burning  body.  It  is  so  in  part,  but  we  now  know  that 
it  depends  more  on  the  intensity  of  the  chemical  action  between  thecombin- 
'ing  substances.  The  water  thrown  upon  quick-lime  to  slake  it,  becomes 
solid  in  combination  with  the  lime,  and  gives  out  its  latent  heat  so  remark- 
ably as  often  to  set  fire  to  a  wooden  vessel  or  ship  containing  it. 

When  dwelling-houses,  green-houses,  manufactories,  &c.,  are  warmed,  as 
is  now  common,  by  the  admission  of  steam  into  systems  of  pipes  which 
branch  over  them,  the  heat  is  chiefly  that  previously  latent  in  the  steam, 
and  which  spreads  around  as  soon  as  the  steam,  by  touching  pipes  of  a  lower 
temperature,  is  condensed  to  a  state  of  water.  The  modes  of  most  profitably 
effecting  these  purposes  have  to  be  considered  in  a  future  chapter. 

For  producing  artificial  cold,  our  processes  generally  involve  the  circum- 
stance either  of  a  solid  changing  into  a  liquid,  during  which  it  absorbs,  and 
hides  in  its  new  constitution  much  of  the  heat  previously  sensible  in  it  and 
in  the  liquid  dissolving  it ;  or  of  a  liquid  changing  into  vapour,  during  which 
heat  equally  becomes  latent.  Thus  by  dissolving  a  salt,  nitre,  for  instance, 
in  water,  we  obtain  a  solution  which  is  very  cold. 

In  India,  the  common  mode  of  cooling  wine  for  table  is  to  surround  the 
bottles  with  nitre  thus  melting ;  and  the  water  of  the  solution  being  evapor- 
ated again  before  next  day,  the  salt  is  left  ready  for  use  as  before.  Such  is 
the  mutual  attraction  of  water  and  many  salts,  that  they  readily  combine, 
assuming  the  liquid  form,  even  when  water  is  used  in  the  solid  state  of 
ice  ;  and  as,  in  that  case,  both  the  water  and  the  salt  render  heat  latent,  the 
fall  of  temperature  is  very  great.  Thus  common  salt  and  snow  (or  ice) 
when  mixed,  dissolve  into  liquid  brine  37°  colder  than  freezing  water,  or  5° 
below  the  zero  of  Fahrenheit. — The  last  mentioned  fact  explains  the  com- 


300  HEAT. 

mon  practice  of  sprinkling  salt  on  an  ice-covered  pavement  before  a  street 
door  to  clear  away  the  ice.  The  salt  and  ice  quickly  combine  and  form 
liquid  brine,  which  either  of  itself  runs  off  into  the  gutter  and  disappears,  or 
is  easily  swept  of,  or  its  water  evaporates,  leaving  only  the  salt  behind.  It 
is  true  that  the  brine  is  at  first  a  refrigerating  mixture,  which  cools  still  more 
the  pavement  and  the  neighboring  ice,  but  all  which  is  touched  by  the  salt 
is  melted.  Servants  usually  err  in  using  a  pickaxe  or  spade  immediately 
after  the  sprinkling,  instead  of  waiting,  and  with  a  broom  spreading  the 
melting  salt  couipleatly  over  the  place. 

Frigorific  Mixtures. 
Substances  mixed.  Thermometer  sinks. 

Common  salt  ....  1  part  j  From  any  temperature  to  5°  below 
Snow  or  pounded  ice  .  .  2  —  j  zero. 

Common  salt  5  •  —    \  ^  ^e 

Snow  or  ice  .....  12  —    I  From  an^  temPerature  to  25 
Nitrate  of  ammonia     .     .     5  —    j 

MuTed  sulphuric  acid      !     2  Z   }  From'32°  abo™  to  23°  bel°* 

Fused  Potass     .     .     .     .     4  —    )  ™        000    ,  Kno  ,   , 

g  o  -from  32°  above  to  51    below  zero. 

Nitrate  of  ammonia  1  —  -       ^ 

From  50°  to  4°  above  zero. 


Sulphate  of  soda     .     .     .     8  —    )  ™        K  A0  ,    A0 
Muriatic  acid     .     .     .     .     5  -    j  From  50    to  °    or  zero" 

We  have  already  described  under  other  heads  the  frigorific  effect  of 
evaporating  in  a  vacuum  or  in  the  air,  and  of  the  operation  of  condensing 
a  gas  to  squeeze  the  heat  out  of  it  before  letting  it  expand  again  to  a  great 
volume. 

for  any  given  substance,  the  changes  of  state  from  solid  to  liquid,  and  from 
liquid  to  air,  happen  under  similar  circumstances,  so  precisely  at  the 
same  temperature,  that  they  mark  fixed  points  in  a  general  scale  of  tem- 
perature, and  enable  us  to  regulate  and  compare  our  various  thermome- 
ters. (See  Analysis,  page  256.) 

As  we  can  neither  weigh  heat,  nor  measure  its  bulk,  nor  see  it,  and  as, 
even  if  our  sense  of  touch  were  a  correct  judge  in  the  matter,  which  it  is 
not,  we  dare  not  touch  things  which  are  very  hot  or  cold,  some  other  means 
were  wanted  for  estimating  the  presence  in  bodies  of  this  very  subtile  prin- 
ciple ;  —  and  a  mean  has  been  found  in  the  measurement  of  its  most  obvious 
and  constant  effect,  namely  that  dilatation  or  expansion  of  bodies,  which 
again  ceases  when  the  heat  is  withdrawn.  Any  substance  so  circumstanced 
as  to  allow  this  expansion  to  be  accurately  measured,  becomes  to  us  a  ther- 
mometer or  measure  of  heat. 

In  solid  substances  the  direct  expansion  by  heat  is  so  small  as  to  be  seen 
or  measured  with  diffculty.  In  airs,  again,  the  expansion  is  very  extensive  ; 
but  there  is  the  objection  that  in  any  apparatus  yet  contrived  which  will 
allow  their  expansion  completely  to  appear,  they  cannot  be  protected  from  the 
varying  pressure  of  the  atmosphere  —  an  influence  which  affects  their  volume 
even  more  than  common  changes  of  temperature.  But  liquids  are  free  from 
both  disadvantages,  and  when  placed  in  a  glass  bulb,  as  «,  having  a  long 


TABLE    OF     TEMPERATURE. 


301 


o 


neck  or  stalk  a  b,  into  which  the  liquid  may  rise  when  expanded  by  heat;  to 
be  measured,  they  form  the  most  generally  convenient 
of  thermometers.     Then,  among  liquids,  mercury  is,  Fig.  134. 

on  several  accounts,  singularly  pre-eminent.     In  mer-         £ 
cury,  the  range  of  temperature  between  freezing  and 
boiling  reaches  a  higher  point  than  in  any  other  liquid,       B 
and  a  lower  point  than  in  all  others  except  alcohol;  its 
little  capacity  for  heat  and  ready  conducting  power, 
causes  it  to  be  very  quickly  affected  by  change  of  tem- 
perature ;  its  expansion  is  singularly  equable  for  equal 
increase  of  heat  through  the  important  middle  part  of 
the  scale,  which  includes  the  common  temperature  on 
earth,  namely,  from  freezing  to  the  boiling  heat  of       F 
water  ;  and  it  is  easy  to  proportion  the  bulb  and  the  \\ 

stalk  to  each  other,  so  that  a  small  difference  of  tern-       a  O 
perature  shall  cause  the  mercurial  column  in  the  stalk 
to  rise  or  fall  very  conspicuously. 

Now,  when  the  important  fact  was  ascertained  that  solid  water  or  ice  melts 
in  every  case  at  precisely  the  same  temperature,  and  that  pure  liquid  water 
in  a  metallic  vessel,  and  under  a  given  atmospheric  pressure,  boils  always  at 
the  same  temperature,  it  followed  that  by  placing  such  a  thermometer,  as 
above  described,  first  in  melting  ice  and  then  in  boiling  water,  and  marking 
upon  the  stalk  the  two  points  at  which  the  mercury  stood,  as  represented 
here  by  F  and  B,  two  fixed  or  variable  points  would  be  obtained,  and  the 
interval  between  them  might  be  divided  on  the  glass,  or  on  a  suitable  scale 
to  be  attached  to  the  glass,  into  any  convenient  number  of  parts  to  be  called 
degrees  ;  it  followed  farther,  that  by  continuing  the  division  to  any  extent 
both  above  and  below  the  fixed  points,  a  general  scale  of  temperature  would 
be  obtained,  with  respect  to  which  all  thermometers  made  on  the  same  prin- 
ciple would  perfectly  agree,  although  the  size  of  the  divisions  on  the  stalks 
would  vary  according  to  the  comparative  capacities  of  the  bulk  and  stalk  in 
the  different  instruments.  Our  Newton  had  the  honor  first  to  propose  the 
regulating  points  of  freezing  and  boiling,  and  they  are  now  universally 
adopted,  but  the  interval  between  them  has  been  variously  subdivided ; — that 
is  to  say,  there  has  not  been  agreement  among  philosophers  as  to  what  they 
would  call  a  degree  of  heat.  In  the  Centigrade  thermometer,  which  is  the 
most  simple,  the  division  is  into  100  equal  parts ;  in  Reaumur's,  which  is 
commonly  used  in  France,  it  is  into  80  parts ;  and  in  Fahrenheit's,  which  is 
used  in  England,  it  is  into  180°.  In  Fahrenheit's,  moreover,  the  freezing 
point,  instead  of  being  called  zero,  as  in  the  others,  is  called  32°,  because 
the  maker  choose  to  begin  counting  from  the  lowest  heat  which  he  met  in 
Iceland,  and  which  was  32°  below  freezing  of  his  scale. — To  turn  the 
degrees  of  any  one  of  these  thermometers  into  degrees  of  any  other,  we 
have  only  to  recollect  that  9°  of  Fahrenheit  are  equal  to  5°  of  the  Centi- 
grade, and  to  4°  of  Reaumer.  Therefore,  multiplying  by  9  and  dividing  by 
5  or  4,  or  the  reverse,  and  adding  or  subtracting  the  32°  of  Fahrenheit, 
gives,  as^the  result,  the  degree  desired.  , 

The  bulb  of  a  mercurial  thermometer  is  formed  by  heating  to  fusion  in 
the  flame  of  a  lamp,  the  end  of  a  glass  tube,  which  has  a  very  small  and 
equable  bore,  and  then  blowing  into  the  tube  until  the  softened  end  swells 
like  a  soap-bubble,  to  the  size  desired.  The  mercury  is  foreed  into  such  a 
bulb  through  its  long  stalk  by  the  pressure  of  the  atmosphere, — thus.  First, 
a  portion  of  the  air  originally  in  the  bulb  being  expelled  by  warming  the 
bulb,  the  open  end  of  the  stalk  is  immersed  in  mercury,  and  when  the  air 


302 


HEAT. 


remaining  in  the  bulb  cools  and  contracts,  a  little  mercury  enters.  Secondly 
this  admitted  mercury  having  been  made  to  boil,  so  as  to  fill  with  its  vapour 
the  whole  capacity  of  the  bulb  and  tube,  and  to  expel  the  air,  on  the  open 
end  being  again  immersed  in  mercury,  and  the  mercurial  vapour  within  being 
condensed,  the  atmosphere  presses  in  fresh  mercury  to  fill  the  whole  vacu- 
um. To  complete  the  making  of  the  thermometer,  the  bulb  is  again  heated 
to  expel  so  much  of  the  mercury  as  that  when  cold,  the  tube  shall  be  about 
one-third  full  of  it,  and  then,  before  the  heated  mercury  begins  to  recede, 
the  end  or  opening  is  hermetically  closed  by  directing  upon  it  the  point  of 
a  blow-pipe  flame  which  fuses  the  glass. 

Although  the  direct  expansion  of  any  solid  body  by  a  moderate  change 
of  temperature  is  so  inconsiderable  as  to  be  with  difficulty  measured,  M. 
Breguet,  of  Paris,  lately  with  much  ingenuity  contrived  a  thermometer 
which  makes  it  very  evident.  Having  soldered  side  by  side  two  very  small 
flattened  wires  of  silver  and  platinum,  or  of  any  other  metals  having  different 
expansibility  by  heat,  he  found  that  all  changes  of  temperature  made  such 
compound  wires  bend  to  a  great  extent,  as  a  sheet  of  damp  paper  curls  on 
being  held  before  the  fire.  The  metal  most  shortened  or  least  lengthened 
acting  like  a  bow-string  to  pull  the  other  into  the  arched  form  ;  and  he  then 
found,  on  giving  to  the  compound  wire  a  spiral  or  cork-screw  form,  and 
securing  the  upper  end  of  it  to  a  fixed  stand,  while  the  lower  was  left  free  to 
move,  that  an  index  like  the  hand  of  a  watch  attached  to  the  lower  end  was 
turned  completely  round  by  a  certain  change  of  temperature,  and  afforded 
on  a  circle  of  degrees  placed  like  a  watch  face  below  it,  indications  which 
perfectly  agreed  with  those  of  good  mercurial  thermometers.  Other  modifi- 
cations of  the  same  principle  have  since  been  successfully  tried,  so  simplified 
anfl  reduced  in  bulk  as  to  be  introduced  into  the  structure  of  a  pocket  watch. 
Air  is  a  substance  on  several  accounts  admirably  adapted  to  the  formation 
of  a  thermometer ;  for  it  has  great  extent  of  dilatation  from  small  increase  of 
heat ;  it  quickly  receives  impressions,  and  its  dilatation  is  equal  for  equal 
increments  of  heat  at  all  temperatures; — but,  as  already  stated,  there  is  the 
strong  objection  that  the  pressure  of  the  atmosphere  cannot  be  excluded, 
without  at  the  same  time  confining  the  air,  and  effecting  its  expansion.  Mr. 
Leslie,  however,  has  used  for  particular  purposes  an  air  thermometer,  which 
he  calls  the  differential  thermometer.  It  consists  of  two  bulbs  a  and  b,  filled 
with  air  and  connected  by  a  bent  tube  d  c,  containing 
liquid,  the  instrument  being  hermetically  sealed,  so  that 
the  atmosphere  cannot  affect  the  air  within.  The  greater 
heat  in  the  bulb  b  than  in  the  other,  as  when  that  bulb  is 
touched  by  the  warm  hand  or  is  exposed  to  the  sun's  ray, 
is  marked  by  the  descending  of  the  liquid  in  the  tube  d, 
which  has  a  scale  attached  to  it. — We  may  observe  that 
equal  divisions  or  degrees  marked  on  the  scale  of  this 
thermometer,  do  not  mark  equal  changes  of  temperature, 
for  the  increasing  condensation  and  resistance  of  the  air  in 
the  cold  bulb  require  the  force  overcoming  it  progressively 
to  increase.  If  the  resistance,  on  the  contrary  ,4  were  un- 
varying, as  in  an  air  thermometer  open  to  a  steady  atmo- 
sphere, equal  extent  of  motion  in  the  fluids  would  mark 
equal  increments  of  heat.  An  air-thermometer  made  of  a 
simple  bulb  and  long  stalk  of  semi-transparent  porcelain, 
with  the  mouth  downwards,  and  containing  in  its  neck 
melted  lead  or  other  fusible  metal  instead  of  mercury,  is 
well  adapted  for  measuring  very  high  temperatures. 


Fig.  135. 


TABLE    OF    TEMPERATURE. 


303 


Temperatures,  below  that  of  freezing  mercury,  are  usually  measured  by 
alcohol  which  substance  has  not  yet  been  frozen  ;  and  temperatures  higher 
than  of  boiling  mercury,  are  measured  by  the  expansion  of  air  or  of  metals, 
as  above  described,  or  by  the  contraction  of  pieces  of  baked  clay,  which, 
when  highly  heated,  lose  water  and  become  semi-vitrified,  The  use  of  baked 
clay  was  proposed  by  Wedgewood,  and  the  apparatus  has  been  called 
Wedgewood's  Pyrometer,  or  fire  measure.  All  contrivances  for  measuring 
heat  may  be  graduated  so  as  to  correspond  with  the  scale  adopted  for  the 
mercurial  thermometer. 

It  is  most  interesting,  while  considering  the  vast  number  and  importance 
of  the  phenomena,  produced  by  heat,  to  observe  the  degrees  in  the  general 
scale  of  temperature  at  which  these  severally  take  place.  In  the  following 
table,  a  selection  of  the  facts  is  classified,  the  temperatures  being  all  referred 
to  the  scale  of  Fahrenheit's  thermometer. 

Table  of  facts  connected  with  the  influence  of  heat  corresponding  to  certain 

temperatures. 


Wedgewood. 


Highest  temperature  measured     .  .  .     240° 

Chinese  porcelain  softened  .  .  .     156 

Cast-iron  thoroughly  melted         .  .  .     150 

Greatest  heat  of  a'commen  smith's  forge  .  .     125 

Mint  glass  surface  ....     11-4 

Stone  ware  baked  in  .  .  .     102 

Welding  heat  of  iron       .  .  .  92  to  95 

Delft  ware  baked  in  .  .  .41 

Fine  gold  melts  .  .  .  .  .32 

Settling  heat  of  flint  glass  .  .  .       29_ 

Fine  silver  melts  .  .  .  .28 

Brass  melts         .  .  .  .  .21 

Full  red  heat  (tlie  beginning  of  Wedgewood1  s 

Pyrometer)      .  .  .  .  .0 

Heat  of  a  common  fire     ...... 

Iron  red  in  the  dark        . 

Quicksilver  boils  .  .  .  .          "  . 

Jiinseed  oil  boils  ...... 

Lead  melts  . 

Sulphur  melts    .  .  . 

Water  boils         ....... 

A  compound  of  three  parts  tin,  five  of  lead,  and  eight  of  bismuth 

melts  .  ..... 

Alcohol  boils     .  .  .  ... 

Bees'-wax  melts  ...... 

Ether  boils         ....... 

The  present  medium  temperature  of  the  globe    ... 
Water  freezes    ....... 

Milk  freezes       ....... 

Vinegar  freezes  ...... 

Strong  wine  freezes        ...... 

Weak  brine  freezes         ....  zero 

Quicksilver  freezes  .  .  .  .  below  zero 

The  air  sometimes  at  Hudson's  Bay      .... 

Greatest  artificial  cold  yet  measured       .... 


Fahrenheit. 

32,277° 
21,357 
20,577 
17,327 
15,897 
14,337 
13,427 
6,407 
5,237 
4,847 
4,717 
3,807 

1,077 
790 

750 
660 
600 
594 
226 
212 

210 
174 
142 

98 
50 
32 
30 
28 
20 
0 

40 
50 
91 


304  HEAT. 

There  is  reason  for  thinking  that  the  higher  temperature  noted  in  this 
table  are  considerably  too  high,  owing  to  the  insufficiency  of  the  thermometer 
or  Pyrometer  (Wedgewood's)  by  which  they  were  estimated. 

It  is  a  curious  inquiry,  suggested  by  contemplating  the  preceeding  table, 
how  much  heat  may  yet  remain  in  bodies  at  the-  lowest  temperature  which 
we  know  ?  No  conjecture  was  hazarded  on  the  subject  until  Dr.  Irving  thought 
it  might  be  elucidated  by  comparing  the  quantity  of  heat  which  becomes 
latent  in  a  body  on  changing  form,  with  the  capacity  of  the  body  before  and 
after  the  change.  For  instance,  with  respect  to  water,  he  said  :  as  it  requries 
one-tenth  more  heat  to  make  a  certain  change  in  the  temperature  of  water 
than  of  an  equal  quantity  of  ice,  probably  ice-cold  water  contains  altogether 
just  one-tenth  more  heat  than  of  an  equal  quantity  of  ice  at  the  melting  point  : 
then  as  we  know  the  water  to  contain  exactly  140°  more  heat  than  the  ice, 
viz.,  its  latent  heat,  the  whole  or  absolute  quantity  of  heat  in  it  will  be  ten 
times  140°,  or  1,400°.  By  applying  this  reasoning,  however,  to  other  sub- 
stances than  water,  it  evidently  is  fallacious  ;  and  the  conclusion  follows  that 
we  have  as  yet  no  means  of  solving  the  question  ;  —  the  thermometer  no  more 
telling  us  the  absolute  quantity  of  heat  in  any  body  than  the  rising  and  falling 
of  the  tide  between  any  two  rocks  tells  us  the  total  depth  of  the  rocky  chasm. 

From  what  is  said  in  the  last  and  in  preceding  paragraphs,  it  is  evident 
that  the  thermometer  gives  very  limited  information  with  respect  to  heat  :it 
merely  indicates,  in  fact,  what  may  be  called  the  tension*  of  heat  in  bodies, 
or  the  tendency  of  the  heat  to  spread  from  them.  Thus  it  does  not  discover 
that  a  pound  of  water  takes  thirty  times  as  much  heat  to  raise  its  temperature 
one  degree  as  a  pound  of  mercury  ;  nor  does  it  discover  the  caloric  of  fluidity 
absorbed  when  bodies  change  their  form,  and  which  is  called  "  latent  heat" 
only  because  hidden  from  the  thermometer  ;  nor  does  it  tell  that  there  is  more 
heat  in  a  gallon  of  water  than  in  a  pint  ;  and  if  an  observer  did  not  make 
allowance  for  the  increasing  rate  of  expansion  with  increasing  temperature, 
in  the  substance  used  as  a  thermometer,  he  would  believe  the  increase  of  heat 
to  be  greater  than  it  is;  and  lastly,  when  a  fluid  is  used  as  a  thermometer, 
the  expansion  observed  is  only  the  excess  of  the  expansion  in  a  fluid  over 
that  in  the  containing  solid,  and  subject  to  the  irregularities  of  expansion  in 
both  substances  ;  —  all  proving  that  the  indications  of  the  thermometer,  unless 
interpreted  by  other  circumstances  and  our  knowledge  of  the  general  laws 
of  heat,  no  more  disclose  the  true  relations  of  heat  to  bodies,  than  the  money, 
accidentally  in  a  man's  pocket  tells  his  rank  and  riches. 


t)  ~by  its  different  relations  to  different  substances,  has  a  powerful  influ- 
ence on  their  chemical  combinations."     (See  Anaylsis,  page  266.) 

By  observations  made  and  recorded  through  past  ages,  man  has  now  come 
to  know  that  the  substances  constituting  the  world  around  him  although 
appearing  to  differ  in  their  nature,  almost  to  infinity,  are  yet  all  made  up  of 
a  few  simple  elements  variously  combined  ;  and  he  has  discovered  that  the 
peculiar  relations  of  these  elements  to  heat,  particularly  their  being  unequally 
expanded  by  it,  and  their  undergoing  fusion  and  vaporization  at  different 
temperatures,  furnish  him  with  ready  means  of  separating,  combining,  and 
new-modifying  them  to  serve  to  him  most  useful  purposes.  Where  the  primi- 
tive savage,  looking  around  on  rocks  and  soils,  saw  in  their  diversified  aspect 
almost  as  little  meaning  as  did  the  inferior  animals  which  participated  with 
him  the  shelter  of  the  wood  or  cave,  his  descendant,  with  penetration  sharp- 
ened by  science,  descries  at  once  the  treasures  of  the  mine,  and  aided  by 


ABSOLUTE    QUANTITY    OP    HEAT.  305  . 

heat,  whose  wonderful  energies  he  has  learned  to  control,  pursues  through 
all  the  Protean  disguises  of  ores,  and  salts,  *and  solutions,  each  of  the  wished- 
for  substance,  until  he  secures  it  apart/  For  instance,  in  what  to  his  fore- 
fathers for  thousands  of  years  appeared  but  a  red  dross,  he  knows  that  there 
lies  concealed  the  precious  iron — king  of  metals  !  and  soon  forcing  tjiis  in 
his  ardent  furnace  to  assume  the  metallic  form,  he  afterwards,  with  imple- 
ments made  of  it,  moulds  all  other  bodies  to  his  will :  the  trees  from  the 
forest,  and  the  rocks  from  the  quarry,  in  obedience  to  these,  are  fashioned 
by  him  as  if  they  were  of  soft  clay,  and  at  his  command  rise  into  magnificent 
structures  of  palaces  and  ships,  with  which  the  earth  and  the  ocean  are  now 
so  thickly  covered. — The  minute  detail  of  the  relations  to  heat  of  particular 
substances  forms  a  great  portion  of  the  department  of  science  called  chemistry 
(a  name  taken  from  an  Arabic  word  signifying  fire;)  but  a  general  review 
of  the  subject  belongs  to  this  work. 

The  most  common  ores  of  metals  are  combinations  of  the  metals  with 
oxygen,  carbonic  acid,  or  sulphur,  substances  all  of  which  are  volatilized  at 
much  lower  temperatures  than  the  metals.  Now  simply  roasting,  as  it  is 
called,  or  strongly  heating  the  ores,  suffices  often  to  drive  away  great  part 
of  these  adjuncts;  and  where  additional  assistance  is  required,  it  is  obtained 
by  mixing  with  the  ore  something  which  when  heated  attracts  the  substance 
to  be  expelled  more  strongly  than  the  metal  does.  Charcoal,  for  instance, 
heated  with  oxide-ore,  takes  the  oxygen,  and  flying  off  with  it  as  carbonic 
acid,  leaves  at  the  bottom  of  the  furnace  or  crucible  the  vivified  or  pure  metal. 

Mercury  mixed  with  the  dross  of  a  mine,  dissolves  any  particles  of  gold 
or  of  silver  existing  in  the  dross,  and  the  ingredients  of  the  solution  may 
afterwards  be  obtained  separate  by  mere  heating — the  mercury  passing  away 
as  vapour  to  where  it  is  cooled  and  again  condensed  for  subsequent  use,  and 
the  more  fixed  gold  or  silver  remaining  in  the  vessel — and  just  as  in  all  other 
distillations,  like  that  of  spirit  from  wine,  or  of  essential  oils  from  water,  &c., 
there  is  the  separation  by  heat  of  a  more  volatile  from  a  less  volatile  sub- 
stance. The  only  difference  between^what  is  called  drying  by  heat  and  dis- 
tilling, is  that  in  the  one  case  the  substance  vaporized,  being  of  no  use,  is 
allowed  to  escape  or  become  dissipated  in  the  atmosphere;  while  in  the  other, 
being  the  precious  part,  it  is  caught  and  condensed  into  the  liquid  form. 
The  vapour  from  drying  linen,  if  caught  would  be  distilled  water.  The 
abundant  vapour  from  wheaten  bread  while  being  baken,  is  chiefly  a  spirit 
like  what  is  obtained  from  malt,  and  by  an  ingenious  apparatus  lately  con- 
trived, may  be  caught  and  preserved. 

A  piece  of  cold  charcoal  lies  in  the  air  for  any  length  of  time  without 
change :  but  if  heated  to  a  certain  degree,  the  mutual  cohesion  of  its  particles 
is  so  weakened,  in  other  words,  the  particles  are  so  repelled  and  separated 
from  each  other,  that  their  attraction  for  the  oxygen  in  the  air  around  is 
allowed  to  operate,  and  they  combine  with  that  oxygen,  so  as  to  produce  the 
phenomenon  of  combustion.  The  same  is  true,  under  similar  circumstances, 
of  almost  any  dry  vegetable  or  animal  substances,  and  of  several  of  the 
metals. 

Nitre,  sulphur  and  charcoal,  while  cold,  may  be  mixed  together  most 
intimately  without  any  change  taking  place ;  but  if  the  mixture  or  any  part 
of  it,  be  heated  to  a  certain  degree,  the  whole  explodes  with  extreme  violence, 
for  it  is  gunpowder.  By  the  change  of  temperature,  and  the  consequently 
altered  relative  attractions  of  the  different  substances,  a  new  chemical 
arrangement  of  them  then  takes  place  with  the  intense  combustion  and 
expansion,  which  constitute  the  explosion. 

20 


306  HEAT. 

Sea  sand  and  soda  very  intimately  mixed,  and  even  ground  together,  if 
remaining  cold,  remain  also  merely  an  opaque  and  useless  powder  j  but  if 
the  mixture  be  heated,  to  diminish  the  cohesion  of  the  particles  of  each  sub- 
stance to  those  of  its  own  kind,  so  that  the  mutual  attraction  of  the  two  sub- 
stances may  come  into  play,  the  two  substances  melt  together,  and  unite 
chemically  into  the  beautiful  compound  called  glass  ;  a  product  than  which 
art  has  formed  none  more  admirable — which,  in  domestic  use  alone,  is  fash- 
ioned into  the  brilliant  chandelier  and  lustre,  into  the  sparkling  furniture  of 
the  side-board,  into  the  magnificent  mirror  plate,  and  when  extended  across 
the  window  opening,  admits  the  light  while  it  repels  the  storm. 

Perhaps  the  influence  of  temperature  on  chemical  union  is  nowhere  more 
remarkably  exhibited,  than  in  retarding  or  hastening  the  decomposition  of 
dead  vegetable  and  animal  substances.  The  functions  of  life  bring  into 
combination  to  form  the  various  textures  of  these  organic  or  living  bodies, 
chiefly  four  substances,  viz.,  carbon,  or  coal ;  the  ingredients  of  water,  or 
oxygen  and  hydrogen  ;  and  lastly,  nitrogen — which  substances,  when  in  the 
proportions  found  in  such  bodies,  have  but  slight  attractions  for  each  other, 
and  all  of  which,  except  the  carbon,  usually  exist  as  airs.  Their  connection, 
therefore,  is  easily  subverted ;  and  particularly  by  a  slight  change  of  tem- 
perature, which  either  so  weakens  their  mutual  hold  as  to  allow  new  arrange- 
ments to  be  formed,  or  altogether  disengages  the  more  volatile  of  "them. — At  a 
certain  temperature,  a  solution  of  sugar  (which  consists  of  the  three  substances 
first  mentioned,  carbon,  oxygen,  and  hydrogen)  undergoes  a  change  into  a 
spirituous  wash,  from  which  spirit  or  alcohol  may  then  be  obtained  by  dis- 
tillation :  but  if  the  heat  be  continued  under  certain  circumstances  the  liquid 
undergoes  a  second  change,  or  new  arrangement  of  constituent  particles,  and 
becomes  vinegar :  under  still  other  circumstances,  it  undergoes  a  third  change, 
which  is  a  destructive  decomposition,  or  rotting,  as  we  call  it,  and  the  oxygen 
and  hydrogen  ascends  away  as  airs.  But  sugar  and  many  similar  vegetable 
compounds,  preserved  at  a  low  temperature,  remain  unchanged  for  ages. 

Again,  as  regards  dead  animal  substances,  we  find  that  although  at  a  certain, 
not  very  elevated  temperature,  they  undergo  that  change  in  the  relations  of 
their  elements  which  we  call  putrefaction,  during  which  nearly  their  whole 
substances  rise  again  to  form  part  of  the  atmosphere,  still  at  or  below  the 
temperature  of  freezing  water,  they  remain  unaltered  for  any  length  of  time. 

In  the  middle  of  summer,  recently  caught  salmon,  or  other  fish,  packed  in 
boxes  with  ice,  may  be  conveyed  fresh  from  the  most  remote  parts  of  Britain 
to  the  capital.  In  our  warmest  weather,  any  meat  or  game  may  be  long 
preserved  in  an  ice-house.  In  Russia,  Canada,  and  other  northern  countries, 
on  the  setting  in  of  the  hard  frosts,  when  food  for  the  cattle  and  poultry  is 
with  difficulty  procured,  the  inhabitants  kill  their  winter  supply,  and  store  up 
their  provender  of  frozen  flesh  or  fowl,  as  in  other  countries  men  store  that 
which  is  salted  or  pickled. — The  most  striking  illustration  which  we  can 
adduce  of  this  kind  is  the  fact  that  on  the  shore  of  Siberia,  in  1801,  in  a  vast 
block  or  island  of  ice,  then  accidentally  broken  and  partially  melted,  the  car- 
cass of  what  has  been  called  the  antediluvian  elephant  was  found,  perfectly 
preserved — an  elephant  differing  materially  from  those  now  existing  on  earth, 
and  having  a  skelton  exactly  similar  to  the  fossil  specimens  found  deep 
buried  in  various  countries.  The  carcass  was  soon  discovered  by  the  hungry 
bears  of  the  district,  which  were  seen  eagerly  feeding  on  its  flesh,  as  if  it  had 
died  but  yesterday,  although  it  must  have  been  of  an  era  long  anterior  to  that 
of  any  existing  monument  on  earth,  of  human  art,  or  even  of  human  being. 
After  it  had  fallen  from  the  ice  to  the  sandy  beach,  and  its  tusks  had  been 


INFLUENCE    OP    HEAT    ON    ANIMA'TED    BEINGS.    307 

carried  away  for  sale  by  a  Tunguisian  Fisherman,  and  much  of  its  flesh  had 
been  devoured,  a  naturalist,  from  St.  Petersburg  who  visited  it  foumd  an  ear 
still  perfect,  and  its  long  mane,  and  a«part  of  its  upper  lip,  and  an  eye  with 
the  pupil,  which  had  opened  on  the  glories  of  a  former  or  younger  world  ! 
About  30  Ibs.  weight  of  its  hair,  which  had  been  trodden  into  the  sand  by 
the  bears  while  eating  the  carcass,  and  part  of  the  skin,  were  preserved,  and 
are  now  distributed  in  different  museums  of  natural  curiosities.  A  piece  of 
the  skin  with  the  hair  upon  it  is  to  be  seen  in  the  museum  of  the  London 
College  of  Surgeons. 

•       "  Heat  has  powerful  influence  also  on  animated  nature,  both  vegetable  and 
animal."     (Read  the  Analysis,  page  256.) 

As  the  detail  of  the  relations  of  heat  to  particular  inanimate  substances 
belongs  to  the  province  of  chemistry,  so  does  the  detail  of  its  relations  to 
particular  living  vegetables  and  animals  belong  to  the  department  of 
Physiology ;  but  a  general  review  of  the  subject  is  required  in  a  treatise  on 
Natural  Philosophy. 

The  influence  which  heat  exerts  on  inanimate  nature,  is,  by  the  common 
mind,  more  immediately  and  completely  perceived  than  its  influence  on  beings 
which  have  life.  Thus,  to  all  it  is  obvious,  that  the  contrast  between  a  winter 
and  summer  landscape,  is  owing  chiefly  to  the  effect  of  heat  on  the  water  of 
the  landscape ; — that  during  the  absence  of  heat,  there  is  the  dry  barren  defor- 
mity of  accumulated  ice  and  snow,  covering  every  thing,  the  roads  impassable, 
the  rivers  bound  up,  perhaps  hidden,  the  air  deprived  of  moisture,  and  loaded 
often  with  powdery  drift : — but  that  on  heat  returning,  the  gliding  streams 
again  appear,  the  cascades  pour,  the  rills  murmur,  the  canals  once  more  offer 
their  bosom  to  the  boats  of  commerce,  the  lake  and  pool  again  show  their 
level  face,  reflecting  the  glories  of  the  heavens,  and  the  genial  shower  falls 
upon  the  bosom  of  the  softened  earth,  become  ready  to  receive  the  spade  or 
the  ploughshare.  But  this  change  is  not  at  all  greater  than  what  happens 
to  a  winter  tree  acted  upon  by  the  warmth  of  spring. — Again,  it  may  be  said 
with  truth,  that  heat  applied  to  the  cold  boiler  of  a  steam-engine,  is  the  cause 
of  all  its  succeeding  motions ;  of  the  heaving  of  its  beam  and  pumps,  the 
opening  and  shutting  of  its  valves,  the  turning  of  its  wheels,  and  its  ultimate 
performance  of  any  work,  as  of  spinning,  or  weaving,  or  grinding,  or  propel- 
ing  vehicles  by  land  and  water;  but  as  truly  may  it  be  said,  that  heat  coming 
to  a  seed  which  has  lain  cold  for  ages,  is  the  cause  of  its  immediate  germina- 
,  tion  and  growth ;  or  coming  to  a  lately  frozen  tree  is  the  cause  of  the  rising 
of  its  sap,  the  new  budding  and  unfolding  of  its  leaves  and  blossoms,  the 
ripening  of  its  fruits.  And  what  is  true  of  one  seed  or  tree,  is  true  of  the 
whole  of  the  vegetable  creation.  When  the  warm  gales  of  spring  have  once 
breathed  on  the  earth,  it  soon  becomes  covered,  in  field  and  in  forest,  with 
its  thick  garb  of  green,  and  soon  opening  flowers  or  blossoms  are  every  where 
breathing  back  again  a  fragrance  to  heaven, — among  these  the  heliotrope  is 
seen  always  turning  its  beautiful  disc  to  the  sun,  and  many  delicate  flowers 
which  open  their  leaves  only  to  catch  the  direct  solar  ray,  closing  them  often 
even  when  a  cloud  intervenes,  and  certainly  when  the  chills  of  night  approach. 
On  the  sunny  side  of  a  hill,  or  in  the  sheltered  crevice  or  a  rock,  or  on  a 
garden  wall,  with  warm  exposure,  there  may  be  produced  grapes,  peaches, 
and  other  delicious  fruits,  which  will  not  grow  in  situations  of  an  opposite 
character — all  acknowledging  heat  as  the  immediate  cause,  or  indispensable 
condition,  of  vegetable  life. 


308  HEAT. 

And  among  animals,  too,  the  effects  of  heat  are  equally  remarkable.  The 
dread  silence  of  winter,  for  instance,  is  succeeded  in  spring  by  one  general 
cry  of  joy.  Aloft  in  the  air  the  lark  is  every  where  caroling;  and  in  the 
shrubberies  and  woods,  a  thousand  little  throats  are  similarly  pouring  forth 
.  their  songs  of  gladness — during  the  day,  the  thrush  and  blackbird  are  heard 
above  the  rest,  and  in  the  evening  the  sweet  nightingale :  for  all  birds  it 
is  the  season*of  love  .and  of  exquisite  enjoyment.  And  it  is  equally  so  for 
animals  of  other  kinds :  in  favoured  England,  for  instance,  in  April  and  May, 
the  whole  face  of  the  country  resounds  with  lowings,  and  bleatings,  and  bark- 
ings of  joy.  And  even  man,  the  master  of  the  whole,  whose  mind  embraces 
all  times  and  places,  is  far  from  being  insensible  to  the  change  of  season. 
His  far-seeing  reason  of  course  draws  delight  from  the  anticipation  of  autumn, 
with  its  fruits;  and  his  benevolence  rejoices  in  the  happiness  observed  among 
all  inferior  creatures;  but  independently  of  these  considerations,  on  his  own 
frame  the  returning  warmth  exerts  a  direct  influence.  In  his  early  life,  when 
the  natural  sensibilities  are  yet  fresh  and  unaltered  by  the  habits  of  artificial 
society,  spring  to  man  is  always  a  season  of  delight.  The  eyes  brighten,  the 
whole  countenance  is  animated,  and  the  heart  feels  as  if  new  life  were  come, 
and  has  longings  for  fresh  objects  of  endearment.  Of  those  who.  have  passed 
their  early  years  in  the  country,  there  are  few  who,  in  their  morning  walks 
in  spring,  have  not  experienced,  without  very  definite  cause,  a  kind  of  tumul- 
tuous joy  of  which  the  natural  expression  would  have  been,  how  good  the 
God  of  nature  is  to  us.  Spring,  thus,  is  a  time  when  sleeping  sensibility  is 
roused  to  feel  that  there  lies  in  nature  more  than  the  grosser  sense  perceives. 
The  heart  is  then  thrilled  with  sudden  ecstacy,  and  wakes  to  aspirations  of 
sweet  acknowledgment. 

Besides  the  effects  of  heat  now  mentioned,  and  which  are  comparatively 
transient  as  being  connected  with  the  seasons,  there  are  other  effects  on  ani- 
mated nature  of  a  more  prominent  character.  Certain  species  of  vegetables 
and  animals,  by  their  relations  to  heat,  are  confined  to  certain  latitudes  or 
climates;  as  the  orange  tree  and  bird  of  paradise,  to  warm  regions;  the  fir  tree, 
and  arctic  bear,  to  those  that  are  colder : — and  when  individuals  of  either 
class  can  support  diversity  of  climate,  they  acquire  a  certain  character  accord- 
ing to  the  climate — as  seen  in  the  sheep  and  dogs  of  the  various  regions  of 
the  earth.  In  this  latter  respect  there  is  no  instance  more  interesting  than 
that  furnished  by  the  varieties  of  the  human  race.  If  we  assume  that  the 
whole  sprung  from  one  stock,  what  a  contrast  is  there  between  the  native  of 
equatorial  Africa,  of  temperate  Europe,  and  of  the  Polar  zone  :  between  the 
Negro,  the  Greek,  and  the  Esquimaux:  or,  again,  between  the  dark  slender 
children  of  Hindostan,  the  strongly-knit  fairer  Roman  or  Spaniard,  and  the 
taller,  ruddy,  powerful  Briton.  And  in  the  female  sex  of  the  last-named 
countries,  we  may  remark  the  gentleness  and  singular  devotedness  of  the 
Indian  woman,  the  more  commanding  dark  eye  and  gesture  of  the  graceful 
nymph  of  Italy  or  Spain,  and  the  happily  attempered  mixture  of  qualities  in 
the  fair  and  much-favoured  daughters  of  Britain. 

The  very  important  influence  of  heat  upon  the  temporary  bodily  state  of 
animals,  becomes  an  object  of  much  study  to  the  physician.  It  explains 
among  many  other  facts,  the  connection  of  temperature  with  the  rise  of  fevers 
and  other  pestilences,  the  powerful  remedial  efficacy  of  hot  and  cold  bathing, 
of  changes  of  climates,  of  regulating  the  temperature  of  air  breathed  by  in- 
valids, the  protection  from  clothes,  houses,  &c. 


THE    SUN    THE    GREAT    SOURCE.  309 

"  The  great  natural  source  of  heat  is  the  sun"     (See  Analysis,  page  256.) 

To  admit  this,  it  is  only  necessary  to  think  of  the  comparative  tempera- 
tures of  night  and  day,  of  seasons  and  of  climates,  and  to  reflect  that  the  sun 
is  the  sole  cause  of  the  differences.  We  need  not  wonder,  then,  that  to 
many  savage  nations,  seeking  the  course  of  their  life  and  happiness,  the  sun 
has  been  the  object,  not  only  of  admiration,  but  of  worship. 

The  heat  comes  from  the  sun  with  all  his  light.  If  a  sunbeam  enter  by  a 
small  opening  an  apartment  otherwise  closed  and  dark,  it  illuminates  intensely 
the  spot  or  object  on  which  it  first  falls,  and  its  light  bein^then  scattered 
around,  all  the  objects  in  the  room  become  feebly  visible.  Again,  a  cold 
thermometer,  held  to  receive  the  direct  ray,  rises  much,  while,  in  any  other 
situation,  it  is  less  affected  ;  proving  the  heat  to  be,  like  the  light,  at  first 
concentrated,  and  then  widely  diffused,  losing  proportionally  of  its  intensity. 
Light  passes  from  the  sun  to  the  earth  in  about  eight  minutes  of  time,  as 
will  be  fully  explained  in  a  future  chapter ;  and  there  is  every  reason  to 
conclude  that  heat  travels  at  the  same  rate. 

Human  art  can  gather  the  sunbeams  together,  and  the  intense  heat 
existing  in  the  focus  of  their  meeting,  is  another  proof  that  the  sun  is  the 
great  source  of  heat.  A  pane  of  glass  in  a  window,  or  a  small  mirror  will 
reflect  the  sun's  rays  so  as  strongly  to  affect  the  eye  at  a  distance  of  miles — 
and  the  heat  accompanies  the  ray,  for  by  many  such  mirrors  directed  towards 
one  point,  a  combustible  object  placed  there  may  be  inflamed  Archimedes 
set  fire  to  the  Roman  ships  by  sunbeams,  returning  from  many  points  to 
one,  his  God-like  genius  thus  rivaling  by  natural  means,  the  supposed  feats 
of  fabled  Jupiter  with  his  thunderbolts.  Again,  when  the  light  of  a  broad 
sunbeam  is  made  by  a  convex  glass  or  lens  to  converge  to  one  point  or  focus, 
the  concentrated  heat  is  also  there — for  a  piece  of  metal  held  in  the  focus 
will  drop  like  melting  wax  :  and  if  the  glass  be  purposely  advanced,  its 
focus  will  similarly  advance,  and  will  pierce  through  the  most  obdurate 
substances,  as  red-hot  wire  pierces  through  paper  or  wood.  A  hunter  on  the 
hill,  and  travelling  hordes  on  the  plains,  often  conveniently  light  their  fires 
at  the  sun  himself,  by  directing  his  energies  through  a  burning  glass. 

The  direct  ray  of  the  sun,  simply  received  into  a  box  which  is  covered 
with  glass  to  exclude  the  cold  air,  and  is  lined  with  charcoal  or  burned  cork 
to  absorb  heat,  and  to  prevent  the  escape  of  heat  once  received,  will  raise  a 
thermometer  in  the  box  to  the  temperature  of  230°  of  Fahrenheit,  a  tem- 
perature considerably  above  that  of  boiling  water.  And  the  experiment 
succeeds  in  any  part  of  the  earth  where  there  is  a  clear  atmosphere,  and 
where  the  sun  attains  considerable  apparent  altitude.  We  see,  therefore, 
that  a  solar  oven  might  in  some  cases  be  used.  In  opAating  with  the 
apparatus  suggested  by  the  author,  and  described  at  page  295,  for  distilling 
water  by  the  heat  of  the  sun,  the  vessel  intended  to  absorb  the  heat,  and  to  act 
as  the  still,  should  be  enclosed  in  a  case  covered  and  lined  as  above  described . 

Reflecting  on  such  facts  as  now  recorded,  and  on  the  globular  form  and  the 
motions  of  our  earth,  we  have  a  reason  and  the  measure  of  the  differences  of 
climate  and  of  season  found  upon  the  earth.  It  is  evident  that  the  part  of  the 
globe  turned  directly  to  the  sun,  receives  his  rays  as  abundantly  as  if  it  were 
a  perfect  plane,  directly  facing  him,  while  on  parts,  which,  as  viewed  from 
the  sun,  would  be  called  the  sides  of  the  globe,  with  the  increasing  obliquity 
of  aspect,  an  equal  breadth  or  quantity  of  rays  is  spread  over  a  larger  and 
a  larger  surface;  and  at  the'  very  edge  the  light  passes  level  with  the  surface, 
and  altogether  without  touching.  The  sunny  side  of  many  a  steep  hill  in 


310  HEAT. 

England  receives  the  sun's  rays  in  summer  as  perpendicularly  as  the  plains 
about  the  equator ;  and  such  hill-sides  are  not  heated  like  these  plains,  only 
because  the  air  over  them  is  colder — just  as  mountain  tops,  even  at  the 
equator,  owing  to  the  rarified,  and,  therefore,  cold  air  around  them,  remain 
for  ever  hooded  in  snow.  In  England,  at  the  time  of  equinoxes,  a  level 
plain  receives  only  about  half  as  much  of  the  sun's  light  and  heat  as  an  equal 
extent  of  level  surface  near  the  equator  :  and  in  the  short  days  of  winter  it 
receives  considerably  less  than  a  third  of  its  summer  allowance. 

There  are  few  contrasts  in  nature  more  striking  than  between  the  conse- 
quences of  diffeilnt  intensity  of  the  sun's  influence :  for  instance,  the  inha- 
bitants of  India,  at  midday,  with  the  thermometer  at  120°,  are  running  to 
the  shade  of  their  bangalows,  darkening  their  windows,  hanging  wetted  mats 
upon  the  walls  and  roofs,  sprinkling  water  on  the  floors,  fanning  themselves 
with  ever  moving  punkas,  and  feeling  the  slightest  covering  or  exertion  too 
much — while,  on  the  other  hand,  the  dwellers  in  Greenland,  with  the  ther- 
mometer below  zero,  are  loaded  with  furs,  and  are  seeking  the  direct  sun- 
shine or  heat  from  a  fire,  as  their  life  and  comfort.  Again,  there  is  the 
contrast  observed  on  passing,  as  the  author  once  did,  very  rapidly,  from 
such  a  paradise  as  Rio  de  Janeiro,  with  all  its  vegetable  riches,  to  Tristan 
de  Cunha,  and  the  Isle  of  Desolation  in  the  Southern  Ocean,  which  exhibit 
only  cold  and  naked  rocks ;  but  yet,  where  the  scene  swarms  with  its  appro- 
priate inhabitants — the  sea  with  seals,  and  the  air  with  clouds  of  sea-fowl, 
playing  over  the  never-resting  waves  like  flakes  of  eddying  snow.  "Were  a 
person  for  a  moment  to  doubt  whether  the  sun  be  the  real  cause  of  such 
differences,  and  of  the  fact  that  certain  creatures  are  found  only  in  certain 
zones  of  the  earth,  let  him  reflect  on  the  extraordinary  migration  of  animals, 
which  have  their  home,  not  in  any  fixed  region,  but  wherever  the  sun  has 
for  a  time  particular  degree  of  influence,  and  which  acccordingly  follow  the 
sun  in  the  changes  of  season.  We  have  the  swallow  in  vast  numbers  coming 
to  visit  the  British  isles  in  the  spring,  to  play  over  our  woods  and  waters, 
in  pursuit  of  the  insects  which  the  heat  breeds  to  fill  the  air, — welcome 
harbingers  of  the  coming  summer  and  its  riches ;  and  in  autumn,  the  same 
creatures  are  seen  congregating  on  our  shores,  to  wing  their  flight  back  in 
united  multitudes  to  more  southern  countries,  where,  in  turn,  there  is  a 
temperate  influence  of  the  sun.  The  same  reason  brings  to  England  the 
nightingale,  and  makes  our  woodlands  resound  with  the  notes  of  the  cuckoo. 
In  the  waters  of  our  bays  and  coasts,  again,  there  appear  with  the  seasons 
the  vast  shoals  of  fish,  as  of  the  herring  and  mackeral,  which  prove  such 
abundant  food  for  millions  of  human  beings ;  and  we  have  salmon,  at  stated 
times,  penetrating  from  the  ocean  far  up  the  mountain  streams,  to  deposit 
its  spawn  for  further  supply ;  all  by  their  movements  contributing  to  the 
harmonious  and  beneficent  system  of  the  universe. 

With  respect  to  the  sun  as  a  source  of  heat,  there  have  been  two  opinions 
among  philosophers,  one  class  believing  that  the  sun  is  an  intensely  heated 
mass,  which  radiates  its  heat  and  light  around,  like  a  mass  of  intensely 
heated  iron ;  and  another  class  holding  that  heat  is  merely  an  affection  or 
state  of  an  ethereal  fluid,  which  occupies  all  space,  as  sound  is  an  affection 
or  motion  of  air,  and  that  the  sun  may  produce  the  phenomena  of  light  and 
heat  without  waste  of  its  temperature  or  substance,  as  a  bell  may  produce 
the  phenomenon  of  sound;  holding,  farther,  that  the  sun,  below  its  luminous 
atmosphere  may  be  habitable,  even  by  such  animals  as  live  on  this  earth. 
Those  who  take  the  first  view,  are  awakened  to  the  dread  contemplation  of 
a  universe  carrying  in  itself  the  seeds  of  its  own  decay,  or  at  least  of  great 


COMBUSTION.  311 

periodical  revolutions  :  the  others  may  view  the  universe  as  destined  to  last 
nearly  unchanged,  until  a  new  act  or  will  of  its  Creator  shall  again  alter  or 
destroy  it. 

Of  one  fact  there  can  be  no  doubt,  namely,  that  the  pregfcnt  temperature 
of  the  surface  of  the  earth  is  much  lower  than  the  temperature  in  remote 
past  time.  The  rocks  called  primitive,  as  granite  and  gneiss,  constituting 
the  interiors  of  our  great  mountain  masses  and  the  substrata  of  our  plains, 
bear  evident  marks  of  there  having  been  at  one  period  a  molten  state,  from 
which  they  have  been  solidified  by  a  very  gradual  cooling  :  and  even  the 
whole  mass  of  the  earth  at  some  time  must  have  been  so  fluid  or  soft,  as,  in 
obedience  to  gravity,  to  have  assumed  its  rounded  form,  and  in  obedience  to 
the  centrifugal  force  of  its  whirling,  to  have  bulged  out,  at  its  great  circum- 
ference or  equator,  the  thirty-four  miles  which  its  equatorial  diameter  exceeds 
the  polar ;  the  same,  by  the  by,  in  degrees  corresponding  to  the  various 
speed  of  rotation,  being  true  of  all  the  other  planets  belonging  to  the  solar 
system.  Again,  while  in  excavating  below  the  surface  of  the  globe,  or  in 
examining  its  structure  as  exposed  to  view  by  volcanic  or  other  convulsions, 
men  encounter,  in  very  many  situations,  a  thickness  of  more  than  a  mile,  of 
the  wreck  and  remains  of  former  states  of  the  world — as,  on  digging  eighty 
feet  under  vineyards  near  Mount  Vesuvius,  they  encounter  the  more  recently 
buried  cities  of  Herculaneum  and  Pompeii — they  farther  discover  that  the 
animal  and  vegetable  remains  buried,  without  number,  in  the  present  cold 
climates  of  the  earth,  and  evidently  near  where  the  creatures  lived,  are  all 
of  kinds  now  inhabiting  only  the  warmer  or  tropical  regions.  Lastly,  in 
the  operations  of  mining,  the  deeper  men  go,  the  higher  they  find  the  tem- 
perature to  be,  at  the  rate  of  a  degree  for  about  200  feet  of  descent ;  which 
fact  as  heat  tends  to  equable  diffusion,  proves  both  that  the  central  heat  of 
our  earth  must  have  had  another  source  than  a  radiation  from  the  sun  of  the 
present  intensity ;  and  that  the  surface  of  the  earth  is  now  radiating  away 
more  heat  than  it  receives  from  the  sun.  The  conclusion  then  follows,  that 
the  temperature  of  the  world  is  still  falling  although  perhaps  so  slowly  that 
a  change  may  not  be  detected  even  within  centuries.  Possibly,  in  very 
remote  antiquity,  that  may  have  been  true  which  the  early  Greeks  errone- 
ously thought  true  in  their  day,  viz.,  that  the  equator  of  the  earth,  by  reason 
of  its  great  heat,  the  sun's  influence  there  being  joined  to  the  heat  from 
within,  was  a  barrier,  impassable  by  man,  between  the  northern  and 
southern  hemispheres. 

"  Electricity  a  source  of  heat."     (See  the  Analysis.) 

This  subject  can  only  be  satisfactorily  entered  upon  in  the  chapter  devoted 
exclusively  to  electricity,  and  is,  therefore  deferred.  Suffice  it  here  to  say, 
that  while  an  electrical  discharge  of  current  passes  from  one  situation  to 
another,  the  substance  serving  as  a  conductor  is  often  heated,  melted,  or  dissi- 
pated, in  such  a  manner  as  to  make  it  doubtful  whether  human  art  has  any 
more  powerful  means  of  producing  these  effects.  We  may  remark,  too,  that 
in  certain  cases  of  the  electrical  current,  the  heat  is  accompanied  by  as 
intense  a  light  as  art  can  exhibit. 

lt  Combustion  and  other  chemical  actions  as  sources  of  heat.  "(See  Analysis, 

page  256.) 

Of  the  phenomena  of  nature  there  is  perhaps  none  which,  to  the  unin- 
gtructed,  appears  so  inexplicable  and  so  wonderful  as  that  of  fire  or  combus- 


312  HEAT. 

tion — whether  contemplated  in  its  beauty  or  in  its  terrors.  Fire  is  seen  in 
its  beauty  when  used  by  man  for  his  domestic  purposes,  as  when  it  blazes 
cheerfully  over  his  parlour  hearth,  or  beams  a  steady  light  around  from  his 
lamps  and  chandeliers.  Tt  is  seen  again  in  its  terrors,  when  spreading  by 
accident  from  some  focus,  it  envelopes  in  sudden  flame  and  quickly  consumes 
the  surrounding  objects,  perhaps  the  draperies  and  other  furniture  of  a  single 
apartment;  or  wider  spread,  theTaluable  contents  of  a  spacious  mansion ;  or 
still  wider  spread,  and  deafening  uproar,  a  whole  town  or  a  forest : — and  it 
is  fire  which,  labouring  within  the  bowels  of  the  earth,  first  prepares  and  then 
urges  up  to  heaven  the  volcanic  eruptions  of  flames  and  red-hot  rocks,  during 
which  the  region  around  often  quakes  and  is  uptorn,  so  that  the  cities  are  de- 
molished into  the  sudden  tombs  of  the  inhabitants,  the  course  of  the  rivers  is 
changed,  the  plains  are  converted  into  lakes,  or  the  lakes'-beds  into  dry  land. 
Fire  is  awfully  seen  also  in  some  meteors,  and  when,  intentionally  lighted  by 
human  hands,  it  bursts  from  the  cannon  to  produce  the  carnage  of  battle.  Fire 
among  many  nations  of  antiquity  was  regarded  with  awe  and  holy  reverence, 
the  sun  himself  being  honoured  chiefly  as  its  concentration  or  supposed  abode. 
There  were  sacred  fires  in  many  of  the  temples,  and  fire  was  used  to  complete 
the  splendour  of  the  most  august  ceremonies.  Nay,  even  Moses,  a  worshipper 
of  the  one  true  God,  has  given  records  of  the  Burning  Bush  and  of  burnt- 
offerings  made  to  that  God  :  and  at  the  present  day,  in  many  Christian  churches 
there  are  ever-burning  lamps  and  frequent  magnificent  illuminations.  Now 
this  principle  of  fire,  which,  when  the  savage  man  first  saw  it  spreading  after 
the  thunder-clap  or  the  rubbing  of  forest  branches  in  a  storni,  so  as  to  threaten 
universal  destruction,  he  sonatuarally  accounted  the  demon,  if  not  the  God  of 
nature ;  this  principle  man's  art  has  now  tamed  to  be  a  most  obedient  and  by 
far  the  most  useful  of  his  servants.  Fire  being  in  truth  but  a  concentration  of 
the  element  of  heat,  which  in  its  tranquil  and  in-visible  diffusion  we  have  al- 
ready contemplated  as  the  beneficent  life  or  soul  of  the  universe — the  cause  of 
seasons  and  climates  and  of  all  the  changes  or  activity  which  distinguish  a 
living  world  from  a  dead  and  frozen  mass ;  man,  by  acquiring  command  over 
it,  commands  heat  when  and  where  he  wills,  and  thus  truly  becomes  in  a 
second  degree  the  ruler  of  nature.  Fire  in  man's  service  may  be  figured 
as  a  legion  of  serving  spirits  to  whom  no  labour  is  difficult,  who,  in  any 
particular  case  have  power  or  magnitude  exactly  proportioned  to  the  quan- 
tity of  food  or  fuel  afforded  ;  of  whom,  moreover,  man  can,  at  any  moment, 
conjure  up  one  or  many  by  the  magic  stroke  of  his  flint  and  steel.  In  every 
private  dwelling  he  has  of  these  fiery  spirits  as  domestic  slaves — in  the 
kitchen  and  in  the  parlour.  In  his  manufactories  they  are  melting  glass  for 
him,  and  reducing  ores,  and  boiling  and  evaporating  for  a  hundred  purposes. 
But  it  is  chiefly  when  chained  to  the  steam-engine,  that  they  show  their 
prodigious  powers  : — ras  when,  putting  forth  a  giant's  strength,  they  heave  a 
river  from  the  bottom  of  a  mine,  or  urge  up  a  vast  ship  through  the  winter 
storni '}  or  when  equaling,  if  not  surpassing,  in  nice  dexterity,  the  work 
of  human  hands,  they  twist  the  silken  or  cotton  threads,  and  weave  them 
into  most  delicate  fabrics.  Men,  grown  familiar  with  such  prodigies,  have 
almost  ceased  to  be  moved  by  them ;  but  even  now  few  persons  can  resist  a 
feeling  of  wonder  and  admiration  when  chemistry  is  calling  forth  the  hidden 
spirit  of  combustion  in  some  new  and  less  familiar  guise : — as,  for  instance, 
when  a  piece  of  iron  wire,  lighted  as  a  taper  in  oxygen  gas,  burns  with  such 
resplendent  brilliancy ;  or  when  phosphorus  similarly  placed,  throws  around 
its  overpowering  flood  of  flame  \  or  when  small  portions  of  the  metal  called 
potassium,  cast  upon  the  surface  of  water,  become  as  beads  of  most  intense 


COMBUSTION.  313 

light  running  about  there,  and  crossing  as  in  a  merry  dance ; — or,  lastly, 
when  flames  produced  from  particular  substances  are  seen  rising  deep-tinged 
with  most  vived  and  beautiful  colours. 

Singularly  interesting,  then,  to  philosophers,  as  in  such  particulars  the 
phenomena  of  combustion  must  always  have  appeared,  one  may  wonder 
that  its  true  nature  could  remain  to  them  so  long  a  mystery  ;  but  until  the 
admirable  researches  of  Davy,  made  only  a  few  years  ago,  their  conjectures 
had  scarcely  approached  the  truth.  An  opinion  long  prevailed,  that  in  every 
combustible  substance  there  was  present  a  certain  quantity  of  a  something 
denominated  phlogiston,  which,  on  being  disengaged  or  separated,  became 
obvious  to  human  sense  as  light  and  heat.  The  white  oxide  of  zinc,  for 
instance,  named  the  flowers  of  zinc,  and  into  which  the  metal  is  changed  by 
burning,  was  supposed  to  be  the  metal  deprived  of  its  phlogiston;  and  when 
on  this  oxide  being  heated  with  charcoal,  the  metal  again  appeared,  it  was 
supposed  simply  to  have  recovered  phlogiston  from  the  charcoal.  The  illus- 
trious Lavoisier  had  the  merit  of  most  clearly  disproving  this  hypothesis,  by 
showing  that  the  flowers  or  powders  obtained  from  a  metal  by  burning  it, 
were  heavier  than  the  piece  of  metal  from  which  they  were  produced,  by  the 
exact  weight  of  the  oxygen  gas,  which  disappeared  in  the  combustion,  &c. ; 
and  he  showed  farther,  that  in  this  and  many  other  cases,  combustion  was 
merely  the  act  of  two  substances  combining  chemically  ;  but  he  fell  into  an 
error  almost  as  great  as  that  which  he  overthrew,  by  supposing  that  for  com- 
bustion, oxygen  had  always  to  be  one  of  the  combining  substances,  and  that 
the  heat  andlight  given  out  in  every»case  had  been  previously  latent  in  the 
oxygen. 

When  Sir  Humphrey  Davy  began  his  labours  on  the  subject,  than  which 
labours  there  is  not,  perhaps  on  record  a  more  perfect  specimen  of  scientific 
research,  it  was  already  known,  first, — that  bodies  when  compressed  or  by 
any  means  reduced  in  bulk,  generally  give  out  a  part  of  their  heat,  as — when 
air  condensed  in  the  match-syringe  lights  tinder, — or  when  water  and  sul- 
phuric acid,  uniting  into  a  compound  of  smaller  volume  than  the  separate 
ingredients,  becoming  very  hot, — or  when  water,  poured  upon  quick-lime  to 
slake  it,  and  becoming  solid  with  it,  produces  heat  sufficient  to  inflame  wood, 
as  has  been  fatally  proved  by  the  burning  of  many  lime-loaded  ships  : — and 
that  in  such  cases,  the  heat  produced  during  the  chemical  union  depended 
more  upon  the  energy  of  the  action  which  united  the  substances  than  upon 
the  change  of  volume  produced. 

Farther,  it  was  known  that  any  substance  having  its  temperature  raised, 
by  whatever  means,  to  800°  or  more  of  Fahrenheit's  thermometer,  became 
incandescent  or  luminous — as  when  iron,  or  stone,  or  any  substance  not  dis- 
sipated by  heat,  is  placed  in  a  common  fire;  in  the  first  degree  the  substance 
being  said  to  be  red-hot,  and  at  higher  temperatures  to  be  white-hot. 

Out  of  these  two  truths  Davy  constructed  his  explanation.  He  asserted — 
that  in  any  case,  combustion  is  merely  the  appearance  produced  when  sub- 
stances, having  a  still  stronger  attraction  for  each  other  than  quick-lime  and 
water,  are,  with  intense  energy,  combining  chemically,  so  as  to  become 
heated  at  least  to  the  degree  of  incandescence ,  and  that  during  the  pheno- 
menon there  is  not,  as  was  formerly  supposed,  something  altogether  con- 
sumed or  destroyed,  or  something  called  phlogiston  escaping,  but  that  the 
substances  concerned  are  only  assuming  a  new  form  or  arrangement.  Thus, 
if  a  piece  of  charcoal  be  enclosed  in  a  glass  vessel  filled  with  air,  of  which 
vessel  the  mouth  dips  into  a  liquid  to  confine  the  air,  and  if  the  charcoal  be 
then  heated  to  a  certain  degree,  by  means  of  a  burning-glass  or  otherwise, 


314  HEAT. 

the  cohesion  of  its  particles  yields  to  their  attraction  for  the  oxygen  of  the 
air  around  them,  and  they  immediately  begin  to  combine  with  the  oxygen  so 
energetically  as  to  produce  a  heat  still  much  greater,  accompanied  by  the 
light  or  incandescence  of  combustion.  The  charcoal,  under  these  circum- 
stances, soon  entirely  disappears,  or  is  dissolved  in  the  air,  as  sugar  may  be 
dissolved  in  water ;  but  if  the  air  be  afterwards  weighed,  it  is  found  to  have 
gained  in  weight  the  exact  weight  of  the  charcoal  which  has  disappeared ; 
and  a  chemist  can  again  separate  the  charcoal  from  the  air,  and  use  either 
for  any  purpose  as  before.  In  like  manner,  if  a  piece  of  iron  wire  be  heated 
at  one  end,  which  wr  then  plunged  into  a  jar  of  oxygen  gas,  it  will  burn  as  a 
most  brilliant  taper,  and  will  gradually  fall  in  the  form  of  oxidized  drops  or 
scales  of  iron,  to  the  bottom  of  the  vessel.  Now  during  this  process,  the 
quantity  of  oxygen  will  be  diminished,  but  if  the  scales  mentioned  be  col- 
lected, they  will  be  found  to  weigh  just  as  much  more  than  the  original  wire 
expended,  as  there  is  of  oxygen  lost  or  combined  with  them.  A  chemist 
can  separate  this  iron  and  oxygen,  and  exhibit  them  apart  as  before,  without 
loss.  Again,  if  iron  and  sulphur,  in  certain  proportions,  be  heated  together, 
they  unite  with  vivid  combustion,  but  the  product  weighs  exactly  as  much 
as  the  original  ingredients. 

"While  every  instance  of  combustion  is  thus  only  a  case  of  chemical  union 
going  on  with  such  intensity  of  action  as  to  produce  incandescence,  still, 
according  to  the  nature  of  the  substances  combining,  the  appearance  varies 
much.  It  may  be,  for  instance,  with  flame  or  without  flame.  The  great 
combining  substance  in  nature,  that  is  to*  say,  the  most  universally  distributed, 
is  oxygen,  of  which  the  name  is  now  become  familiar  even  to  the  ears  of  the 
unlearned.  It  forms  four-fifths  of  the  substance  of  water  and  one-fifth  of  our 
atmosphere,  being  on  the  latter  account  present  wherever  man  can  be,  and 
ready  to  unite  itself  with  any  matter  exposed  to  it  at  the  necessary  tempera- 
ture. Now,  of  substances  burning  in  air,  those  which  are  originally  aeriform, 
as  coal-gas,  or  which  on  being  heated,  are  rendered  aeriform,  or  vapourized 
before  the  union  takes  place  as  oil  or  wax,  assume  the  appearance  of  flame — 
which  means  that  the  aeriform  particles  usually  invisible  are  raised  to  the 
incandescent  temperature;  but  when  the  substance  combining  with  the 
oxygen  remains  solid,  while  its  particles  are  gradually  lifted  away  by  the 
oxygen  acting  only  at  the  surface  of  the  mass,  it  appears  during  the  whole 
time,  only  as  a  red  hot  stone.  The  latter  is  the  case  of  charcoal,  coke,  Welch 
stone-coal,  &c.,  while  in  the  case  of  wood,  common  coal,  &c.,  a  greater  or 
less  portion  of  the  inflammable  matter  is  by  the  heat  of  the  combustion  con- 
verted into  vapour,  and  produces  the  beautiful  appearance  of  flame. 

Of  the  substances  called  combustible,  and  thus  called  because  they  combine 
with  oxygen  so  energetically  as  to  become  incandescent,  there  are  only  a 
few,  as  the  metals  called  potassium,  sodium,  &c.,  which  will  begin  to  unite 
with  oxygen,  or  to  burn  at  the  common  temperature  of  our  globe,  the  others 
requiring  to  be  at  some  higher  temperature.  Thus  phosphorous  begins  to 
burn  at  150°,  sulphur  at  550°,  charcoal  at  750°,  hydrogen  at  800°,  &c.,  it 
appearing  that  up  to  these  temperatures  the  attraction  of  the  atoms  of  the 
substances  among  themselves  is  sufficient  to  resist  the  other  attraction  or  that 
of  oxygen.  But  when  the  combustion  once  begins,  the  temperature,  from 
the  effect  of  the  combustion  itself,  rises  instantly  much  beyond  the  degree 
necessary  for  the  commencement  of  the  process.  Oxygen  and  hydrogen, 
which  begin  to  burn  or  combine  at  800°,  produce  a  flame  of  as  intense  heat 
as  human  art  can  excite. 

On  the  circumstance  that  bodies  require  to  have  certain  preparatory  tern- 


COMBUSTION.  315 

perature  before  beginning  thus  to  combine  with  oxygen,  depend  many  im- 
portant facts  of  nature  and  art.  Hence  the  safety  with  which  most  com- 
bustibles may  be  exposed  at  ordinary  temperatures  to  the  contact  of  atmo- 
spheric air :  otherwise,  coal,  wood,  &c.,  in  the  moment  of  being  exposed  to 
the  air,  would  catch  fire,  as  really  happens  to  phosphorated  hydrogen  gas ; 
or  to  the  metal  called  potassium,  even  when  thrown  into  cold  water,  the 
metal  attracting  the  oxygen  from  the  water  instantly,  and  with  intense  com- 
bustion. If  a  fire  or  flame  be  so  small  that,  from  the  rapid  absorption  or 
heat  by  bodies  around,  it  does  not  produce  heat  enough  to  maintain  the 
inflaming  temperature  of  the  substance,  the  combustion  will  soon  be  extin- 
guished. Thus  a  common  coal  fire,  if  not  watched,  and  the  remaining  fuel 
occasionally  gathered  together  to  reduce  the  surface  of  wasteful  radiation, 
will  be  extinguished  long  before  the  whole  fuel  is  expended  : — but  not  so 
with  a  fire  of  wood  or  of  paper,  which  substances  burn  more  readily  than 
coal.  The  Welch  stone-coal  can  only  be  made  to  burn  in  very  large  masses, 
or  when  mixed  with  a  more  inflammable  coal  or  other  fuel,  or  when  fed  by 
air  already  heated.  Some  of  our  manufactures  have  lately  been  improved 
by  causing  the  air  which  feeds  the  working  fire  to  pass  through  metal  tubes 
heated  by  lying  in  another  fire.  In  common  fires  much  coal  is  burned  at 
temperatures  so  low  as  to  be  nearly  useless.  A  substance  placed  in  pure 
oxygen  gas  burns  with  much  greater  intensity,  and  will  begin  burning  at  a 
lower  temperature  than  if  placed  in  atmospheric  air,  which  contains  only 
one-fifth  of  oxygen  and  four-fifths  of  another  substance,  nitrogen,  which 
does  not  aid  the  combustion — because,  in  the  latter  case,  the  nitrogen  by 
absorbing  much  of  the  heat  of  the  combustion,  lowers  the  temperature.  Iron 
wire  will  burn  as  a  taper  in  oxygen,  but  not  in  common  air;  and  a  common 
taper  or  flaming  piece  of  wood  just  extinguished  by  blowing  on  it,  will  imme- 
diately be  rekindled  if  placed  in  oxygen.  Again,  a  lamp  with  a  very  small 
wick,  as  of  one  thread,  and  producing,  therefore,  very  little  heat,  will  not 
burn  in  cold  weather,  or  in  an  ice-house,  and  at  any  time  will  be  extinguished 
by  a  foreign  body  brought  near  it  so  as  to  cool  it — a  piece  of  ice,  for  instance, 
or  a  small  metallic  nob,  presented  to  it  on  the  end  of  a  wire,  or  a  metallic 
ring  let  down  over  it ;  but  if  the  ball  or  ring  be  hot,  the  effect  will  not  fol- 
low. By  more  powerful  refrigerating  processes,  even  a  considerable  lamp  or 
candle  may  be  put  out.  These  discoveries  led  Davy  to  the  construction  of 
his  miner's  safety-lamp,  which  is  merely  a  lamp  surrounded  by  a  wire  gauze, 
of  which  the  meshes  are  of  such  size  that  aflame  of  hydrogen  gas  attempting 
to  pass  through  is  so  cooled  by  the  heated  absorbing  and  heat-conducting 
power  of  the  metal  as  to  be  extinguished.  A  wire  gauze  gradually  let  down 
upon  any  common  flame,  annihilates  the  part  of  it  which  should  appear  above 
the  gauze  ;  but  the  combustible  vapour  passing  invisibly  through  the  gauze, 
may  be  lighted  afresh  on  its  upper  side.  Oxygen  and  hydrogen,  which  are 
the  constituents  of  water,  when  uniting,  produce  such  intense  heat  that  the 
momentary  expansion  of  the  newly  formed  water,  then  in  the  state  of  steam, 
is  such  as  to  constitute  a  violent  explosion ;  and  when  a  jet  of  the  two  gases 
mixed  gives  a  continued  flame,  the  most  refractory  substances  melt  in  it  like 
wax  in  a  common  taper — yet  these  gases  may  be  kept  mixed  together  in  the 
cold  reservoir  of  a  blow  pipe  without  combining,  and  when  they  are  set  on 
fire,  while  issuing  as  a  jet,  the  flame  does  not  travel  inwards  through  the 
opening,  as  might  be  feared,  because  it  is  cooled  by  the  metal  of  the  orifice. 
A  mixture  of  oxygen  and  hydrogen  passing  through  the  small  channels  left 
in  a  tube  filled  with  fire,  may  be  lighted  at  the  end  of  the  tube  without 
danger  of  explosion. 


316  HEAT. 

While  solid  bodies  become  very  visible  or  incandescent  at  about  100°  of 
Fahrenheit,  airs,  owing  to  their  tenuity  of  condition,  require  to  be  heated 
much  farther  before  they  take  on  the  vivid  appearance  of  flame  ;  and  airs  of 
light  atoms,  like  hydrogen,  require  to  be  heated  still  more  than  heavier  airs. 
Thus  the  flame  of  pure  hydrogen  is  pale  and  blue,  but  a  wire  held  in  it  be- 
comes much  more  luminous  than  the  flame  itself;  and  the  flame  of  mixed 
oxygen  and  hydrogen  escaping  from  a  very  minute  orifice  in  a  glass  tube, 
may  itself  be  scarcely  visible,  while  the  extremity  of  the  tube  heated  by  it 
becomes  like  a  brilliant  star.  Hence  the  light  of  many  flames  may  be  in- 
creased by  placing  a  wire  gauze  or  other  solid  body  in  the  flame ;  as  is  seen 
•when  a  piece  of  lime  is  placed  in  a  flame  of  mixed  oxygen  and  hydrogen. 
Consideration  of  this  subject  enables  us  to  explain  why  common  coal  gas, 
which  consists  of  hydrogen  holding  a  quantity  of  carbon  in  solution,  gives  a 
stronger  light  than  pure  hydrogen  ;  and  why  oil  gas,  which  contains  about 
twice  as  much  carbon  as  the  coal  gas,  gives  also  about  twice  as  much  light; 
— or  it  appears  that  the  atmospheric  air,  which  first  mixes  with  these  gases 
as  they  issue  to  burn,  is  sufficient  to  combine  with  all  their  hydrogen  (which 
it  most  strongly  attracts)  but  not  at  the  same  time  with  all  their  carbon  : 
the  particles  of  the  carbon,  therefore,  are  first  separated  or  precipitated  in 
the  flame,  and  become  so  many  solid  particles  most  intensely  heated  and 
luminous ;  and  afterwards,  when  they  have  ascended  a  little  higher,  they 
meet  with  new  oxygen  and  burn  in  their  turn,  giving  a  second  quantity  of 
light.  That  this  decomposition  of  the  gas  really  occurs  is  proved  by  placing 
a  wire  gauze  in  the  flame,  for  then  we  find  that  if  it  be  held  near  the  middle 
of  the  flame,  it  becomes  immediately  loaded  with  particles  of  charcoal  sepa- 
rated there,  and  cooled  by  it  so  as  to  cohere ;  while  if  held  at  the  bottom  of 
the  flame  where  the  carbon  is  not  yet  separated,  it  retains  none,  and  if  held 
at  the  top  of  the  flame,  where  they  are  already  burned,  it  retains  none.  A 
candle  or  lamp  is  said  to  smoke  when  the  heat  produced  by  it,  or  the  quan- 
tity of  oxygen  allowed  to  approach  the  flame,  is  not  sufficient  to  effect  the 
total  combustion  of  the  carbon  which  rises  in  the  flame.  The  hollow  or  cir- 
cular wick  of  the  common  Argand  lamp,  and  the  similar  form  given  to  gas 
flames  is  useful,  by  admitting  air  to  the  inside  as  well  as  to  the  outside  of 
the  flame,  and  the  lofty  glass  chimney  is  to  quicken  the  currents  of  air. 

When  oxygen  mixed  with  certain  of  the  inflammable  gases  or  vapours  is 
heated,  although  only  to  a  temperature  considerably  below  that  of  common 
burning  or  explosion,  a  union  still  takes  place,  but  very  slowly,  so  that  the 
temperature  never  rises  to  what  is  necessary  to  exhibit  flame.  This  pheno- 
menon has  been  called  invisible  combustion.  It  is  remarkably  exemplified 
•  on  plunging  plantinum  or  gold  wire  moderately  heated  into  such  a  mixture : 
the  combination  then  goes  on  in  the  immediate  vicinity  of  the  rhot  wire ; 
and  although  without  flame,  still  with  sufficient  disengagement  of  heat  to 
maintain  the  wire  in  an  incandescent  or  luminous  state,  so  long  as  there  are 
gases  left  to  combine.  Thus  the  vapour  always  arising  at  a  common  tem- 
perature from  the  mouth  of  a  phial  of  ether,  (ether  consists  chiefly  of  hydrogen 
and  carbon),  if  made  to  pass  through  a  coil  of  heated  plantinum  wire,  while 
combining,  by  this  slow  combustion,  with  the  oxygen  of  the  air  around  it, 
gives  out  heat  enough  to  keep  the  wire  so  luminous  as  to  serve  as  a  little  lamp 
by  which  to  read  from  the  dial-plate  of  a  watch  through  the  night.  A  beau- 
tiful modification  of  this  principle  has  been  adopted  in  the  miner's  safety- 
lamp  ;  and  when  the  air  of  the  mine  is  too  impure  to  maintain  the  flame,  it 
still  suffices  thus  to  produce  a  continued  light  from  the  incandescent  metal. 


• 


FUEL.  31T 


•  "fuel" 

Heat  being,  in  the  sense  already  explained,  the  life  of  the  universe,  and 
man  having  command  over  nature  chiefly  by  his  power  of  controlling  heat, 
which  power  again  comes  to  him  with  the  ability  to  produce  combustion,  it 
is  of  great  interest  to  inquire  what  substances  he  can  most  easily  procure  as 
food  for  combustion,  or  fuel,  and  how  these  may  be  most  advantageously  em- 
ployed. To  speak  on  this  subject  at  all  fully  in  reference  to  the  various  arts 
of  life  would  be  to  compose  an  extensive  work,  but  an  interesting  sketch 
may  be  comprised  within  narrow  limits. 

Although  there  are  a  great  number  of  substances,  which,  in  the  act  of 
their  chemical  union,  occasion  the  heat  and  light  which  constitute  the  com- 
bustion, still  by  far  the  greater  part  of  these,  in  an  uncombined  state,  are  so 
sparingly  distributed  in  nature,  and  are  therefore  procurable  with  such  diffi- 
culty, that  heat  obtained  by  sacrificing  them  would  be  much  too  expensive 
to  be  within  common  means.  Providence,  however,  has  willed  that  the  ele- 
mentary substance  in  nature  which  has  the  most  energetic  attraction  for 
almost  all  other  substances,  and  which  therefore  produces  in  uniting  with 
them  the  most  intense  heat,  is  also  of  all  the  most  universally  distributed. 
This  substance  is  oxygen.  It  forms  part  of  our  atmosphere,  and  therefore 
penetrates,  and  is  present  wherever  man  can  breathe,  offering  itself  at  once 
at  his  service.  Then  for  the  purpose  of  combining  with  the  oxygen,  there 
are  chiefly  two  other  substances  also  very  widely  scattered,  and  therefore 
easily  procurable  and  cheap.  These  are  carbon  and  hydrogen,  the  great  ma- 
terials of  all  vegetable  bodies,  and  therefore  of  our  forest  trees,  and  of  coal 
beds,  many  of  which  are  evidently  the  remains  of  antediluvian  forests.  Car- 
bon is  found  nearly  alone  in  hard  ctrnl,  but  it  is  united  with  a  large  proportion 
of  hydrogen  in  caking  coal,  and  in  wood,  wax,  resins,  tallow  and  oils.  The 
gases  obtained  from  these  last-mentioned  substances  and  now  used  for  illu- 
mination are  merely  hydrogen,  holding  in  solution  certain  proportions  of 
carbon ;  and  all  bodies  which  burn  with  flame  give  out  such  gases  in  the  act 
of  combustion.  In  the  mass  of  the  earth,  as  far  as  known  to  man,  the 
stones,  earths  and  water,  forming  its  surface,  are  already  combinations  of 
oxygen  with  other  substances,  and  are  therefore  not  in  a  state  to  produce 
fresh  combustion ;  but  carbon  and  hydrogen,  by  various  processes  of  vege- 
table and  animal  life,  are  in  numberless  situations  becoming  accumulated, 
so  as  to  be  fit  for  fuel ; — as  by  other  processes  the  atmosphere  is  always 
preserved  with  its  due  proportion  of  oxygen. 

The  name  fuel  is  given  only  to  the  substances  which  combine  with  oxygen, 
and  not  to  the  oxygen  itself,  probably  because  the  former  being  solid  or 
liquid,  and  therefore  more  obvious  to  sense,  had  attracted  human  notice  as 
producers  of  combustion  long  before  the  existence  of  the  aeriform,  agent, 
oxygen,  was  even  suspected. 

Oils,  fat,  wax,  &c.,  from  becomin^eriform  in  their  combustion,  exhibit 
the  appearance  of  flame,  as  already  explained,  and  hence  these  substances  are 
chiefly  used  for  the  purpose  of  giving  light ;  while  wood  and  coal  are  more 
frequently  used  for  mere  heating.  But  the  chemist's  lamp,  by  which  he 
distills  and  evaporates,  and  his  common  blow-pipe  for  directing  the  point  of 
any  flame  upon  a  substance  to  melt  it,  and  his  blow-pipe  fed  with  mixed 
oxygen  and  hydrogen,  whose  flame  is  capable  of  melting  the  most  refractory 
substances — prove  that  it  is  chiefly  the  expense  of  the  former  kinds  of  fuel 
which  has  limited  them  so  much  to  the  office  of  light-giving.  Lately  an 


318  HEAT. 

• 

important  application  of  coal-gas  and  of  oil  or  fat  as  heat-giving  fuel  has 
been  made  in  a  general  cookiDg  apparatus,  which  promises  to  effect  a  con- 
siderable diminution  of  house-keeping  expense. 

Wood  was  the  common  fuel  of  the  early  world  when  coal-mines  were  not 
yet  known,  and  still  in  many  countries  it  is  so  abundant  as  to  be  the  cheap- 
est fuel.  Charcoal  is  the  name  given  to  what  remains  of  wood  after  it  has 
been  heated  in  a  close  place;  during  which  operation  the  hydrogen  and  other 
minor  ingredients  are  driven  away  in  the  form  of  vapour.  Charcoal  is  nearly 
pure  carbon.  Coke,  again,  is  the  charcoal  obtained  by  a  similar  process  from 
coal.  The  wood  and  coal,  if  much  heated  in  the  air,  would  burn  or  combine 
with  the  oxygen  of  the  air ;  but  heated  in  a  vessel  or  place  which  excludes 
air,  they  merely  give  out  their  more  volatile  parts. 

Good  coal,  where  it  abounds,  is  now  for  ordinary  purposes  by  much  the 
cheapest  kind  of  fuel ;  and  since  within,  a  few  years  men  have  learned  to 
separate  from  it,  and  to  use,  instead  of  oil  and  wax,  its  illuminating  gas, 
namely,  its  hydrogen  holding  in  solution  a  little  carbon,  it  has  become  dou- 
bly precious  to  them.  A  person  reflecting  that*  heat  is  the  magic  power 
which  vivifies  nature,  and  that  coal  is  what  best  gives  heat  for  the  endless 
purposes  of  human  society,  cannot  without  admiration  think  of  the  rich  stores 
of  coal  which  exists  treasured  up  in  the  bowels  of  the  earth  for  man's  use. 
And  Britain,  in  this  respect,  is  singularly  favored.  Her  extensive  coal  mines 
are  in  effect  mines  of  latent  labour  or  power  vastly  more  precious  than  the 
mines  of  gold  and  silver  in  Peru, — for  a  hundred  men  in  England,  of  whom 
a  part  dig  coal,  and  the  remainder  apply  it  in  manufactures,  can  get  in  ex- 
change for  the  merchandize  they  produce  more  of  either  gold  or  silver  than 
a  hundred  men  working  in  the  American  mines  are  able  to  extract.  These 
coal-mines,  therefore,  may  be  said  to  produce  abundantly  every  thing  which 
labour  and  ingenuity  can  produce,  or  \fhich  our  money  can  buy,  and  they 
have  essentially  contributed  to  render  Britain  the  mistress  of  the  industry 
and  commerce  of  the  earth.  Britain  has  become  to  the  civilized  world  around, 
nearly  what  an  ordinary  town  is  to  the  rural  district  in  which  it  stands,  and 
of  this  vast  and  glorious  city  the  mines  in  question  are  the  coal-cellars, 
stored  at  the  present  rate  of  consumption  for  above  1,000  years;  a  supply 
which,  as  coming  improvements  in  the  arts  of  life  will  naturally  bring 
economy  of  fuel,  or  substitution  of  other  means  to  effect  similar  purposes  or 
changes  in  the  channels  of  industry, — may  be  regarded  as  exhaustless. 

The  coal  in  many  mines  is  evidently  the  remains  of  forests,  which  have 
existed  in  very  remote  time,  swept  together  either  during  convulsions  of 
nature  or  by  the  more  gradual  operation  of  rivers,  (as  may  be  seen  at  present 
where  the  great  Mississippi  is  carrying  an  uninterrupted  stream  of  floating 
timber  to  the  sea,  and  depositing  it  at  the  bottom  of  the  Mexican  gulf)  the 
accumulated  wood  being  afterwards  compressed  and  solidified  by  superin- 
cumbent deposit  sof  earthy  matters,  aided  probably  by  the  action  of  heat.  In 
many  coal-beds  the  trees  yet  retain  their  form,  so  that  their  species  can  be 
easily  distinguished,  and  there  are  buried  among  them  other  vegetable  and 
animal  remains  of  cotemporaneous  inhabitants  of  the  earth.  Coal  is  found 
'of  different  qualities.  In  some  places  it  is  almost  unmixed  carbon,  ^nd 
exceeding  solid,  as  if  it  had  been  coked  by  subterranean  heat.  Such  is 
the  stone-coal  of  Wales,  which  in  100  parts  contains  97  of  pure  carbon,  with 
only  three  of  hydrogen  and  earthy  matter.  In  other  places  the  coal  con- 
tains hydrogen  in  nearly  as  large  proportion  as  wood  does,  so  combined  with 
part  of  the  carbon  as  to  form  the  oily  or  pitchy  substances  existing  in  the 


FUEL.  319 

coal  which,  when  burning,  produce  flame,  and  when  rising  unburned,  con- 
stitute smoke. 

The  comparative  values,  as  fuel,  of  different  kinds  of  carbonaceous  matter, 
have  been  found  to  be  as  in  the  following  tables. 

One  pound  of  Melts  of  ice 

Good  coal     -----  90  fbs. 

Coke 84 

Charcoal  of  Wood     -  -  95 

Wood 32 

Peat  -----  19 

Lavoisier,  in  making  experiments  on  combustibles  generally,  to  ascertain 
the  quantities  of  oxygen  expended,  and  of  heat  given  out  during  the  combus- 
tion of  a  given  quantity  of  each,  obtained  the  following  results  : 

One  pound  of  Melts  of  ice    Takes  of  oxygen 

Hydogen  gas  370  Ibs.  7J  Ibs. 

Carburetted  hydrogen    -  85  4 

Olive  oil      ---  120  3 

Wax     ---  -       110  3 

Tallow                     -            -  105  3 

Charcoal  ...         95  2f 

Phosphorus-                         -  100  H 

Sulphur            -  25  1 

The  following  remarks  with  respect  to  fuel,  and  the  modes  of  using  it,  seem 
to  demand  a  place  here. 

A  pound  of  coke  produces  nearly  as  much  heat  as  a  pound  of  coal ;  but 
a  pound  of  coke  is  the  produce  of  a  pound  and  a  third  of  coal,  although  the 
coke  is  more  bulky  than  the  coal. 

It  is  wasteful  to  wet  fuel,  because  the  moisture  in  being  evaporated  carries 
off  with  it  as  latent,  and  therefore  useless  heat,  a  considerable  proportion  of 
what  the  combustion  produces.  It  is  a  very  common  prejudice,  that  the 
wetting  of  coal,  by  making  it  last  longer,  is  effecting  a  great  saving ;  but  it 
does  so  merely  by  restraining  the  combustion  or  producing  a  smaller  fire,  and 
with  the  bad  fire,  there  is  also  much  waste  of  heat.  To  illustrate  the  influ- 
ence of  watery  vapour  upon  combustion,  we  may  mention  the  fact,  that  a 
manufacturer  who  tried  to  blow  his  fire  by  forcing  steam  into  the  furnace 
with  the  air,  extinguished  his  fire ;  and  the  analogous  fact,  that  ordinary  fires 
burn  better  in  winter  than  in  summer,  although  the  temperature  be  lower, 
because  cold  air  is  generally  much  drier  than  that  which  is  warm. 

Coal  which  contains  much  hydrogen,  as  all  flaming  coal  does,  is  used 
wastefully  when  any  of  the  hydrogen  is  allowed  to  escape  unburned ;  for, 
first,  the  great  heat  which  the  combustion  of  such  hydrogen  would  produce 
is  not  obtained ;  and,  secondly,  the  hydrogen  while  becoming  gas,  absorbs 
into  the  latent  state  still  more  heat  than  an  equal  weight  of  water  would. 
Now  the  smoke  of  a  fire  is  the  hydrogen  of  the  coal  rising  in  combination 
with  a  portion  of  carbon.  We  see,  therefore,  that  by  causing  a  fire  to 
destroy  or  burn  its  smoke,  we  not  only  prevent  a  nuisance,  but  effect  a  great 
saving.  The  reason  that  common  fires  give  out  so  much  smoke  is,  either 
that  the  smoke  or  vaporized  pitch  is  not  sufficiently  heated  to  burn;  or  that 


320  HEAT. 

the  air  mixed  with  it  as  it  ascends  in  the  chimney,  has  already,  by  passing 
through  the  fire,  been  deprived  of  its  free  oxygen.  If  the  pitch  be  very 
much  heated,  its  ingredients  assume  a  new  arrangement,  becoming  transpa- 
rent, and  constituting  the  common  coal-gas  of  our  lamps ;  but  at  lower  tem- 
peratures, it  is  seen  jetting,  from  cracks  or  openings  in  the  coal,  as  a  dense 
smoke — a  smoke,  however,  which  immediately  becomes  a  brilliant  flame  if 
lighted  by  a* piece  of  burning  paper  or  the  approximation  of  the  combustion. 
The  alternate  bursting  out  and  extinction  of  these  burning  jets  of  pitchy 
vapour,  contribute  to  render  a  common  fire  the  lively  and  agreeable  object 
which  it  is  in  the  winter  ^evenings.  When  coal  is  first  thrown  upon  a  fire, 
a  great  quantity  of  vapourized  pitch  escapes  as  a  dense  smoke,  too  cold  to 
burn,  and  for  a  time  the  flame  is  smothered,  or  there  is  none ;  but  as  the 
fresh  coal  becomes  heated,  its  vapour  reproduces  the  flame  as  before.  In 
close  fire-places,  as  those  of  steam-engines,  brewing  and  dyeing  apparatus, 
&c.,  all  the  air  which  enters  after  the  furnace-door  is  shut,  must  pass  through 
the  grate  and  the  burning  fuel,  so  that  its  oxygen  is  consumed  by  the  red« 
hot  coal  before  ascending  to  where  the  smoke  is.  The  smoke,  therefore, 
however  hot,  passes  away  unburnt;  unless,  sometimes,  as  over  foundry  fur- 
naces, where  the  heat  is  very  great  indeed,  and  it  burns  as  a  flame  or  great 
gas  lamp  at  the  chimney-top  on  reaching  the  oxygen  of  the  open  atmosphere. 
There  have  been  many  modes  proposed  of  destroying  smoke :  one  has 
been  to  admit  to  the  space  above  the  fire,  by  a  suitable  opening,  a  certain 
quantity  of  fresh  air,  the  oxygen  of  which  may  inflame  the  smoke.  This 
plan  is  efficient  at  a  certain  point  of  time  after  the  addition  of  fresh  fuel, 
and  then  for  a  time  effects  the  saving  of  fuel ;  but  the  difficulty  of  admitting 
just  the  quantity  of  air  required  to  suit  the  varying  demand  for  it,  has  not 
been  overcome,  and  hence,  from  there  being  no  saving  on  the  whole,  the^ 
plan  has  been  abandoned.  When  just  enough  air  entered,  the  flame  pro- 
duced gave  so  intense  a  heat  as  in  several  cases  to  have  burned  or  destroyed 
the  parts  of  valuable  boilers  exposed  to  it ;  and  when,  on  the  contrary,  too 
much  air  entered,  it  injuriously  cooled  the  boiler.  A  contrivance  at  present 
commonly  adopted  for  burning  smoke,  is  that*  of  Mr.  Brunton,  namely,  a 
circular  fire-grate,  kept  turning  like  a  horizontal  wheel,  on  which,  by  ma- 
chinery, coal  is  made  to  fall  in  a  gradual  manner,  so  as  to  be  uniformly 
spread  over  it.  The  coal  falls  so  gradually,  that  although  there  is  generally 
a 'little  smoke  from  it,  there  is  never  much, — the  oxygen  which  finds  en- 
trance through  and  around  the  grate,  being  always  in  quantity  the  same, 
and  nearly  sufficient.  A  smoke-consuming  fire  would  be  perfect,  in  which 
the  fuel  were  made  to  burn  only  at  the  upper  surface  of  its  mass,  so  that 
the  pitch  and  gas  disengaged  from  it,  as  the  heat  spread  downwards,  might 
have  to  pass  through  the  burning  coals  where  fresh  air  were  mixing  with 
them ;  and  thus  the  gas  and  smoke,  being  the  most  inflammable  parts, 
would  burn  first  and  be  all  consumed.  This  was  the  principle  proposed 
in  a  fire-place  suggested  by  the  author  for  the  great  brewery  of  Mr.  Meux 
in  his  neighbourhood,  and  tried  at  the  time  when  attempts  were  extensively 
made  to  abate  the  nuisance  of  srnoke  in  towns.  The  experiment  proved 
the  theoretical  perfection  of  the  method,  and  that  it  would  produce  a  saving 
of  15  or  20  per  cent,  on  the  expenditure  of  coal ;  but  before  a  durable  grate 
of  the  kind  was  completed,  the  Welch  stone-coal  was  introduced,  which 
has  97  per  cent,  of  pure  carbon,  and  therefore  has  no  pitch  to  evaporate, 
and  no  smoke, — and  it  was  at  once  adopted  there  and  in  many  other  places. 
A  little  common  coal  is  added  to  it  to  make  it  more  easily  combustible. 


cl 


FUEL.  321 

Coal  in  a  deep  narrow  trough,  as  a  b  c  d,  if 
lighted  at  its  surface  a  b}  burns  with  a  lofty  Fig.  136. 

flame  as  if  it  were  the  wick  of  a  large  oil- 
lamp  ;  for  all  the  gas  given  out  from  the  coal 
below,  as  that  is  gradually  heated,  passes 
through  the  burning  fuel  and  becomes  a  flame. 
Now,  if  we  suppose  several  such  troughs 
placed  together  with  intervals  between  them, 
like  the  fire  bars  of  a  common  grate  or  furnace, 
there  would  be  a  perfect  non-smoking  fire- 
place. Such  was  that  made  on  the  occasion 

mentioned  ;  and  although,  as  a  mere  experimental  apparatus,  it  was  flimsy 
and  imperfect,  it  put  beyond  a  doubt  the  possibility  of  accomplishing  its 
object.  The  reason  of  the  vast  saving  of  fuel  by  such  a  grate  is,  that  the 
smoke  instead  of  stealing  away  latent  heat — being  itself  also  the  most  com- 
bustible and  precious  part  of  the  fuel,  here  gives  all  its  powers  and  worth  to 
the  purpose  of  the  combustion.  The  coal  rested  on  moveable  bottoms  in 
the  troughs  (here  shown  by  dotted  lines)  and  was  moved  up  like  the  wick 
of  a  lamp  by  its  screw.  The  bottoms  may  be  lifted  in  many  ways.  Or, 
without  bottoms  the  coal  may  be  raised  in  each  trough  successively  by  an 
iron  lever  or  bar  introduced  by  the  fire-man  under  the  coal,  fresh  coal  being 
at  the  same  time  put  into  the  trough  to  fill  the  space  under  the  lever.  The 
author  believes  that  this  principle  of  construction  will  still  be  extensively 
adopted  for  the  Newcastle  or  flaming  coal, — the  consequences  would  be  so 
important.  The  principle  has  been  already  introduced  for  common  parlour 
fires  by  Mr.  Cuttler  in  his  stove,  which  is  merely  a  common  grate,  having, 
instead  of  bottom-bars,  a  deep  box  to  hold  the  coal  for  a  whole  day,  with  a 
moveable  bottom,  which  lifts  the  coal  up  as  wanted.  From  such  a  fire  there 
is  always  ascending  a  long  beautiful  flame  ;  and  much  more  heat  is  given 
out  than  from  the  same  quantity  of  coal  burned  in  the  common  way ;  the 
chimney  never  requires  sweeping,  for  there  is  absolutely  no  smoke  and  there- 
fore no  soot. 

Smoke  is  effectually  consumed  also  in  a  fire-place  in  which  the  air  feeding 
the  combustion  is  caused  to  pass  downwards  through  the  burning  fuel,  so  as 
to  carry  the  smoke  with  it,  instead  of  upwards  as  usual.  This  result  is  at- 
tained by  having  the  chimney  to  communicate  with  the  ash-pit.  The  chief 
objection  is,  that  the  fire-bars  are  quickly  destroyed  by  the  intense  heat  to 
which  they  are  exposed ;  and  to  obviate  this  instead  of  solid  bars,  tubes 
have  been  used  with  water  passing  through  them,  and  admitting  -the  feeding 
air  only  above. 

It  is  evident  that  if  a  house  or  apartment  with  the  air  in  it,  were  once 
warmed  to  a  certain  degree,  it  would  for  ever  retain  its  temperature,  but  for 
the  escape  of  heat  through  the  walls  and  windows,  or  with  the  air  from 
within,  whether  passing  away  as  necessary  ventilation  or  as  waste.  A  per- 
fect system  of  heating,  therefore,  would  consist  in  diminishing  as  much  as 
possible  these  causes  of  loss,  with  reference,  however,  both  to  the  expense 
of  the  means  and  the  salubrity  of  the  dwelling,  and  in  producing  and  distri- 
buting the  heat  judiciously.  It  may  be  asserted  that  a  fourth  part  of  the  fuel 
generally  expanded  in  English  houses,  if  more  skilfully  used,  would  better 
secure  comfort  and  health  than  all  that  is  now  expanded.  But  it  does  not 
accord  with  the  character  of  this  general  work  to  enter  into  minute  detail  on 
the  subject.  Remarks  were  made  upon  it  in  the  chapter  on  "  Pneumatics," 

21 


322  HEAT. 

under  the  head  of  "  warming  and  ventilating/'  and  more  minute  informa- 
.  tion  may  be  obtained  from  Mr.  Tredgold's  work,  expressly  devoted  to  it. 

The  detailed  consideration  of  furnaces,  blow-pipes,  &c.,  may  appear  to 
some  so  clo?ely  connected  with  our  present  subject  as  to  demand  a  place 
here,  but  by  so  treating  of  them  we  should  be  encroaching  on  the  province 
of  the  chemist,  &c.  We  may  state  generally,  that  furnaces  are  merely  struc- 
tures by  which  coal  or  other  fuel  heated  to  the  degree  at  which  it  combines 
rapidly  with  the  oxygen  of  the  atmospheric  air,  is  placed  in  circumstances 
favourable  to  the  rapid  renewal  of  the  air, — and  a  common  blow-pipe  is 
merely  a  jet  of  air  thrown  from  a  minute  opening  into  any  flame,  so  as  with 
great  decision  to  direct  the  point- of  the  flame  upon  the  body  to  be  heated. 
The  sand-bath  and  water-bath  of  the  chemist  are  merely  means  of  insuring 
a^more  uniform  or  steady  temperature : — a  vessel  imbedded  in  sand,  so  that 
heat  can  reach  it  only  through  the  sand,  cannot  be  very  suddenly  heated  or 
cooled,  because  sand  is  a  slow  conductor  ,  and  a  vessel  immersed  in  boiling 
water,  can  never  have  greater  heat  than  212°,  or  the  boiling  heat  of  water. 
For  certain  purposes,  hotter  baths,  as  of  high-pressure  steam,  or  of  solutions 
of  certain  salts,  or  of  vapour  of  oil  of  turpentine,  or  of  boiling  whale  oil,  have 
been  used.  On  such  subjects,  readers  may  consult  works  on  "  chemistry 
applied  to  the  arts." 

t(  Condensation  and  friction  as  causes  of  heat"  (Read  Analysis,  page  256.) 

A  soft  iron-nail  laid  upon  an  anvil,  and  receiving  in  rapid  succession  three 
or  four  powerful  blows  of  a  hammer  becomes  hot  enough  to  light  a  match, 
and  if  longer  hammered,  will  become  incandescent  or  red  hot, — parti v  from 
the  diminished  volume  or  condensation  of  the  iron,  on  the  principle  already 
explained,  and  partly  from  the  percussion  or  friction,  in  a  way  not  yet  well 
understood,  but  probably  electrical 

In  the  familiar  case  of  the  mutual  percussion  of  flint  and  steel,  small  por- 
tions of  one  or  both  are  struck  off  by  the  violence  of  the  collision,  in  a  state 
of  white  heat,  and  the  particles  of  the  iron  burn  in  passing  through  the  air  j 
in  a  vacuum,  the  heated  particles  are  equally  produced,  but  are  scarcely 
visible  from  this  combustion  not  occurring.  In  both  cases,  they  suffice  to 
inflame  gunpowder,  or  to  light  tinder.  When  the  materials  are  good  the 
shower  or  burst  of  sparks  from  the  sudden  blow  is  most  brilliant. 

The  heat  produced  by  friction  alone  without  perceivable  condensation  of 
the  bodies  concerned,  is  exemplified  in  many  facts.  Two  dry  branches  of  a 
tree  kept  strongly  rubbing  against  each  other  by  the  wind,  have  sometimes 
set  a  wood  on  fire.  Savages  light  their  fires  by  analogous  means.  Men  warm 
their  cold  hands  in  winter,  by  rubbing  them  against  each  other,  or  against 
their  coat  sleeves.  Again,  the  axle-tree  of  a  heavily  laden  wagon  or  other 
carriage,  if  left  without  oil,  is  often  heated  so  as  to  inflame  wood  near  it. 
The  line  attached  to  a  whale-harpoon  as  it  runs  over  the  side  of  the  boat 
when  the  whale  dives  after  the  harpoon  has  entered  his  flesh,  requires  that 
water  be  constantly  thrown  upon  it  to  prevent  its  setting  fire  to  the  boat. 
The  cable  of  a  ship  drawn  very  rapidly  through  the  hawse-hole  by  the  falling 
anchor  produces  there  intense  heat  and  smoke.  When  a  great  ship  is 
launched  from  the  builder's  yard  into  the  deep,  and  glides  along  the 
sloping  beams,  a  dense  smoke  usually  rises  from  the  points  of  rubbing 
contact. 


ANIMAL    FUNCTIONS    A    SOURCE    OF    HEAT.         323 


"  The  functions  of  animal  life  a  source  of  heat."     (Read  the  Analysis. 

p.  256.) 

It  is  one  of  the  remarkable  facts  in  nature,  that  living  animal  bodies,  and 
to  a  certain  degree  living  vegetables  also,  have  the  property  of  maintaining 
in  themselves  a  peculiar  temperature,  whether  surrounded  by  bodies  that  are 
hotter  or  colder  than  they.  Captain  Parry's  sailors,  during  the  polar  winter, 
where  they  were  breathing  air  that  froze  mercury,  still  had  in  them  their 
natural  warmth  of  98°  of  Fahrenheit :  and  the  inhabitants  of  India,  where 
the  same  thermometer  stands  sometimes  at  115°  in  the  shade  have  their 
blood  at  no  higher  temperature. 

It  was  at  one  time  the  favourite  explanation, of  this,  that  animal  heat  was 
produced  in  the  lungs,  during  respiration,  from  the  oxygen  then  admitted. 
This  oxygen  combines  with  carbon  from  the  blood,  and  becomes  carbonic 
acid  as  in  combustion,  and  it  was  supposed  to  give  out  a  portion  of  its  latent 
heat,  as  in  actual  combustion ;  which  heat,  being  then  spread  o\er  the  body 
by  the  circulating  blood,  maintained  the  temperature.  We  now  know  that 
if  such  a  process  assist,  which  it  probably  does, — for  the  animal  heat  has 
generally  a  relation  to  the  quantity  of  oxygen  required  in  any  particular 
case,  and  when  an  animal  being  already  much  heated  needs  no  increase, 
very  little  oxygen  disappears, —  still  much  of  the  effect  is  dependent  on 
the  influence  of  the  nerves  either  directly  or  indirectly,  through  the  vital 
functions  governed  by  them.  Mr.  Brodie,  in  his  important  experiments 
upon  the  subject  found  that,  although  in  animals  apparently  dead  from  injury 
done  to  the  nervous  system,  he  could  artificially  continue  the  action  of  res- 
piration, with  the  usual  formation  of  carbonic  acid,  still  the  temperature  fell 
very  quickly.  Again,  the  maintenance  of  low  temperature  in  an  animal 
immersed  in  air  hotter  than  itself,  is  partly  attributable  to  the  copious  per- 
spiration and  evaporation  which  then  take  place,  and  which  absorb  into  the 
latent  form  the  excess  of  heat  then  existing.  Perspiration,  both  from  the 
skin  and  internal  surface  of  the  lungs,  occurs  generaly  in  proportion  to  the 
excess  of  heat.  Dogs  and  other  animals  when  much  heated,  as  they  cannot 
throw  oif  or  diminish  their  natural  covering,  increase  the  evaporating  surface 
by  protruding  a  long,  humid  tongue. 

The  power  in  animals  of  preserving  their  peculiar  temperature  has  its 
limits.  Intense  ct»ld  coming  suddenly  upon  a  man  who  has  not  sufficient 
protection,  first  causes  a  sensation  of  pain,  and  then  brings  on  an  almost 
irresistible  sleepiness,  which  if  indulged  in  proves  fatal.  Sir  Joseph  Banks 
-having  gone  on  shore  one  day  near  the  cold  Cape  Horn,  and  being  fatigued, 
was  so  overcome  by  the  feeling  mentioned,  that  he  entreated  his  companions 
to  let  him  sleep  but  for  a  little  while.  His  prayer,  if  granted,  might  have 
allowed  to  come  upon  him  that  sleep  which  ends  not — as,  under  similar 
circumstances,  it  came  upon  so  many  thousands  of  the  army  which  Buona- 
parte led  into  Russia,  and  lost  there  during  the  disastrous  retreat  through 
the  snows.  The  celebrated  bulletin  which  allowed  that,  in  one  night,  when 
the  thermometer  of  Reaumur  stood  at  19°  below  zero,  30,000  horses  died, 
declared  not  the  number  of  human  victims — tenderly  loved  husbands  and 
brothers,  and  children  of  thousands  anxiously  waiting  their  return,  but  doomed 
never  to  see  them  more.  Cold  in  inferior  degrees,  and  longer  continued, 
acting  on  persons  imperfectly  protected  by  clothing,  &c.,  induces  a  variety 
of  diseases  which  destroy  more  slowly ;  as  many  of  the  winter  diseases  of 
England.  A  great  excess  of  heat;  again;  may  at  once  excite  a  fatal  appo- 


324  HEAT. 

plexy,  and  heat  in  inferior  degrees,  "but  long  continued,  may  cause  those 
fevers,  &c.,  which  prevail  in  warm  climates,  and  which  are  so  destructive  to 
strangers. 

Each  species  of  animal  "has  a  temperature  natural  to  it,  and  in  the  diversity 
are  found  creatures  fitted  to  live  in  all  parts  of  the  earth,  what  is  wanting  in 
internal  bodily  constitution  being  found  in  the  admirable  covering  which  has 
been  provided  to  protect  them — covering  which  grows  from  their  bodies, 
with  form  of  fur  or  feather,  in  the  exact  degree  required,  and  even  so  as  in 
the  same  animal  to  vary  with  climate  and  season.  Such  covering,  however, 
has  been  denied  to  man;  but  the  denial  is  not  one  of  unkindness, — it  is  an 
indication  of  his  superior  nature  and  destinies.  God-like  reason  was  be- 
stowed on  him,  by  which  he  subjects  all  nature  to  his  use,  and  he  was  left 
to  clothe  himself. 

The  human  race  is  naturally  inhabitant  of  a  warm  climate,  and  the  para- 
dise described  as  Adam's  first  abode,  may  be  said  still  to  exist  over  vast 
regions  about  the  equator.  There  the  sun's  influence  is  strong  and  uniform, 
producing  a*rich  and  warm  garden,  in  which  human  beings,  however  igno- 
rant of  the  world  which  they  had  come  to  inhabit,  would  have  their  necessi- 
ties at  once  supplied.  The  ripe  fruit  is  there  always  hanging  from  the 
branches;  of  clothing  there  is  required  only  what  moral  feelings  may  dictate, 
or  what  may  be  supposed  to  add  grace  to  the  form ;  and  as  shelter  from  the 
weather,  a  few  broad  leaves  spread  on  connected  reeds,  complete  the  Indian 
hut.  The  human  family,  in  multiplying  and  spreading  in  all  directions  from 
such  a  centre,  would  find  to  the  east  and  west,  only  the  lengthened  paradise, 
with  slightly  varying  features  of  beauty ;  but  to  the  north  and  south,  the 
changes  of  season,  which  make  the  bee  of  high  lattitudes  lay  up  its  winter 
store  of  honey,  and  send  migrating  birds  from  country  to  country  in  search 
of  warmth  and  food,  would  also  rouse  man's  energies  to  protect  himself. 
His  faculties  of  foresight  and  contrivance  would  come  into  play,  awakening 
industry :  and  as  their  fruits,  he  would  soon  possess  the  knowledge  and  the 
arts  which  secure  to  him  a  happy  existence  in  all  climates,  from  the  equator 
almost  to  the  pole.  And  it  is  chiefly  because  man  has  learned  to  produce  at 
will,  and  to  control,  the  wonder-working  principle  of  heat,  that  in  the  rude 
winter,  which  seems  the  death  of  nature,  he,  and  other  tropical  animals  and 
plants  which  he  protects,  do  not  in  reality  perish — as  exemplified  when  a 
Canary  bird  escapes  from  its  cage,  or  an  infant  is  exposed  among  the  snow- 
hills.  By  producing  heat  from  his  fire,  he  then  obtains  a  novel  but  most 
pleasurable  sort  of  existence  :  and  in  the  night,  while  the  dark  and  freezing 
winds  are  howling  over  his  roof,  he  basks  in  the  presence  of  his  mimic  sun, 
surrounded  by  his  friends  and  all  the  delights  of  society ;  while  in  his  store- 
rooms, or  in  those  of  merchants  at  his  command,  he  has  the  treasured  deli- 
cacies of  every  season  and  clime.  He  soon  becomes  aware,  too,  that  the 
dreary  winter,  instead  of  being  a  curse,  is  really  in  many  respects  a  bleseing, 
by  arousing  from  the  apathy  to  which  the  eternal  serenity  of  a  tropical  sky 
so  much  disposes.  He  sees  that  in  climates  where  labour  and  ingenuity 
must  precede  enjoyment,  every  faculty  of  mind  and  body  is  invigorated ;  and 
that  hence  the  sterner  climates  produce  the  perfect  man  ; — that  in  them  the 
arts  and  sciences  have  reached  their  present  advancement,  and  the  brightest 
examples  have  arisen  of  intellectual  and  moral  excellence  :  while  from  them, 
as  centres,  knowledge  and  example  are  spreading  over  all  the  earth  and 
promising  soon  to  render  the  whole  of  human  kind  but  one  large  and  happy 
family. 


LIGHT.  325 

• 

• 

PART  IV. 

(CONTINUED.) 
SECTION  II.— OX  LIGHT,  OR  OPTICS. 

ANALYSIS   OF   THE    SECTION. 

Light  is  an  emanation  from  the  sun  and  other  self-luminous  bodies,  becom- 
ing less  intense  as  it  spreads,  and  which,  I y  falling  on  other  bodies,  and 
being  reflected  from  them  to  the  eye,  renders  them  visible.  Its  absence  is 
called  darkness.  It  moves  with  great  velocity,  and  in  straight  lines 
where  there  is  no  obstacle — leaving  shadows  ichere  it  cannot  fall.  It 
passes  readily  through  some  bodies — which  are  therefore  called  transpa- 
rent— but  when  it  enters  or  leaves  their  surfaces  obliquely,  it  suffers,  at 
them  a  degree  of  bending  or  refraction  proportioned  to  the  obliquity. 
And  a  beam  of  white  light  thus  refracted  or  bent,  does  not  all  bend  equally, 
but  is  divided  or  resolved  into  beams  of  what  are  called  the  elementary 
colours,  which  colours,  on  being  again  blended,  become  the  white  light  as 
before. 

Transparent  bodies,  as  glass,  may  be  made  of  such  form  as,  by  the  powers 
of  refraction  thence  received,  to  cause  all  the  rays  which  pass  through 
them  from  any  given  point  to  bend  and  meet  again  in  another  point 
beyond  them  ; — the  body,  then,  because  usually  in  form  somewhat  resem- 
bling a  flat  bean  or  lentil,  being  called  a  LENS.  And  when  the  light  thus 
proceeding  from  every  point  of  an  object  placed  before  a  lens  is  collected 
at  corresponding  points  behind  it,  a  perfect  image  of  the  object  is  there 
produced,  which  may  be  seen  from  any  situation  on  a  white  screen  placed 
to  receive  it,  or  in  the  air,  if  viewed  from  behind.  Now  the  most  important 
optical  instruments,  and  even  the  living  eye,  are  merely  various  arrange- 
ments af  parts  for  producing  and  examining  such  images  as  now  described. 
When  this  image  is  received  upon  a  suitable  white  surface  or  screen  in  a 
dark  room,  the  arrangement  is  called,  according  to  minor  circumstances, 
a  CAMERA  OBSCURA,  a  MAGIC  LANTERN,  or  a  SOLAR  MICROSCOPE.  And 
the  EYE  itself  is,  in  fact,  but  a  small  camera  obscura,  enabling  the  mind 
to  judge  of  external  objects,  by  the  size,  brightness,  colour,  &c.,  of  the  very 
minute  but  most  perfect  images  or  pictures  formed  at  its  back  part,  on  the 
smooth  screen  of  nerve  called  the  retina.  The  art  of  painting  aims  at 
producing  on  a  larger  scale  such  a  picture  as  is  formed  on  the  retina,  and 
which,  when  afterwards  held  before  the  eye,  and  reproducing  itself  in  mini- 
ature upon  the  retin-i,  may  excite  the  same  impression  as  the  original 
objects. —  When  the  image  beyond  a  lens,  formed  as  above  described,  is 
viewed  in  the  air,  by  looking  at  it  from  behind,  that  is,  from  a  situation 
where  the  light  continued  from  it  passes,  then  there  is  exhibited  the  arrange- 
ment  of  parts  constituting  the,  TELESCOPE  or  COMMON  MICROSCOPE. 


326  LIGHT. 

• 

Li gh  falling  on  very  smooth  or  polished  surfaces,  is  reflected  so  nearly  in 
the  order  in  which  it  falls,  as  to  appear  to  the  eye  receiving  it  as  'if  coming 
directly  from  the  objects  originally  emitting  it — and  such  surfaces  are 
called  mirrors.  Mirrors  may  be  plane ,  convex,  or  concave  ;  and  certain 
forms  will  concentrate  light,  to  produce  images  by  reflection,  just  as 
lenses  produce  them  by  refraction  ;  so  that  there  are  reflecting  telescopes, 
&c.,  a9  there  are  refracting  instruments  of  the  same  name.  Light,  again, 
falling  on  bodies  of  rougher  or  irregular  surface,  or  which  have  other 
peculiarities^  is  so  modified  as  to  produce  all  those  phenomena  of  colour 
and  varied  brightness  seen  among  natural  bodies,  and  giving  them  their 
distinctive  characters  and  beauty. 


"  Light:'     (See  the  Analysis.) 

The  phenomena  of  light  and  vision  have  always  been  held  to  constitute  a 
most  interesting  branch  of  natural  science ;  whether  in  regard  to  the  beauty 
of  light,  or  its  utility.  The  beauty  is  seen  spread  over  a  varied  landscape — 
among  the  beds  of  the  flower-gardens,  on  the  spangled  meads,  in  the  plumage 
of  birds,  in  the  clouds  around  the  rising  and  setting  sun,  in  the  circles  of  the 
rainbow.  And  the  utility  may  be  judged  of  by  the  reflection,  that  if  man 
had  been  compelled  to  supply  his  wants  by  groping  in  utter  a.nd  unchangeable 
darkness,  even  if  originally  created  with  all  the  knowledge  now  existing  in 
the  world,  he  could  scarcely  have  secured  his  existence  for  one  day.  Indeed, 
without  light,  the  earth  would  have  been  an  unfit  abode,  even  for  grubs, 
generated  and  living  always  amidst  their  food.  Eternal  night  would  have 
been  universal  death.  Light,  then,  while  the  beauteous  garb  of  nature, 
clothing  the  garden  and  the  meadow — glowing  in  the  ruby — sparkling  in  the 
diamond,  is  also  the  absolutely  necessary  medium  of  communication  between 
living  creatures  and  the  universe  around  them.  The  rising  sun  is  what  con- 
verts the  wilderness  of  darkness  which  night  covered,  and  which,  to  the 
young  mind  not  yet  aware  of  the  regularity  of  nature's  changes,  is  so  full  of 
horror,  into  a  visible  and  lovely  paradise  ] — no  wonder,  then,  if  in  early  ages 
of  the  world,  man  has  often  been  seen  bending  the  knee  before  the  glorious 
luminary,  and  worshipping  it  as  the  God  of  Nature.  When  a  mariner,  who 
has  been  toiling  in  midnight  gloom  and  tempest,  at  last  perceives  the  dawn 
of  day,  or  even  the  rising  of  the  moon,  the  waves  seem  to  him  less  lofty, 
the  wind  is  only  half  as  fierce,  and  hope  and  gladness  beam  on  him  with  the 
light  of  heaven.  A  man,  wherever  placed  in  light,  receives  by  the  eye  from 
every  object  around,  nay,  from  every  point  in  every  object  and  at  every 
moment  of  time,  a  messenger  of  light  to  tell  him  what  is  there,  and  in  what 
condition.  Were  he  omnipresent,  or  had  he  the  power  of  flitting  from  place 
to  place  with  the  speed  of  the  wind,  he  could  scarcely  be  more  promptly 
informed.  Then,  in  many  cases  where  distance  intervenes  not,  light  can 
impart  at  once  knowledge,  which,  by  any  other  conceivable  means,  could 
come  only  tediously,  or  not  at  all.  For  example,  when  the  illuminated  coun. 
tenance  is  revealing  the  secret  workings  of  the  heart,  the  tongue  would  in 
vain  try  to  speak,  even  in  long  phrases,  what  one  smile  of  friendship  or 
affection  can  in  an  instant  convey; — and  had  there  been  no  light,  man  never 
could  have  suspected  the  existence  of  the  miniature  worlds  of  life  and  activity 
which,  even  in  a  drop  of  water,  the  microscope  discovers  to  him ;  nor  could 
he  have  formed  any  idea  of  the  admirable  structure  of  many  minute  objects. 


EMANATION    FROM    THE     SUN,     ETC.  327 

It  is  light,  again,  which  gives  the  telegraph,  by  which  men  readily  converse 
from  hill  to  hill,  or  across  an  extent  of  raging  sea — and  it  is  light  which, 
pouring  upon  the  eye  through  the  optic  tube,  brings  intelligence  of  events 
passing  in  the  remotest  regions  of  space. 

"  Emanation  from  the  sun,"  &c.     (See  the  Analysis,  page  325.) 
\ 

The  relation  of  the  sun  to  light  is  most  strikingly  marked  in  the  contrast 
between  night  and  day.  In  tropical  countries  where  the  sun  rises  and  sets 
almost  perpendicularly,  and  allows  not  the  long  dawn  and  twilight  of  tem- 
perate latitudes,  the  change  from  perfect  darkness  to  the  overpowering  efful- 
gence of  day,  and  the  contrary  change,  are  so  sudden  as  to  be  most  impres- 
sive. An  eye  turned  in  the  morning  to  the  east  has  scarcely  noted  a  com- 
mencing brightness  there,  when  that  brightness  has  already  become  a  glow; 
and  the  clouds  floating  near  so  as  to  meet  the  upward  rays,  appear  like 
masses  of  gold  fleece  suspended  in  the  sky :  a  little  after  the  whole  atmos- 
phere is  bright,  and  as  the  stream  of  light  reaches  the  lofty  mountain-tops, 
it  makes  them  shine  like  burnished  pinnacles ;  then  as  that  stream  descends 
to  lower  and  lower  levels,  the  inhabitants  in  succession  see  the  radiant  orb 
first  rising  above  the  horizon  like  a  tip  of  flame,  and  soon  displaying  all  its 
breadth  and  glory,  too  bright  for  the  eye  to  dwell  upon.  With  evening  the 
same  appearances  recur  in  a  reversed  order,  ending,  as  in  the  morning  they 
began  by  complete  darkness. 

Light  emanates  also  from  the  stars,  but  they  are  so  distant  as  in  that  re- 
spect to  be  of  little  importance  to  this  earth.  And  all  bodies  in  combustion 
are  self-luminious,  as  exemplified  in  our  common  fires  and  lamps.  And 
there  are  still  other  transient  sources  in  animal  and  vegetable  nature,  and 
among  solar  phosphori. 

There  have  been  two  opinions  respecting  the  nature  of  light;  one,  that  it 
consists  of  extremely  minute  particles  darting  all  around  from  theluminious 
body ;  the  other,  that  the  phenomenon  is  altogether  dependent  on  an  undula- 
tion among  the  particles  of  a  very  subtile  elastic  fluid  diffused  through  space 
— as  sound  is  dependent  on  an  undulation  among  air-particles.  To  admit  the 
first  opinion,  the  particles  of  light  must  be  held  to  be  most  wonderfully 
minute,  for  a  common  taper  can  fill  with  them  during  hours  a  space  of  four 
miles  in  diameter;  and  with  the  extreme  velocity  of  light,  if  its  particle  pos- 
sessed at  all  the  property  of  matter  called  inertia,  their  momentum  should  be 
very  remarkable; — yet,  even  a  large  sunbeam  collected  by  a  burning-glass, 
and  with  the  precautions  necessary  in  the  case,  thrown  upon  the  scale  of  a 
most  delicate  balance,  has  sot  the  slightest  effect  upon  the  equilibrium. 
Such,  and  many  other  facts  to  be  treated  of  in  subsequent  parts  of  this  work, 
lead  to  the  opinion  that  there  is  an  undulation  of  an  elastic  fluid  concerned 
in  producing  the  phenomenon  of  light, — although  the  fact  of  light  spreading 
so  nearly  in  straight  lines,  as  if  only  the  crown  of  the  wave  had  existence, 
instead  of  being  diffused  like  sound,  is  an  important  difference. 

"  Becoming  less  intense  as  it  spreads"     (See  the  Analysis,  page  325.) 

Any  emanation  from  a  central  point  in  spreading  through  water  space, 
becomes  proportionally  thinner  or  less  intense.  Thus,  if  a  taper  be  placed 
in  the  centre  of  a  box,  each  side  of  which  is  a  foot  square,  the  light  falling 
on  the  sides  of  the  box  will  have  a  certain  intensity  there  : — if  the  taper  be 
then  placed  in  a  box  with  sides  of  two  feet  square,  there  will  be  only  the 


328  LIGHT. 

same  quantity  of  light,  but  it  will  be  spread  over  four  times  the  surface,  (a 
square  of  two  feet  is  made  up  of  four  squares  of  one  foot,)  and  will,  therefore, 
on  any  part  of  that  surface,  be  only  one-fourth  part  as  strong  or  intense  as 
in"  the  first  box : — and  so  for  any  other  size  of  box  or  space,  the  intensity 
will  diminish  as  the  square  of  the  distance  increases. 

Hence  four  times  as  much  light  and  heat  fall  upon  a  foot  of  this  earth's 
surface  as  upon  a  foot  of  the  surface  of  the  planet  Mars,  which  is  twice  as 
distant  from  the  sun  : — as  four  times  as  much  light  and  heat  fall  on  a  man 
who  is  at  one  yard  from  the  fire,  as  on  another  who  is  distant  two  yards. 

"  Falling  on  other  bodies  makes  them  visible."     (Read  the  Analysis, 

page  325.) 

If  the  window-shutter  of  an  apartment  be  perfectly  closed,  an  eye  there 
turns  upon  an  absolute  blank  :  it  perceives  nothing. 

If  a  ray  of  the  sun  be  then  admitted,  and  made  to  fall  upon  any  object,  that 
object  becomes  bright,  and  affects  the  eye  as  if  it  were  itself  luminious.  It 
returns  a  part  of  the  light  which  falls  upon  it,  and  it  is  visible  in  all  directions, 
proving  that  it  scatters  the  received  light  all  around.  This  scattered  light, 
again  falling  on  other  objects,  and  reflected  from  and  among  them  until  ab- 
sorbed, like  echo  repeated  many  times  and  lost  between  perpendicular  rocks, 
makes  all  of  them  visible,  although  in  a  less  degree,  and  the  whole  apartment 
is  said  to  be  lighted.  If  the  sun's  ray  be  made  to  fall  upon  a  thing  which, 
from  its  nature,  reflects  much  of  the  light,  as  a  sheet  of  white  paper,  the 
apartment  will  be  well  lighted : — if,  on  the  contrary,  it  be  received  on  black 
velvet,  which  returns  hardly  any  light,  the  apartment  will  remain  dark ; — 
and,  again,  if  received  on  a  polished  surface  or  mirror,  which  returns  nearly 
the  whole  light,  but  in  one  direction  only,  and  therefore  throws  it  upon  some 
other  single  object,  the  effect  will  be  according  to  the  nature  of  that  object, 
and  nearly  as  if  the  ray  had  fcillen  directly  upon  it. 

Now  all  bodies  on  earth,  and  among  these  the  constituent  particles  of  the 
mass  of  atmosphere  surrounding  the  earth,  retain  and  diffuse  among  them- 
selves, for  a  time,  the  light  received  directly  from  the  sun,  and  by  so  doing, 
maintain  every  where  that  milder  radiance  so  agreeable  to  the  sight,  which 
renders  objects  visible  when  the  sun's  direct  ray  does  not  fall  upon  them. 
But  for  this  fact  indeed,  all  bodies  shadowed  from  the  sun,  whether  by  in- 
tervening clouds  or  by  any  other  more  opaque  masses  on  earth,  would  be 
perfectly  black  or  dark;  that  is,  totally  invisible.  And  without  an  atmos- 
phere, the  sun  would  appear  a  round  luminious  mass  in  a  perfectly  black  sky. 
On  lofty  mountain  summits,  where  half  the  atmosphere  is  below  the  level,  the 
direct  rays  of  the  sun  are  painfully  intense,  and  the  sky  is  of  darkest  blue. 

A  shadow  is  the  name  given  to  the  comparative  darkness  of  places  or 
objects  which  are  prevented  by  intervening  things  from  receiving  the  direct 
rays  of  some  luminious  body  shining  on  the  things  around.  The  apparent 
darkness 'of  a  shadow,  however,  is  not  proportioned  to  its  real  darkness, 
but  the  intensity  of  the  surrounding  lights.  A  landscape  may  be  very 
bright,  even  when  the  sun  is  veiled  by  a  cloud,  and  then  little  or  no  shadow 
is  perceived ;  but  as  soon  as  the  cloud  passes  away,  deep  shadows  are  cast 
behind  or  beyond  every  projecting  object.  Yet  the  objects  and  places  then 
appearing  so  dark,  are  in  reality  more  illuminated  than  before  the  shadow 
existed,  for  they  are  receiving,  and  again  scattering  new  light  from  all  the 
more  intensely  illuminated  objects  around  them.  A  finger  held  between  a 
candle  and  the  wall  casts  a  shadow  of  a  certain  intensity;  if  another  candle 


LIGHT    MOVES    WITH    GREAT    VELOCITY.  329 

be  then  placed  in  the  same  line  from  the  shadow,  the  shadow  will  appear  doubly 
dark,  although  in  fact  more  light  will  be  reaching  it  and  reaching  the  eye  from 
it  than  before ;  it  will  be  more  dark  only  by  comparison.  If  the  candles  be 
separated  laterally,  so  as  to  produce  two  shadows  of  the  finger,  but  which 
coincide  or  overlap  in  one  part,  that  part  will  be  of  double  darkness,  as 
compared  with  the  remainder,  The  most  accurate  mode  of  comparing  lights 
is  to  place  them  at  such  distances  from  a  screen  or  wall,  as  to  make  them  at 
the  same  time  throw  equally  dark  shadows  of  the  same  object ;  and  then 
according  to  the  law  of  decreasing  intensity  explained  above,  to  calculate  the 
intensities  of  the  sources  of  light  by  the  difference  of  their  distances  from 
the  wall.  The  eye  judges  very  easily  of  the  equal  intensity  of  compared 
shadows  of  the  same  object. 

The  real  darkness  of  a  shadow,  then,  depends  on  the  number  and  nature 
of  the  light  reflecting  objects  around  it.  Thus  shadows  are  less  remarkable 
opposite  to  any  white  surface,  as  that  of  a  recently  painted  wall,  than  in  other 
situations.  The  reason  why  the  moon,  when  eclipsed,  that  is,  as  will  be 
afterwards  explained,  when  passing  behind  the  earth,  or  through  the  shadow 
cast  by  the  earth  in  a  direction  away  from  the  sun,  becomes  almost,  if  not 
quite  invisible,  is  that  there  are  no  other  moons  or  bodies  bearing  laterally  on 
the  moon  to  share  their  light  with  it.  And  the  reason  why  our  nights  on 
earth  are  darker  than  the  shadows  behind  a  house  or  rock  in  the  sunshine  of 
day,  is  merely  that  there  are  not  other  earths  near  us  to  reflect  light  into  the 
great  night-shadow  of  the  earth,  as  there  are  other  houses  and  rocks  to  illu- 
minate the  day-shadow  of  these.  The  moon  is  the  only  light-reflecting  body 
which  the  earth  has  near  it ;  and  we  perceive  how  much  less  dark  the  earth's 
night-shadow  is  when  the  moon  is  so  placed  as  to  bear  upon  it.  The  eclipsed 
moon,  again,  is  invisible  to  men  on  earth,  because  it  receives  neither  sunshine 
nor  reflected  light  from  this  earth,  for  the  side  turned  towards  us  faces  the 
shadowed  part  of  the  earth ;  but  when  the  moon  is  near  the  situation  in 
which  it  is  called  new  moon,  or  between  us  and  the  sun,  the  shaded  side  of 
the  moon  is  then,  in  a  degree,  visible  to  us  because  facing  the  enlightened 
side  of  the  earth;  the  bright  crescent,  or  part  of  the  moon  illuminated  by 
the  sun,  appearing  to  embrace  the  non-illuminated  part,  and  giving  occasion 
to  the  popular  saying,  that  the  new  moon  holds  the  old  moon  in  its  arms. 

Many  persons  have  doubted  whether  the  light  of  the  moon  could  be 
altogether  reflected  light  of  the  sun  ;  the  moon  appearing  to  them  more 
luminous  than  any  body  on  earth  merely  exposed  to  the  sunshine.  Their 
error  has  arisen  from  contrasting  the  moon  while  returning  direct  sunshine 
with  the  shadows  of  night  on  the  earth  around  them.  But  could  they  at 
night  see  on  a  hill  near  them,  a  white  tower  or  other  object  scattering  light 
as  when  it  receives  the  rays  of  the  sun,  that  object  being  nearer  than  the 
moon,  would  appear  to  them  almost  to  be  on  fire,  and  much  brighter  than 
the  moon.  The  moon  when  above  the  horizon  in  the  day  time,  is  perfectly 
visible  on  earth,  and  is  then  throwing  towards  the  earth  just  as  much  light 
as  during  the  night;  but  the  day  moon  does  not  appear  more  luminous  than 
any  small  white  cloud :  and  although  visible  every  day,  except  near  the 
change,  many  persons  have  passed  their  lives  without  ever  observing  it.  The 
full  moon  gives  to  the  earth  only  about  a  one-hundred-thousandth  part  as 
much  light  as  the  sun. 

"  Light  moves  with  great  velocity"     (See  Analysis,  page  325.) 

The  extraordinary  precision  with  which  the  astronomical  skill  of  modern 
days  enables  men  to  foretell  the  times  of  remarkable  appearances  or  changes 


330  LIGHT. 

among  the  heavenly  bodies  has  served  for  the  detection  of  the  fact,  that 
light  is  not  an  instantaneous  communication  between  distant  objects  and  the 
eye,  as  was  formerly  believed,  but  is  a  messenger  which  requires  time  to 
travel :  and  the  rate  of  travelling  has  been  ascertained. 

The  eclipses  of  the  satellites  or  moons  of  the  planet  Jupiter  had  been 
carefully  observed  for  some  time,  and  a  rule  was  obtained  which  foretold  the 
instants  in  all  future  time  when  the  satellites  were  to  glide  into  the  shadow 
of  the  planet  and  disappear,  or  when  again  to  immerge  into  view.  Now  it 
was  found,  that  these  appearances  took  place  16J  minutes  sooner  when  Ju- 
piter was  near  the  earth,  or  on  the  same  side  of  the  sun  with  the  earth  than 
when  it  was  on  the  other  side,  that  is  to  say  more  distant  from  the  earth 
by  one  diameter  on  the  earth's  orbit  •  and  at  all  intermediate  stations,  the 
difference  diminished  from  the  16 2  minutes,  in  exact  proportion  to  the  less 
distance  from  the  earth.  This  proves,  then,  that  light  takes  16£  minutes 
to  travel  across  the  earth's  orbit,  and  81  minutes  for  half  that  distance,  or 
to  come  to  us  from  the  sun. 

The  velocity  of  light,  ascertained  in  this  way,  is  such,  that  in  one  second 
of  time,  viz :  during  $  single  vibration  of  a  common  clock  pendulum,  it 
would  go  and  come  from  London  to  Edinburgh  200  times,  the  distance 
between  these  being  400  miles.  This  velocity  is  so  surprising  that  the 
philosophic  Dr.  Hooke,  when  it  was  first  asserted  that  light  was  thus  pro- 
*  gressive,  said  he  could  more  easily  believe  the  passage  to  be  absolutely  in- 
stantaneous, even  for  any  distance,  than  that  there  should  be  a  progressive 
moment  so  prodigiously  rapid.  The  truth,  however,  is  now  put  quite  beyond 
a  doubt  by  any  collateral  facts  bearing  upon  it. 

As  regards  all  phenomena  upon  earth,  they  may  be  considered  as  happen- 
ing at  the  very  instant  when  the  eye  perceives  them  ;  the  difference  of  time 
being  too  small  to  be  appreciated ;  for,  as  shown  in  the  preceding  paragraph, 
if  our  sight  could  reach  from  London  to  Edinburgh,  we  should  perceive  a 
phenomenon  there  in  the  four-hundredth  part  of  a  second  after  its  occurrence. 

It  is  hence  usual  and  not  sensibly  incorrect,  when  we  are  measuring  the 
velocity  of  sound,  as  when  a  cannon  is  fired,  by  observing  the  time  between 
the  flash  and  the  report,  to  suppose  that  the  event  takes  places  at  the  very 
moment  when  it  is  perceived  by  the  eye. 

In  using  a  telegraph,  no  sensible  time  is  lost  on  account  of  light  requiring 
time  to  travel.  A  message  can  be  sent  from  London  to  Portsmouth  in  a 
minute  and  a  half;  and  at  the  same  rate  a  communication  might  pass  to  Rome 
in  about  half  an  hour,  to  Constantinople  in  forty  minutes,  to  Calcutta  in  a 
few  hours,  and  so  on.  A  telegraph  is  any  object  that  can  be  made  to  as- 
sume different  forms  or  appearances  at  the  will  of  an  attendant,  and  so  that 
the  changes  may  be  distinguished  at  a  distance.  A  pole  with  movable  arms 
is  the  common  construction,  each  position  of  the  arms  standing  for  a  letter, 
or  cypher,  or  word,  or  sentence,  as  may  be  agreed  upon.  Telegraphic  signals 
between  ships  at  sea  are  generally  made  by  a  few  flags,  the  meanings  of  each 
being  varied  by  the  mast  on  which  it  is  hoisted,  and  by  its  combination  with 
others. 

"  Light  proceeds  in  straight  lines"  &c.     (Read  the  Analysis,  page  325.) 

"We  have  scarcely  a  clearer  notion  of  a  straight  line  than  that  received  from 
the  direction  in  which  light  moves  : — but  we  can  verify  a  line  so  obtained  by 
other  means,  as  by  stretching  a  cord  between  the  two  extremes,  or  by  sus- 
pending a  weight  by  a  cord,  and  making  a  moveable  solid  measure  to  corres- 
pond with  the  cord,  which  standard  may  be  used  in  any  other  case. 


LEAVING  SHADOWS   WHERE   IT   CANNOT   FALL.     331 

We  can  see  through  a  straight  tube,  but  not  through  a  crooked  one.  The 
vista  through  a  long  straight  tunnel  is  striking  as  an  illustration  of  this  fact 
and  of  the  diminution  of  the  apparent  size  of  objects  as  they  are  more  distant. 
If  a  person  enter  one  end  of  the  canal  tunnel  two  miles  long,  cut  through  the 
chalk-hills  near  Rochester  as  part  of  the  canal  which  joins  the  Thames  and 
Medway  rivers,  the  opening  at  the  distant  end  is  seen  as  a  minute  luminous 
speck,  having  the  form  of  a  general  arch ;  and  a  person  who  has  advanced 
half  way  through  the  tunnel  may  see  the  luminous  speck  at  each  end,  then 
appearing  a  little  larger  than  in  the  former  case. 

In  taking  aim  with  gun  or  arrow,  we  are  merely  trying  to  make  the  pro- 
jectile go  to  the  desired  object  nearly  by  the  path  along  which  the  light  comes 
from  the  object  to  the  eye. 

A  carpenter  looks  along  the  edge  of  a  plank,  &c.,  to  see  whether  it  be 
straight. 

Because  light  moves  in  straight  lines,  if  a  number  of  similar  objects  be 
placed  in  a  row  from  the  eye,  the  nearest  one  hides  the  others.  In  a  wood 
or  city,  a  person  sees  only  the  trees  or  houses  that  are  next  him. 

He  who  believes  that  a  squinting  person  can  see  round  a  corner,  may  also 
believe  that  a  crooked  gun  can  shoot  round  a  corner. 

All  astronomical  and  trigonometrical  observations  are  made  on  the  faith  of 
this  property  of  light,  the  observer  holding  that  any  object  is  situated  from 
him  in  the  direction  in  which  the  light  comes  to  him  from  it.  When  the 
mariner,  after  watching  for  hours  in  cloudy  weather  has  caught  a  glimpse  of 
the  sun  or  star  through  his  sextant-glass,  he  has  ascertained  his  place  among 
the  trackless  waves,  and  boldly  advances  through  the  mist  of  hidden  dan- 
gers. And  the  beam  darting  from  the  light-house  across  the  stormy  sea, 
would  be  useless  if  the  light  moved  not  in  a  straight  line. 

"Leaving  shadows  where  it  cannot  fall."     (See  the  Analysis,  page  325.) 

The  form  of  shadows  proves  that  light  moves  in  straight  lines,  for  the  out- 
line of  the  shadow  is  always  correctly  that  of  the  object  as  seen  from  the 
luminous  body.  If  the  light  bent  round  the  body,  this  could  not  be. 

The  shadow  of  a  face  on  the  wall  is  a  correct  profile. 

As  a  wheel  presented  edgeways  to  the  eye  appears  only  like  a  broad  line, 
becomes  oval  or  round  as  it  is  more  turned,  so  a  wheel  presented  edgeways 
to  the  sun  or  other  light  casts  a  linear  shadow  on  the  wall  behind  it,  the 
shadow  becoming  oval  or  round  as  the  position  is  changed. 

A  globe,  a  cylinder,  a  cone,  and  a  flat  circle,  will  all  throw  the  same  round 
shadow  if  held  with  their  axes  pointing  to  the  luminous  body,  and,  there- 
fore, by  the  shadow  only,  these  objects  could  not  be  distinguished. 

The  figure  of  a  rabbit  cut  in  paste-board,  will  throw  the  same  shadow  on 
the  wall  as  the  animal  itself;  and,  again,  that  shadow  may  be  well  imitated 
by  a  certain  position  of  the  two  hands  joined,  as  is  known  to  those  who  find 
pleasure  in  witnessing  the  surprise  and  delight  of  a  child  who  beholds  such 
a  shadow  made  to  mimic  the  actions  of  life. 

A  man  under  the  vertical  sun  stands  upon  his  little  round  shadow ;  but 
as  the  sun  declines  in  the  afternoon,  the  shadow  juts  out  on  the  opposite  side, 
and  at  last  may  extend  across  a  whole  field. 

A  distant  cloud  which  appears  to  the  eye  of  an  observer  only  as  a  streak 
along  the  sky,  may  yet  be  broad  enough  to  shadow  a  whole  region ;  for 
clouds  generally  form  in  level  strata,  and  when  viewed  by  a  spectator  on 
earth  at  a  distance  are  seen  nearly  edgeways. 


332  LIGHT. 

The  velocity  of  the  wind  may  be  ascertained  by  marking  the  time  which 
the  shadow  of  a  cloud  takes  to  pass  over  a  plain  or  other  space  of  known 
dimension. 

A  body  held  between  a  candle  and  the  wall  darkens  a  portion  of  the  wall, 
or  casts  its  shadow  there ;  and  the  whole  space  between  it  and  the  wall 
is  a  shadowed  space,  for  anything  introduced  there  is  as  much  shadowed  as 
the  portion  of  the  wall.  Thus,  also,  all  the  heavenly  bodies  which  revolve 
about  the  sun  cast  a  shadow  beyond  them  or  away  from  the  sun,  as  is  seen 
when  one  of  them,  before  brightly  visible,  passes  where  the  shadow  of  an- 
other is.  The  satellites  or  moons  of  Jupiter,  when  they  suddenly  disappear 
to  our  glasses,  or  are  eclipsed  as  we  term  it,  have  generally  only  plunged 
into  the  shadow  of  the  planet,  and  are  not  hidden  by  being  then  on  the 
other  side  of  the  planet,  as  many  suppose.  When  our  own  moon  is  eclipsed, 
that  phenomenon  so  awful  in  the  early  ages  of  the  world,  she  is  only  passing 
through  the  long  shadow  which  the  earth  casts  beyond  it. 

When  in  the  case  of  a  luminous  centre  and  a  body  casting  a  shadow,  the 
centre  is  larger  than  the  body,  then  the  cross  section  of  the  shadowed  space, 
or  the  shadow  as  thrown  on  a  plane  surface,  will  be  less  than  the  body,  and 
less,  moreover,  the  farther  the  surface  is  from  the  body,  for  the  shadowed 
space  terminates  in  a  point.  This  is  true  of  the  shadows  of  all  the  planets 
and  of  the  earth,  because  they  are  less  than  the  sun.  On  the  contrary,  if  the 
light-giving  surface  is  smaller  than  the  opaque  body,  the  shadow  will  be  larger 
than  the  body.  The  shadow  of  a  hand  held  between  a  candle  and  the  wall 
is  gigantic ;  and  a  small  pasteboard  figure  of  a  man  placed  near  a  narrow 
centre  of  light,  throws  a  shadow  as  big  as  a  real  man.  The  latter  fact  has 
been  amusingly  illustrated  by  the  art  of  making  phantasmagoric  shadows. 

When  the  surface  which  receives  a  shadow  is  not  directly  exposed  to  the 
light,  the  shadow  may  be  much  larger  than  the  object,  even  although  the 
sun  himself  be  throwing  the  light; — as  is  seen  when  a  slightly  projecting 
roof,  or  a  veranda,  shadows  from  the  high  sun  of  summer  noon  the  whole 
front  of  a  house ;  or,  as  is  proved  by  the  long  evening  shadows  of  all  coun- 
tries, a  low  wall  will  shadow  from  the  setting  sun  a  whole  field. 

"Light  posses  readily  through  some  bodies — which  are,  therefore,  called 
transparent ,  but  when  it  enters,  or  leaves  their  surfaces  obliquely,  its 
course  is  bent."  (Read  the  Analysis,  page  325.) 

It  may  well  excite  the  surprise  of  inquirers  that  light,  of  which  the  con- 
stitution is  so  fine  or  flimsy,  should  still  be  able  to  dart  readily  and  in  every 
direction  through  great  masses  of  solid  matter,  but  such  is  the  truth.  Thick 
plates  of  solid  glass,  blocks  of  rock  crystal,  mountains  of  ice,  &c.,  are  in- 
stantly pervaded  by  the  beam  of  the  sun. 

What  it  is  in  the  constitution  of  ono  mass  as  compared  with  another,  which 
fits  the  one  to  transmit  light,  and  the  other  to  obstruct  it,  we  cannot  clearly 
explain,  but  we  perceive  that  the  arrangement:  of  the  particles  has  more  in- 
fluence than  their  peculiar  nature.  Nothing  is  more  opaque  than  thck  masses 
of  the  metals,  but  nothing  is  more  transparent  than  equally  thick  masses  of 
the  same  metals  in  solution,  nor  than  the  glasses  of  which  a  metal  forms  a 
large  proportion.  The  thousand  salts  formed  by  the  union  of  the  metals  or 
earths  with  the  diluted  acids,  are  all  transparent,  when  in  cooling  from  the 
fluid  to  the  solid  state,  their  particles  have  been  allowed  to  arrange  themselves 
according  to  the  laws  of  their  mutual  attraction,  that  is  to  say,  to  form  crys- 
tals; but  the  same  substances  in  other  states,  as  when  reduced  to  powder, 


REFRACTION. 


333 


are  opaque.  Even  the  pure  metals  themselves,  when  reduced  to  leaves  of 
great  thinness,  are  transparent,  as  may  be  perceived  by  looking  at  a  lamp 
through  a  fine  gold  leaf.  It  is  to  be  remarked,  however,  that  even  the  most 
transparent  bodies  intercept  a  considerable  part  of  the  light  which  enters 
them  :  a  depth  of  seven  feet  of  pure  water  intercepts  about  one-half,  so  that 
the  bottom  of  the  sea  is  very  dark.  And  of  the  sun's  light,  when  passing 
obliquely  through  the  atmosphere  towards  the  earth,  as  when  the  sun  has 
lately  risen  or  is  about  to  set,  only  a  small  part  arrives. 
'  Light  having  once  entered  a  transparent  mass  of  uniform  nature  passes 
forward  in  it  as  straightly  as  in  a  vacuum ;  but  at  the  surface,  whether  on 
entering  or  leaving  it,  if  the  passage  be  oblique,  and- if  the  mafs.be  of  a 
different  density  from  the  transparent  medium  around  it,  a  very  curious  and 
most  important  phenomenon  occurs,  namely,  the  light  suffers  a  degree  of 
bending  from  its  antecedent  direction,  or  a  refraction,  proportioned  to  the 
obliquity. 

But  for  this  fact,  which  to  many  persons  might  at  first  appear  a  subject  of 
regret,  as  preventing  the  distinct  vision  of  objects  through  all  transparent 
media ;  light  could  have  been  of  little  utility  to  man.  There  could  have 
been  neither  converging  lenses  as  now,  nor  any  optical  instruments,  of  which 
lenses  form  a  part,  as  telescopes  and  microscopes ;  nor  even  the  eye  itself, 
which  has  its  crystalline  lens. 

Light  falling  from  the  air  directly  or  per- 
pendicularly upon  a  surface  of  water,  glass,  Fig.  137. 
or  any  such  transparent  body,  passes  through 

without  suffering  the  least  bending; — a  ray  ^i -^ d> 

for  instance,  shot  from  a  to  the  point  c,  in  the 
surface  of  a  piece  of  glass  g  li,  would  reach 

directly  across  to  o  and  b ;  but  if  the  ray  fell        I/ r    cy r 

obliquely,  as  from  d  to  c,  then,  instead  of 
continuing  in  its  first  direction  to*  and,  7c}  it 
would  at  the  moment  of  its  entrance  be  bent 
downwards  in  the  path  c  e,  nearer  to  a  line  c 
o,  called  the  perpendicular  to  the  surface  at 
the  point  of  entrance, — and  the  moving 
straightly  while  in  the  substance  of  the  glass 
it  would,  when  it  passed  out  again  at  e,  in  the 
opposite  surface,  he  bent  just  as  much  as  at  first,  but  in  the  contrary  direc- 
tion, or  away  from  a  similar  perpendicular  at  that  surface,  viz.,  into  the  line 
e  f,  instead  of  e  n.  A  ray,  therefore,  passing  obliquely  through  a  transparent 
body  with  parallel  surfaces,  has  its  course  shifted  a  little  to  one  side  of  the 
original  course,  but  still  proceeds  in  the  same  direction,  or  in  a  line  parallel 
to  the  first — as  here  shown  in  the  line  ef,  parallel  and  near  to  the  line^&  ;  if 
the  surfaces  of  the  body  are  not  parallel,  the  ray  is  ultimately  bent  as  will  be 
explained  some  pages  hence. 

The  degree  of  bending  or  refraction  of  light  in  traversing  a  single  trans- 
parent surface  is  measured  by  comparing  the  obliquity  of  its  approach  to  the 
surface  with  the  obliquity  of  its  departure  after  passing ;  and  for  this  purpose 
a  line  is  supposed  to  be  drawn  perpendicularly  through  the  surface  of  the 
point  where  the  ray  passes  (as  a  b  in  the  above  figure  drawn  through  c,  where 
the  ray  d  c  passes)  and  the  relative  positions  of  the  ray  to  this  line  on  both 
sides  of  the  surface,  are  easily  ascertained.  Thus  the  line  a  d,  drawn  from 
any  point  of  the  ray  before  passing  to  such  perpendicular,  is  a  measure  of  the 
original  obliquity  or  angular  distance  of  the  ray,  and  is  called  the  sine  of  the 


7, 


334  LIGHT. 

angle  of  incidence,  and  the  other  line  o  c  drawn  from  a  corresponding  point 
of  the  ray  after  passing  to  the  perpendicular,  is  a  measure  of  the  jobliquity 
after  refraction,  and  is  called  the  sine  of  the  angle  of  the  refraction : — by 
comparing  these  two  lines  in  any  case,  the  problem  is  solved. 

When  light  passes  obliquely  from  air  into  water,  the  refraction  or  bending 
produced  is  such,  that  the  line  a  d  measuring  the  obliquity  before  refraction, 
is  always  longer  than  the  line  o  e  measuring  it  after  refraction,  by  nearly  one- 
third  of  the  latter,  and  the  refractive  power  of  water  is,  therefore,  signified  by 
the  index  1J  or  1,33  ;  in  like  manner  the  greater  refractive  power  of  common 
glass  has  the  index  £,  of  diamond  the  index  1£,  and  so  on.  And  it  is  im- 
portant to  remark,  that  for  the  same  substance  the  same  relation  holds, 
whatever  the  obliquity  of  the  incidence  ray  may  be.  If,  for  instance,  where 
the  obliquity,  as  measured  by  its  sine,  is  40,  and  the  refraction,  is  half  or  20, 
then  in  the  same  substance  an  obliquity  of  10  will  occasion  a  refraction 
of  5,  and  obliquity  of  4  will  occasion  a  refraction  of  2,  and  so  on. 

As  a  general  rule,  the  refractive  power  of  transparent  substances  of  media 
is  proportioned  to  their  densities.  It  increases,  for  instance,  through  the 
list  of  air,  water,  salt,  glass,  &c.  But  Newton,  while  engaged  in  his  experi- 
ments upon  this  subject,  observed  that  inflammable  bodies  had  greater  re- 
fractive powers  than  others,  and  he  then  hazarded  the  conjecture,  almost  of 
inspired  sagacity,  which  chemistry  has  since  so  remarkably  verified,  that 
diamond  and  water  contained  inflammable  ingredients.  We  now  kncrw  that 
diamond  is  merely  crystallized  carbon,  and  that  water  consists  of  hydrogen 
or  inflammable  air  and  oxygen.  Diamond  has  nearly  the  greatest  light- 
bending  power  of  any  known  substances,  and  hence  comes  in  part  its 
brilliancy  as  a  jewel. 

No  good  explanation  has  been  given  of  the -singular  fact  of  refraction;  but 
to  facilitate  the  conception  and  remembrance  of  it,  we  say  that  jit  happens 
as  if  it  were  owing  to  an  attraction  between  the  light  and  the  refracting  body 
or  medium.  The  light  approaching  from  d  to  c,  for  instance  (in  the  last 
figure,)  may  be  supposed  to  be  attracted  by  the  solid  body  below  it,  so  as  at 
the  surface  to  be  bent  into  the  direction  c  e  ;  and,  again,  on  leaving  the  body 
to  be  still  equally  attracted  and  bent  back,  so  as  to  take  the  direction  ef, 
instead  of  e  n  ;  and  we  see  why  the  attraction  and  bendingjjshould  be  greater, 
the  greater  the  obliquity. 

The  following  are  familiar  examples  of  this  bending  of  light  in  passing  from 
one  medium  to  another, 

If  an  empty  basin  or  other  vessel  6  c/e,  be  in  the  sun's  light,  so  that  the 
rays  falling  within  it  may  reach  low  on  the  side  as  to  <f,  but  not  to  the  bot- 
tom, then,  on  filling  the  vessel  with  water,  the  sun  will  be  found  to  be  shin- 
ing on  the  bottom  or  down  to  e,  as  well  as  on 
Fig.  138.  the  side.     The  reason  of  this  phenomenon  is, 

that  water  being  a  denser  medium  than  air, 
#f  the  light,  on  entering  it  at  c,  is  bent  towards 
the  perpendicular  (c  /,)  at  the  point  of  inci- 
dence, and  so  reaches  the  bottom.  Again,  if 
a  coin  or  metal,  were  laid  on  the  bottom  of 
such  a  vessel  at  e,  it  would  not,  while,  the  ves- 
sel were  empty,  be  seen  by  an  eye  at  a,  but 
would  be  visible  there  immediately  on  the 
vessel  being  filled  with  water ; — because  then, 
the  light  leaving  the  coin  in  the  direction  e  c} 


REFRACTION.  335 

towards  the  edge  of  the  vessel,  would  at  c,  on  passing  from  the  water  into 
air,  be  bent  away  from  the  perpendicular,  and  instead  of  going  to  g,  would 
reach  the  eye  at  a.  The  coin,  moreover,  would  appear  to  the  eye  to  be  in 
the  direction  of  c  d,  instead  of  the  true  direction  c  e  :  for  the  eye  not  being 
able  to  discover  that  the  light  had  been  bent  in  its  course,  would  judge  the 
object  to  be  in  the  line  by  which  the  light  came  from  it. 

It  is  thus  because  objects  at  the  bottom  of  water,  when  viewed  obliquely, 
do  not  appear  so  low  as  they  really  are,  tfrat  a  person  examining  a  river  or 
pond,  or  any  clear  water,  from  its  bank,  naturally  judges  its  depth  to  be  less 
than  it  is.  Many  a  young  life  has  been  sacrificed  to  this  error.  A  person 
looking  from  a  boat  directly  down  upon  the  objects  at  the  bottom  of  water, 
sees  them  in  their  true  directions,  but  even  then  not  in  their  true  distances, 
as  will  be  afterwards  explained ;  and  if  he  view  them  more  and  more  ob- 
liquely, the  appearance  becomes  more  and  more  deceiving,  until  at  last  it 
represents  them  as  at  much  less  than  half  of  their  true  depth. 

The  ship  in  which  the  author  sailed,  once  in  the  middle  of  the  China  Sea, 
where  no  danger  was  apprehended,  entered  by  a  narrow  passage  a  large 
hore-shoe  enclosure  of  coral  rocks.  When  the  looker-out  gave  the  alarm, 
the  predicament  had  become  truly  terrific.  On  every  side,  in  water  most 
singularly  transparent,  the  rocks  appeared  to  be  almost  at  the  surface  of  the 
water,  and  the  anchor,  which  in  the  first  moment  had  been  let  go  to  arrest 
the  ship,  appeared  to  have  been  dragged  to  a  shallow  place.  It  seemed  that 
if  the  ship,  then 'drawing  24  feet,  or  the  depth  of  a  two-storied  house,  moved 
but  a  little  way  in  almost  any  direction,  she  must  inevitably  meet  her 
destruction  On  sending  boats  around  to  sound  and  to  search,  the  place  of 
entrance  was  again  discovered,  and  was  safely  traversed  a  second  time  as  an 
outlet  from  that  terrible  prison. 

On  account  of  this  bending  of  light  from  objects  under  water,  there  is 
more  difficulty  in  hitting  them  with  a  bullet  or  spear.  The  aim  by  a  person 
not  directly  over  a  fish,  must  be  made  at  a  point  apparently  below  it,  other- 
wise the  weapon  will  miss  by  flying  too  high.  The  spear,  sometimes  used 
in  this  country  for  killing  salmon,  is  a  common  weapon  among  the  islanders 
of  the  Atlantic  and  Pacific  Oceans  for  killing  the  albacore;  the  use  of  it,  like 
that  of  the  fly-hock  in  England,  affording  to  the  fishermen,  sport  as  well  as 
profit.  The  author  once  witnessed  at  St.  Helena,  this  employment  of  the 
spear.  A  small  fish  previously  stunned,  that  it  might  notrtry  to  escape,  was 
every  minute  or  two  thrown  upon  the  water  as  a  bait,  in  the  sight  of  perhaps 
a  hundred  great  albacores,  greedily  waiting  for  it  at  one  side  below,  and 
knowing  the  danger  to  which  they  exposed  themselves  by  darting  across  to 
seize  it.  Some  albacore  bold  enough,  soon  made  at  the  mouthful,  apparently 
with  the  speed  of  lightning,  but  yet  with  speed  which  did  not  save  him,  for 
every  now  and  then  the  thrown  spear  met  him,  and  held  him  writhing  there 
in  a  cloud  of  his  death-blood  After  a  victim  so  destroyed,  the  scene  of 
action  was  changed. 

The  bending  of  light  when  passing  obliquely  from  water,  is  also  the  reason 
of  the  following- facts.  A  straight  rod  or  stick,  of  which  a  portion  is  immersed 
in  water,  appears  crooked  or  broken  at  the  surface  of  the  water,  the  portion 
immersed  seeming  to  be  bent  upwards.  That  part  of  a  ship  or  boat  visible 
under  water,  appears  much  flatter  and  shallower  than  it  really  is.  A  deep- 
bodied  fish  seen  near  the  surface  of  water,  appears  almost  a  flat  fish.  A 
round  body  there  appears  oval.  A  gold  fish  in  a  vase  may  appear  as  two 
fishes,  being  seen  as  well  by  light  bent  through  the  upper  surface  of  the 
water,  as  by  straight  rays  passing  through  the  side  of  the  glass.  To  see 


336 


LIGHT. 


OB 


bodies  under  water,  in  their  true  directions  and  nearly  of  their  true  propor- 
tions, the  eye  must  view  them  through  a  tube,  of  which  the  lower  end  closed 
with  a  plate  glass,  is  held  in  the  water. 

.  As  light  is  bent  on  entering  from  air  into  water,  glass,  or  other  substance 
denser  than  air,  so  it  is  also  bent  on  coming  from  void  space  into  the  ocean 
of  our  atmosphere.  Hence  none  of  the  heavenly  bodies,  except  when  directly 
over  our  heads,  are  seen  by  us  in  their  true  situations.  They  all  appear  a 
little  higher  than  they  really  are,  and  more  so  the  nearer  they  are  to  the  hori- 
zon ;  as  when  to  a  spectator  at  d, 

Fig.  139.  suppose  on  the  surface  of  the  earth, 

a  star  really  at  A  appears  to  be  at 
a,  because  its  ray,  on  reaching  the 
atmosphere  ate,  is  bent  downwards. 
In  astronomical  books  there  is  al- 
ways introduced  a  table  of  refraction 
as  it  is  called,  showing  what  correc- 
tion must  be  made  on  this  account 
for  different  apparent  altitudes. 
This  effect  of  our  atmosphere  so 
bends  the  rays  of  the  sun,  that  we 
see  him  in  the  morning  before  he 
is  really  above  the  horizon,  and  we 

see  him  in  the  evening  after  he  is  really  below  it — for  the  ray  coining  hori- 
zontally from  e  to  rf,  appears  to  come  from  6,  although  in  truth  it  really 
comes  from  the  lower  situation  B,  and  is  bent  into  the  level  line  only  at  e. 
Our  atmosphere  thus,  by  the  bending  of  light  as  well  as  by  itself  becoming 
luminous,  lengthens  at  dawn  and  twilight  the  durations  of  the  precious  day. 
As  the  atmosphere  is  denser  near  the  surface  of  the  earth  than  higher  up, 
the  light  is  more  and  more  bent  as  it  descends,  and  hence  describes  a  course 
which  is  sensibly  curved,  and  therefore  unlike  the  course  of  light  in  water. 
Certain  states  of  the  atmosphere,  depending  chiefly  on  its  humidity  and 
warmth,  change  very  considerably  its  ordinary  refractive  power ;  hence,  in 
one  state,  a  certain  hill  or  island  may  appear  low  and  scarcely  rising  above 
the  intervening  heights  or  ocean,  while  in  another  state,  the  same  object  will 
be  seen  towering  above  ;  and  from  a  certain  station,  a  city  in  a  neighbouring 
valley  may  be  either  entirely  visible,  or  it  may  show  only  the  tops  of  its 
steeples,  as  if  the  bed  on  which  it  rested  had  sunk  deeper  into  the  earth. 
In  days  of  ignorance  and  superstition,  such  appearances  occasionally  excited 
a  strange  interest. 

Owing  to  the  bending  of  light  in  passing  through  the  media  of  different 
densities,  a  beautiful  phenomena  is  often  observable  in  a  day  of  warm  sun- 
shine. Black  or  dark-colored  substances,  by  absorbing  much  light  and 
heat  from  the  sun's  rays,  and  warming  the  air  in  contact  with  them,  until 
it  dilates  and  rises  in  the  surrounding  air,  as  oil  rises  in  water,  cause  the 
light  from  more  distant  objects,  reaching  the  eye  through  the  rarefied  me- 
dium, to  be  bent  a  little ; — and  owing  to  the  heated  air  rising  irregularly 
under  the  influence  of  the  wind  and  other  causes,  these  objects  acquire  the 
appearance  of  having  a  tremulous  or  a  dancing  motion.  In  a  warm,  clear 
day,  the  whole  landscape  at  last  appears  to  be  thus  dancing. 
•f  The  same  phenomenon  is  to  be  observed  at  any  time,  by  looking  at  an 
object  beyond  the  top  of  a  chimney  from  which  hot  air  is  rising.  An  illicit 
distillery  has  been  discovered  by  the  exciseman  happening  thus  to  look  across 


REFRACTION. 


33T 


a  hole  used  as  the  chimney,  although  charcoal  was  the  fuel,  and  there  was 
no  vestige  of  smoke. 

This  bending  of  light  by  the  varying  states  of  the  atmosphere  renders 
precaution  necessary  in  making  very  nice  geometrical  observations : — as  in 
measuring  base  lines  for  the  construction  of  maps  or  charts. 

As  it  is  the  obliquity  with  which  a  ray  traverses  the  surface,  which,  in  any 
case  of  refraction,  determines  the  degree  of  bending,  a  body  seen  through  a 
medium  of  irregular  surface  appears  distorted  according  to  the  nature  of  that 
surface.  It  is  because  the  two  surfaces  of  common  window-glass  are  not  as 
in  the  case  of  plate-glass  perfect  planes,  and  perfectly  parallel  to  each  other, 
that  objects  seen  through  a  common  window  appear  generally  more  or  less 
out  of  a  shape;  and  hence  come  the  elegance  and  beauty  of  plate-glass  win- 
dow: and  hence  the  singular  distortion  of  things  viewed  through  that  swell- 
ing or  lump  of  glass,  which  appears  at  the  centre  of  certain  very  coarse 
panes  and  which  remains  where  the  glass-blower's  instrument  was  attached. 

The  refraction  or  bending  of  light  is  interestingly  exemplified  in  the  effect 
of  the  glass  called  a  prism,  viz.,  a 

wedge  or  three-sided  rod  of  glass  Fig.  140. 

such  as  that  of  which  the  end  is 
here  represented  at  b  c.  A  ray 
from  a  falling  on  the  surface  at  b  is 
bent  towards  the  internal  perpen- 
dicular,  and  therefore  reaches  c,  but 
on  escaping  again  at  c,  it  is  bent 
away  from  the  external  perpendicu- 
lar and  thus  with  its  original  deviation  doubled,  goes  on  to  d. 

The  law  of  lights  bending,  according  to  the  obliquity 
with  which  it  traverses  the  surfaces  of  a  transparent  body, 
is  well  elucidated  by  the  effect  of  what  is  called  a  multi- 
plying glass  \  that  is  to  say,  a  piece  of  glass  like  a  b  c  e, 
having  many  distinct  faces  cut  upon  it  at  angles  with  each 
other.  If  a  small  object,  a  coloured  bead  for  instance,  be 
placed  at  d,  an  eye  at  e  will  see  as  many  beads  as  there 
are  distinct  surfaces  or  faces  at  the  glass ;  for  first,  the 
ray  d  a,  passing  perpendicularly,  and  therefore  straight 
through,  will  form  an  image  as  if  no  glass  intervened ; 
then,  the  rays  from  d  to  the  surface  b,  will  be  bent  by  the 
oblique  surface,  and  will  show  the  object  as  if  it  were  in 
the  direction  e  b;  and  the  light  falling  on  the  still  more 
oblique  surface,  c,  will  be  still  more  bent,  and  will  reach 
the  eye  in  the  direction  c  e,  exhibiting  a  similar  object 
also  in  that  direction — and  so  of  all  the  other  surfaces. 
If  the  eye  were  at  d,  and  the  object  at  e,  the  result  would 
still  be  the  same.  A  plate  of  glass  roughened,  or  cut  into 
cross  furrows,  becomes  a  very  good  screen  or  window- 
blind,  by  disturbing  the  passage  of  light  through  it  so  that 
objects  beyond  it  are  not  distinguishable. 

t(  And  a  beam  of  while  light  thus  made  to  bend,  is  resolved  into  beams  of  the 
various  primary  colours  ;  which  beams,  however,  on  being  again  blended, 
become  white  light  as  before."  (Read  the  Analysis,  page  325.) 

The  most  extraordinary  fact  connected  with  the  bending  of  light  is  that  a 
pure  ray  of  white  light  from  the  sun  admitted  into  a  darkened  room  by  a  hole 

22 


Fig.  141. 


338  LIGHT. 

in  the  window-shutter,  and  made  to 

Fig.  142.  bend  by  passing  through  transparent 

surfaces  which  it  meets  very  oblique- 
ly (as  the  ray  a,  admitted  and  made 
to  bend  by  passing  through  the  prism 
of  glass  b  c,  to  fall  upon  the  wall  at 
d,]  instead  of  bending  altogether  and 
appearing  still  as  the  same  white 
ray,  is  divided  into  several  rays, 

which  falling  on  the  white  wall,  are  seen  to  be  of  different  most  vivid 
colours.  The  original  white  ray  is  said  thus  to  be  analyzed,  or  divided  into 
its  elements. 

This  solar  spectrum,  as  it  is  called,  formed  upon  the  wall,  consists,  when 
the  light  is  admitted  by  a  narrow  horizontal  slit,  of  four  coloured  patches 
corresponding  to  the  slit,  and  appearing  in  the  order,  from  the  bottom  of  red, 
green,  blue,  and  violet.  If  the  slit  be  then  made  a  little  wider,  the  patches 
at  their  edges  overlap  each  other,  and  produce  by  the  mixture  of  their  ele- 
mentary colours,  certain  new  tints.  Then  the  spectrum  consists  of  the  seven 
colours  commonly  enumerated  and  seen  in  the  rainbow,  viz.,  red,  orange, 
yellow,  green,  blue,  indigo,  and  violet. — Had  red,  yellow,  blue,  and  violet 
been  the  four  colours  obtained  in  the  first  experiment,  the  occurrence  of  the 
others,  viz.,  of  the  orange,  from  the  mixing  edges  of  the  red  and  yellow — of 
the  green,  from  the  mixture  of  the  yellow  and  blue, — and  of  the  indigo, 
from  the  mixture  of  blue  and  violet,  would  have  been  anticipated.  But  the 
facts  of  the  case  not  being  such,  we  see  that  they  are  not  yet  well  under- 
stood. When  Newton  first  made  known  the  phenomenon  of  the  many- 
coloured  spectrum,  and  the  extraordinary  conclusions  to  which  it  led,  he  ex- 
cited universal  astonishment ;  for  the  common  idea  of  purity,  the  most  un- 
mixed was  that  of  white  light.  In  farther  corroboration  of  the  notion  of 
the  compound  nature  of  light,  he  mentioned,  that  if  the  colours  which  ap- 
pear on  the  spectrum  be  painted  separately  around  the  rim  of  a  wheel,  and 
the  wheel  be  then  turned  rapidly,  the  individual  colours  cease  to  be  distin- 
guished and  a  white  band  only  appears  where  they  are  whirling ;  also,  that 
if  the  rays  of  the  spectrum,  produced  by  a  prism,  be  again  gathered  together 
by  a  lens,  they  reproduce  white  light.  The  red  is  the  kind  of  light  which 
is  least  bent  in  refraction,  and  the  violet  that  which  is  most  bent.  It  was  at 
one  time  said,  as  an  explanation,  that  the  differently  coloured  particles  in 
light  had  different  degrees  of  gravity  or  inertia,  and  were  therefore,  not  all 
equally  bent.  It  is  farther  remarkable,  with  respect  to  the  solar  spectrum, 
that  much  of  the  heat  in  the  ray  is  still  less  refracted  than  even  the  red  light, 
for  a  thermometer  held  below  the  red  light  rises  higher  than  in  any  part  of 
the  visible  spectrum  ', — and  there  is  an  influence  or  something  in  the  beam 
more  refrangible  than  even  the  violet  rays,  and  capable  of  producing  power- 
ful chemical  and  magnetical  effects.  The  different  spots  of  colour  in  the 
spectrum  are  not  all  the  same  size,  and  there  is  a  difference  in  this  respect 
according  to  the  refracting  substance. 

All  transparent  substances  in  bending  light  produce  more  or  less  of  the 
separation  of  colour ;  but  it  is  an  important  fact,  that  the  quality  of  merely 
bending  a  beam,  or  of  refraction,  and  that  of  dividing  it  into  coloured  beams, 
or  of  dispersion,  are  distinct  qualities,  and  .not  having  the  same  proportions 
to  each  other  in  different  substances.  Newton,  from  not  discovering  this, 
concluded  that  a  perfect  telescope  of  refraction  could  never  be  made ;  he  sup- 
posed that  the  bent  light  would  always  become  coloured,  and  so  render  the 


REFRACTION    BY    LENSES.  339 

objects  indistinct.  We  now  know,  however,  that  by  combining  two  or  more 
media,  we  may  obtain  bending  of  light  without  dispersion,  —  thus,  by  oppos- 
ing a  glass  which  bends  five  degrees  and  disperses  one  degree,  to  another 
glass  which  bends  three  degrees  and  disperses  one,  the  opposing  dispersions 
will  just  counterbalance  or  neutralize  .  each  other,  while  the  two  degrees  of 
excess  of  bending  will  remain  to  be  applied  to  use. 

The  diversified  colours  of  the  substances  around  us  depend  merely  upon 
their  fitness,  from  texture  or  other  cause,  to  reflect  or  transmit  other  modi- 
fications of  common  light,  and  the  colour  is  not  a  part  or  property  of  the 
body  itself.  We  shall  soon  find  that  the  vivid  colours  of  the  rainbow  are 
merely  the  white  light  of  the  sun,  reflected  to  us  after  being  bent  and 
modified  by  the  colourless  drops  of  falling  rain  ;  and  that  the  sparkling  with 
appearance  of  rubies  and  emeralds,  which  we  see  in  cut-glass  lustre,  is  a 
phenomenon  of  the  same  kind  :  —  and  that  by  scratching  the  surface  of  a 
piece  of  metal  so  as  to  have  a  given  number  of  lines  in  a  given  space,  we 
can  cause  the  same  substance  to  appear  of  any  colour  we  please. 

lt  Transparent  bodies,  as  glasSj  'may  be  made  of  such  form  as  to  cause  all 
the  raijs  of  light  which  pass  through  them  from  any  one  point,  to  bend  so 
as  to  meet  again  in  another  corresponding  point  beyond  them,  —  the  body 
itself,  from  the  required  form  generally  resembling  that  of  a  bean  or 
lentil,  being  then  called  a  LENS."  (Read  the  Analysis,  page  325.) 

The  innumerable  rays  of  light  (of  which  five  only  are  here  represented,) 
issuing  from  any  point  as  c,  towards  any  surface  in  the  situation  a  b}  are  said 
to  form  a  cone  or  pencil  of  diverging  light.  Now  it  is  evident  that  to  make 

Fig.  143. 


z  ••>::- 


all  such  rays  converge  or  meet  again  in  one  place  as/,  beyond  a  transparent 
body  place  at  a  6,  it  would  be  necessary,  while  the  middle  ray  or  axis  of 
the  pencil  c  df  did  not  bend  at  all,  for  the  others  to  be  bent  more  and  more,  in 
proportion  as  they  fell  upon  the  body  farther  and  farther  from  the  centre  d. 
Recollecting,  then  the  law  of  refraction,  that  light  entering  from  air  through 
the  surface  of  any  denser  medium,  as  glass,  is  bent  there  towards  the  perpen- 
dicular at  the  internal  surface,  in  proportion  to  the  obliquity  of  incidence, 
and  on  leaving  the  opposite  surface,  is  correspondingly  bent  away  from  its 
external  perpendicular,  (see  the  case  of  the  prism  at  p.  337,)  we  see  that  if 
a  piece  of  glass  were  placed  at  a  b,  of  such  form  that  the  rays  falling  upon 
it  from  c  should  meet  and  leave  its  surfaces  with  greater  and  greater  obliquity 
in  some  regular  proportion,  as  the  points  of  incidence  were  more  distant  from 


340  LIGHT. 

the  centre  d,  the  purpose  would  "be  obtained.  And  we  have  the  satisfaction 
of  knowing  that  a  glass,  of  which  the  surface  is  ground — which  it  easily  may 
be — to  have  a  regular  convexity  or  bulging,  as  if  it  were  a  portion  cut  off 
from  the  surface  of  the  globe,  can  be  shown  to  answer  very  correctly  the  re- 
quired condition.  Such  a  glass  similarly  ground  on  both  sides,  is  here  repre- 
sented edgeways  between  a  and  b,  where  the  ray  c  d  falling  on  its  middle,  or 
perpendicularly,  and  similarly  leaving  it,  is  seen  going  straight  through  to/, 
but  the  ray  c  e  meeting  the  surface  with  a  certain  degree  of  obliquity  is  bent 
down  a  little,  first  on  entering  the  surface  at  e,  and  then  as  much  more  on 
leaving  the  opposite  surface  with  equal  obliquity,  and  so  arrives  at/;  then 
the  rayc  or,  for  corresponding  reasons  is  still  more  bent,  and  equally  arrives 
at/; — and  the  case  would  be  similar  of  any  other  rays  that  might  be  exam- 
ined. The  point /is  usually  called  a,  focus  (meaning  a  fire-place,)  because 
when  the  light  of  the  sun  is  thus  gathered,  the  heat  concentrated  with  it  is 
powerful  enough  to  make  combustibles  inflame. — We  have  here  to  remark 
farther,  that  in  accordance  both  with  calculation  and  experiment,  the  direction 
in  which  a  pencil  of  rays  falls  upon  a  lens  does  not  affect  the  result  of  the 
covergence  to  a  focus,  only  the  focus  is  always  in  the  direction  of  the  cen- 
tral ray  of  the  pencil  or  beam  ;  it  will  be  atp,  for  instance,  for  light  issuing 
from  o,  and  at  z  for  light  issuing  from  x. 

The  lens  represented  at  a  b  above  or  in  the  annexed  diagram,  at  fig.  1, 
having  both  sides  convex,  is  called  a  double  convex  lens.  A  glass  convex 
only  on  one  side;  and  plane  or  flat  on  the  other,  as  shown  at  fig  2,  would 


Fig.l 


as  effectually  gather  the  rays,  but  with  half  the  power,  and  the  point  of 
meeting  or  focus  would  be  therefore  proportionably  more  distant.  Such  a 
glass  is  called  a  plano-convex  lens.  Then  the  gathering  or  converging 
power  of  any  glass,  whether  doubly  or  singly  convex,  is  in  proportion  to 
the  degree  of  its  convexity  or  bulging  of  surfaces,  for  the  less  it  bulges, 
the  more  nearly  does  it  approach  to  a  plane  glass,  and  the  more  it  bulges, 
the  more  obliquely  will  the  rays,  at  any  distance  from  the  centre,  fall  upon 
its  surface,  and  the  sooner,  therefore,  in  consequence  of  their  being  more 
bent,  will  they  all  meet  the  axis  ray ;  hence,  fig.  1  would  converge  much 
more  quickly  than  fig.  3,  which  represents  nearly  a  common  spectacle  glass; 
and  a  very  minute  globe  is  the  form  most  powerfully  converging  of  all. 
The  surfaces  of  fig.  1  are  proportions  of  a  small  globe;  those  of  fig.  3,  are 
smaller  portions",  but  of  a  globe  much  larger.  Concave  lens  as — fig.  4, 
a  double  concave,  and  at  fig.  5,  a  plano-concave  lens,  in  obedience  to  the 
same  law  of  refraction,  spread  rays,  or  bend  them  away  from  the  axis  of 
the  pencil,  in  the  same  degree  that  similarly  convex  lenses  gather  them. 
A  concave  lens,  therefore,  receiving  the  converging  pencil  of  rays  from  a 
convex  lens,  might  restore  them  to  their  former  direction.  Very  useful 
purposes,  as  will  be  afterwards  explained,  are  served  in  optics,  by  certain 


CAMERA    OBSCURA.  341 

combinations  of  differently  formed  lenses.  A  lens  may  be  convex  on  one 
side  and  concave  on  the  other,  as  at  fig.  6,  called  a  meniscus  lens,  because 
it  resembles  the  crescent  moon,  and  its  effect  will  be  according  to  the  curve 
which  predominates. 

A  person  collecting  the  case  of  the  "  multiplying  glass,"  described  a  few 
pages  back,  might  say, — but  is  not  a  convex  lens  merely  a  multiplying  glass 
of  a  much  greater  number  of  faces,  and  if  so,  why  instead  of  one  image, 
does  it  not  make  thousands  ?  The  answer  is,  that  the  multiplying  glass,  by 
every  face,  bends  a  set  of  rays,  capable  of  forming  a  distinct  and  complete 
image  j  but  the  lens  has  no  surface  large  enough  to  bend  more  than  a 
single  ray  and  it  concentrates  all  the  single  rays  into  one  place,  to  form 
there  one  image  of  great  vividness  and  beauty. 

"  And  ivhen  the  light  proceeding  from  every  point  of  an  object  placed  before 
a  lens  is  collected  in  corresponding  points  behind  it,  a  perfect  image  of 
the  object  is  there  produced.  When  the  image  is  received  upon  a  suitable 
white  surface  in  a  dark  place,  the  arrangement  is  called,  according  to 
minor  circumstances,  the  CAMERA  OBSCURA,  SOLAR  MICROSCOPE  or  MAGIC 
LANTERN."  (Read  the  Analysis,  page  326.) 

Words  are  wanting  to  express  the  admirable  consequences  to  man  of  the 
curious  property  of  a  lens  that  it  can  bring  together  to  focal  points  behind 
all  the  rays  of  light  which  traverse  it  from  any  points  of  an  object  placed 
before  it.  The  following  instance  will  lead  to  the  understanding  of  others. 
If  a  lens  as  a,  be  placed  so  as  to  fill 

up  an  opening  made  in  the  window-  Fig.  145. 

shutter  of  a  darkened  room,  then,  from 
any  object  before  that  opening — as  the 
cross  here  presented,  all  the  light 
which  each  point  emits  towards  the 
lens  will  be  concentrated  or  gathered 
together  in  a  corresponding  focal  point 
behind  the  lens  or  within  the  room 
and  if  a  sheet  of  paper  be  held  there  at  the  distance  of  the  focal  points,  a 
beautiful  image  of  the  object  will  be  seen  upon  the  paper. 

In  these  few  words,  we  have  described  the  interesting  contrivance  called 
the  camera  obscura  or  dark  chamber ;  and  when  a  glass  is  chosen  of 
proper  size  and  focal  distance,  and  a  screen  or  the  wall  of  the  chamber  (if  at 
the  required  distance,)  is  properly  prepared  to  receive  the  light,  the  most  en- 
chanting portraiture  is  instantly  produced  of  the  whole  scene  which  the  win- 
dow commands.  With  what  rapture  does  the  school-boy  first  view  this 
lovely  picture  drawn  by  nature's  own  pencil,  and  with  colors  taken  directly 
from  the  sun's  bright  ray — with  what  rapture,  as  his  eyes  search  over  it, 
does  he  recognize,  perhaps,  his  playmates  there,  and  the  river  in  which  he 
bathes,  and  where  he  sails  his  boat,  and  the  wood  in  whose  solitudes  he 
loves  to  wander,  and  the  mountain  heights  which  he  climbs  to  meet  the  fresh 
breeze,  and  at  a  distance  from  the  world,  to  allow  his  young  fancy  to  work, 
beginning  to  shoot  far  into  time  and  space.  The  great  peculiarity  of  such 
a  picture  is,  that  it  does  not,  like  others,  portray  still-nature,  but  every  thing 
with  appropriate  motion  or  changes  :  the  playmates  are  all  in  action  :  the 
leafy  trees  wave  in  the  wind,  the  clouds  sail  along,  the  sun  may  rise  or  may 
set,  and  even  the  lightning's  gleam  may  dart  across  :  or,  again  commenced 
enterprize  may  be  brought  to  a  close ;  the  traveller  may  climb  the  distant 


342  LIGHT. 

hill  and  disappear;  the  fisherman  may  draw  his  net  and  secure  his  prize; 
the  contested  race  may  be  won  or  lost.  A  Maylan  chief  in  the  island  of 
Sumatra,  was  so  surprised  and  pleased  by  a  small  portable  camera  obscura 
which  the  author  happened  to  have  among  his  apparatus,  that  he  seemed 
disposed  to  give  for  it  almost  any  thing  he  possessed. 

It  appears  in  the  last  diagram  that  the  image  formed  beyond  a  lens  by  the 
gathered  light,  is  in  a  contrary  position  to  the  object  itself, — that  is,  inver- 
ted,— because  the  light  from  the  top  of  the  object  darts  through  the  opening 
or  glass  in  a  descending  direction,  and  that  from  the  bottom  rises  to  the 
opening,  and  in  the  same  direction  passes  beyond  it.  It  is  usual,  therefore 
in  a  camera  obscura,  to  place  a  small  mirror  immediately  behind  the  lens, 
so  as  to  throw  all  the  light  which  enters  downwards,  to  a  whitened  table, 
upon  which  the  picture  may  be  conveniently  contemplated. 

The  camera  obscura  often  gives  very  useful  assistance  to  young  painters, 
by  enabling  them  to  trace  correctly  the  outlines  of  the  objects  placed  before 
it,  and  also  to  study  effects  of  light,  shade  and  colour,  more  profitably  than 
they  at  first  can,  by  looking  at  the  objects  themselves.  The  laws  of  per- 
spective are  most  intelligibly  illustrated  in  this  most  true  picture 

An  effect,  approaching  in  a  degree  to  that  of  the  complete  camera  obscura 
now  described,  is  produced  by  merely  making  a  small  hole  in  the  shutter  of 
a  dark  room,  and  letting  the  light  which  enters  by  it  fall  on  any  white 
surface  beyond.  The  whole  landscape  is  then  dimly  portrayed  upon  the 
surface.  Barry,  the  painter,  while  lying  on  a  sick  bed,  mistook  such  a 
scene  appearing  on  the  ceiling  of  his  room  for  a  supernatural  vision.  If  a 
cross  be  held  before  the  opening  as  in  the  last  figure,  it  is  evident  that  from 
every  point  of  the  cross  light  will  enter  by  the  opening,  and  will  foil  on 
corresponding  parts  of  a  sheet  of  paper  held  behind, — but  as  the  light  from 
each  point  is  not  a  single  ray,  but  a  spreading  pencil  or  cone  of  light,  it  will 
fall  on  the  paper,  not  on  one  point,  but  on  a  surface  at  least  as  large  as  the 
opening,  and  thus  the  light  from  adjoining  points  will  mix  at  the  edges,  and 
will  render  the  images  misty  and  indistinct,  somewhat  like  those  on  the 
back  of  tapestry.  If  the  opening  be  very  small,  the  picture  will  be  well 
defined,  but  very  feebly  illuminated ;  and  if  the  opening  be  of  considerable 
size,  the  mixing  of  the  pencils  will  be  so  great  as  to  leave  no  particular 
object  distinguishable.  But,  in  the  latter  case,  and  however  large  the 
opening  be,  if  a  lens  be  introduced,  it  will  converge  every  pencil  of  light  to 
an  exact  point,  and  the  picture  will  instantly  be  rendered  perfectly  clear. 
A  lens  is  never  held  up  in  the  light  without  forming  beyond  it  pictures  such 
as  now  described,  of  every  visible  object  about  it,  and  the  pictures  are  not 
seen,  only  because  there  are  no  screens  placed  to  receive  them,  and  because 
they  are  so  numerous  as  to  confuse  one  another, — in  other  words,  because 
they  are  not  admitted  singly  into  a  dark  chamber. 

The  distance  from  a  lens  at  which  an  image  is  formed  or  the  rays  of  tbe 
light  meet,  depends  first  upon  the  refractive  or  bending  power  of  the  lens, 
and  therefore,  on  its  form  and  on  the  nature  of  its  substance  ;  and,  secondly, 
upon  the  direction  of  the  rays  of  light  when  they  reach  the  lens,  as  to  whether 
they  are  divergent,  parallel  or  convergent.  We  have  already  explained  that 
glass  refracts  about  twice  as  much  as  water,  and  that  diamond  refracts  about 
twice  as  much  as  glass  :  and  we  have  considered  the  effect  of  different  degrees 
of  convexity  in  lenses — arising  equally  whether  the  lens  be  of  water 
enclosed  between  glasses  like  watch-glasses,  or  of  solid  glass,  or  of  rock- 
crystal  or  of  diamond  itself.  We  now  proceed  to  consider  the  joint  effect 
of  the  refractive  power,  and  of  the  direction.of  the  incident  rays. 


CAMERA    OBSCURA.  343 

Rays  falling  from  a  on  a  comparatively  flat  or  weak  lens  at  L,  might  meet 

Fig.  146. 

•i* 


only  at  d,  or  even  farther  off:  while,  with  a  stronger  or  more  convex  lens, 
they  might  meet  at  c  or  at  b  ;  a  lens  weaker  still  might  only  destroy  the  diver- 
gence of  the  rays,  without  being  able  to  give  them  any  convergence  or  to  bend 
them  enough  to  bring  them  to  a  point  at  all — and  then  they  would  proceed  all 
parallel  to  each  other,  as  seen  at  e  and/; — and  if  the  lens  were  yet  weaker, 
it  might  only  destroy  a  part  of  the  divergence,  causing  the  rays  from  a  to  go 
to  g  and  h,  after  passing  through,  instead  of  to  i and  k,  in  their  original  direction. 

In  an  analogous  manner,  light  coming  to  the  lens  in  contrary  directions 
from  bed,  &c.,  might,  according  to  the  strength  of  the  lens,  be  all  made 
to  come  to  a  focus  at  a  or  at  /,  or  in  some  more  distant  point ;  or  the  rays 
might  become  parallel,  as  m  and  n,  and,  therefore,  never  come  to  a  focus, 
or  they  might  remain  divergent. 

It  may  be  observed  in  the  figure  above,  that  the  farther  an  object  is  from 
the  lens,  the  less  divergent  are  the  rays  which  fall  from  it  upon  the  lens ; 
or  the  more  nearly  do  they  approach  to  being  parallel.  From  b  there  is 
much  divergence  in  the  exterior  rays,  from  c  less,  from  d  less  still,  and  rays 
from  a  great  distance,  as  those  represented  by  e  and/,  appear  quite  parallel. 
If  the  distance  of  the  radiant  point  be  very  great,  they  really  are  so  nearly 
parallel,  that  a  very  nice  test  is  required  to  detect  the  non-accordance.  Rays 
for  instance,  coming  from  the  earth  to  the  sun,  do  not  diverge  the  millionth 
of  an  inch  in  a  thousand  miles.  Hence,  where  we  wish  to  make  experiments 
with  parallel  rays,  we  take  those  of  the  sun. 

Any  two  points  so  situated  on  the  opposite  sides  of  a  lens,  as  that  when 
either  becomes  the  radiant  point  of  light,  the  other  is  the  focus  of  such  light, 
are  called  conjugate  foci.  An  object  and  the  image  of  it  formed  by  a  lens, 
must  always  be  in  conjugate  foci,  and  as  the  one  is  nearer  the  lens,  the  other 
will  be  in  a  certain  proportion  more  distant. 

What  is  called  the  principal  focus  of  a  lens,  and  by  the  distance  of  which 
from  the  glass  we  compare  or  classify  lenses  among  themselves,  is  the  point 
at  which  the  sun's^rays,  or  any  parallel  rays,  are  made  by  it  to  meet;  and 
thus,  by  holding  the  glass  in  the  sun,  and  noting  at  what  distance  behind  it 
the  little  luminous  spot  or  image  of  the  sun  is  formed,  we  can  at  once  ascer- 
tain the  focus  of  a  glass — as  at  a  for  the  rays  e  and  / 

It  is  a  remarkable  coincidence  that  the  bending  power  of  the  common  glass 
used  for  lenses  should  be  such,  that  the  focus  of  a  double  lens  is  just  where 
the  centre  of  the  sphere  would  be,  of  which  the  surface  of  the  lens  is  a  por- 
tion. This  gives  us  another,  fact  with  which  to  associate  the  recollection 
that  the  focus  is  nearer  as  the  convexity  of  the  lens  is  greater,  that  is  to  say, 
as  the  surface  is  a  portion  of  a  smaller  sphere,  And  such  being  the  law,  it 
may  be  proved  by  calculation  as  well  as  by  the  fact,  that  if  a  candle  be  held 
in  relation  to  a  lens  at  twice  the  principal  focal  distance,  suppose  at  c  for  a 


344  LIGHT. 

lens  with  the  focus  at  cr,  the  image  of  the  candle  will  be  formed  at  I  just  as 
far  on  the  other  side  Thus,  then,  by  trying  with  the  lens  until  the  image 
of  a  candle  is  formed  at  the  same  distance  from  it  as  the  object  is,  we  have 
a  second  mode  of  ascertaining  the  focal  distance  of  a  lens.  Other  kinds  of 
glass  and  other  substances  refract  with  different  power ;  but  the  facts  now 
stated  should  be  retained  in  the  memory  as  standards  of  comparison. 

Because  the  focal  point  of  light  passing  through  a  lens  is  at  the  same  dis- 
tance from  the  centre  of  the  lens,  in  whatever  direction  the  light  passes 
through,  a  surface  placed  to  receive  the  image  of  any  broad  object  should 
really  be  concave,  that  is  to  say,  all  parts  of  it  should  be  at  the  same  distance 
from  the  centre  of  the  lens,  otherwise  the  image  will  be  more  perfect  either 
at  its  middle  than  towards  its  edges,  or  vice  versa — but  it  is  not  found  neces- 
sary to  attend  to  this  in  common  practice,  where  the  object  and  its  image 
are  not  of  great  extent. 

The  size  of  an  image  formed  behind  a  lens  is  always  proportioned  to  the 
distance  of  the  image  from  the  lens,  and  the  image  is  much  larger  or  smaller 
than  the  object  as  it  is  farther  from  or  nearer  to  the  lens  than  the  object.  This 

will  be  evident  from  considering  the  an- 

Fig.  147.  nexed  figure  c  represents  the  place  of  a 

lens,  and  the  lens,  according  to  its  power, 
will  form  an  image  of  the  cross  a  b,  in  some 
situation,  as  at  d,  e,  g,  &c.  Now  when- 
ever the  image  is  formed  and  by  whatever 
lens,  one  end  of  it  must  be  in  contact  with 
the  line  a  g,  and  the  other  end  with  the 
line  b  h  ;  and  as  these  lines  cross  each 
other  at  c,  and  widen  regularly  afterwards, 
a  line  adjoining  them  (and  the  image  is  such  a  line,)  must  always  be  shorter 
the  nearer  it  is  to  c,  that  is  to  say,  shorter  in  proportion  to  the  converging 
power  of  the  lens. 

Many  persons  may  not  have  reflected,  that  the  luminous  circle  called  the 
focus  of  a  burning  glass,  is  really  but  the  image  or  picture  of  the  sun  formed 
by  the  glass  or  lens.  The  intensity  of  the  heat  and  of  the  light  is  of  course 
in  proportion  as  the  image  is  smaller  than  the  glass  which  forms  it,  and  the 
nearer  that  the  image  is  formed  to  the  lens,  or  the  more  powerfully  convergent 
that  the  lens  is,  the  smaller  will  the  image  be.  Mr.  Parker's  famous  burning 
lens,  which  cost  £700,  and  is  now  the  property  of  the  Emperor  of  China, 
was  three  feet  in  diameter,  and  the  diameter  of  the  sun's  image  formed  by  it 
was  one  inch  :  it  concentrated  the  light  and  heat,  therefore,  about  1,800  times. 
To  render  the  effect  still  more  powerful,  a  smaller  lens  was  placed  behind 
the  larger,  farther  reducing  the  size  of  the  image  to  one-sixth.  Very  sur- 
prising effects  were  produced  by  this  lens,  in  the  melting  of  metals,  inflaming 
of  combustibles,  &c.  The  size  of  burning  lenses,  until  lately,  was  limited 
by  the  difficulty  of  obtaining  the  great  pieces  of  glass  required  to  form  them  : 
but  they  are  now  built  up  of  many  pieces  suitably  united  together.  Some 
large  lenses  have  been  made  of  water,  that  is,  of  water  enclosed  between 
rnenicus  glasses,  like  watch-glasses.  A  common  goblet  of  water,  or  a  vase 
holding  gold-fishes,  has  in  some  cases  acted  as  a  burning  glass,  setting  fire  to 
the  curtains,  near  which  it  had  been  left  in  the  sunshine. 

And  the  nearer  that  an  object  is  brought  to  a  lens,  the  more  distant,  and 
therefore  the  larger  will  its  image  be ;  for,  as  the  rays  falling  upon  a  lens  are 
divergent  in  proportion  to  the  nearness  of  the  object,  and  therefore  with  the 
same  power  of  lens,  must  meet  farther  behind  (as  seen  in  the  figure  at  page 


MAGIC    LANTERN.  345 

343,)  then  the  axis  of  the  rays,  as  the  lines  c  a  and  c  I  in  the  last  figure, 
will  have  separated  far  before  the  rays  meet,  and  will  have  made  the  image 
proportionally  larger.  If  we  suppose  little  d  in  the  same  diagram  to  be  the 
object,  its  image  would  be  a  b.  The  sun  is  exactly  as  much  larger  than  his 
image  formed  by  a  burning-glass,  as  he  is  more  distant  from  it  than  the 
image ;  and  if  we  had  a  screen  of  sufficient  size  hung  up  in  a  distant  space, 
a  very  bright  object  of  a  quarter  of  an  inch  in  diameter  might  be  made  by  a 
lens  to  form  an  image  as  broad  as  the  sun. 

From  all  these  considerations,  we  see  that,  in  a  camera  obscura,  the  screen 
should  be  from  the  lens,  at  the  distance  of  its  principal  focus  for  distant  ob- 
jects, and  a  little  farther  than  this  for  near  objects  Accordingly  the  lens  is 
generally  fixed  in  a  sliding  piece,  which  allows  the  distance  from  the  screen 
to  be  adjusted  to  circumstances.  If  the  representation  be  desired  large,  the 
lens  must  be  of  a  long  focus ;  if  small  the  lens  must  be  of  a  short  focus. 
Again,  when  the  reversed  use  of  the  lens,  a  small  object  as  d  is  to  be  mag- 
nified to  such  a  size  as  a  b,  then  the  object  must  be  placed  a  little  beyond 
the  focus  of  the  glass ;  for  if  placed  nearer,  the  pencils  of  rays  from  it  would 
never  be  gathered  to  focal  points  at  all,  and  no  image  would  be  formed  at 
any  distance. 

When,  as  alluded  to  in  the  last  sentence,  a  small  object  is  placed  very  near 
a  lens,  and  the  image  of  it  is  thrown  upon  the  wall  of  a  dark  room,  perhaps 
a  hundred  times  farther  from  the  lens  than  the  object  is,  the  image  is  a  greatly 
magnified  representation  of  the  object,  viz.t  it  is  a  hundred  times  longer  and 
a  hundred  times  broader,  and  therefore  has  ten  -thousand  times  as  much  sur- 
face as  the  object;  but  if  in  this  experiment  the  object  be  illuminated  only 
in  an  ordinary  degree,  the  light  from  it  is  so  scattered  as  not  to  suffice  for 
distinct  division.  Hence,  to  attain  fully  in  this  manner  the  purpose  of  a 
microscope,  a  very  strong  light,  concentrated  by  a  suitable  mirror  or  glass, 
must  be  directed  upon  the  object.  When  the  light  of  the  sun  is  used  in 
such  a  case,  the  complete  apparatus  is  called  a  solar  microscope,  and  serves 
beautifully  to  display  the  structure  of  any  minute  objects.  When  artificial 
light  is  used,  as  of  a  lamp,  the  apparatus  is  called  the  lucernal  microscope 
or  magic  lantern. 

A  good  solar  microscope  becomes  one  of  the  most  interesting  presents 
which  science  has  made  to  man,  for  aiding  him  in  his  researches  into  the  secrets 
of  nature.  With  the  late  improvements  in  the  construction  of  lenses  by  which 
the  dispersion  of  light  or  the  rainbow-fringe,  is  prevented,  (as  will  be  ex- 
plained under  the  head  of  Telescopes,)  objects  may  be  magnified  two  or 
three  hundred  thousand  times,  and  still  be  so  luminous  as  to  be  beautifully 
distinct;  thus  a  cheese-mite  will  appear  of  the  dimensions  of  a  hog,  and  crea- 
tures altogether  invisible  to  the  naked  eye.  or  perceived  by  it  only  as  minute 
white  points  are  discovered  to  be  animated  beings,  having  the  perfect  pro- 
portions, and  often  the  beauty  of  larger  animals,  and  endowed  with  similar 
appetites,  passions,  and  apparent  ingenuity,  but  with  an  activity  far  surpass- 
ing that  met  with  in  the  more  bulky  creation.  A  judicious  selection  of  objects 
for  the  solar  microscope  is  calculated  exceedingly  to  surprise  the  mind  on  its 
first  attending  to  them,  and  to  fill  it  with  high  conceptions  of  the  infinity  of 
creation.  With  the  common  microscope  only  one  person  at  a  time  can  feast 
his  wonder;  but  with  a  solar,  a  whole  roomfull  of  company  may  at  once  con- 
template the  same  objects  and  witness  the  same  actions,  and  thus  have  their 
admiration  increased  by  the  consciousness  of  sympathy. 

The  magic  lantern,  we  have  said,  consists  of  a  powerful  lens,  with  objects, 
highly  illuminated  by  lamp-light,  placed  so  near  it  that  their  images  are 


346  LIGHT. 

formed  far  off,  and  arc  therefore  proportionally  larger.  For  the  magic  lan- 
tern the  objects  are  generally  paintings  made  on  thin  plates  of  glass  with 
transparent  colours ;  and  each  plate  is  formed  to  slide  through  a  slit  or  pas- 
sage behind  the  lens.  The  lens  itself,  or  what  may  be  called  half  of  it,  (for 
there  are  often  two  lenses  joined  to  give  greater  power,)  is  moveable  with 
the  tube  which  is  seen  projecting  from  the  lantern,  so  that  its  distance  from 
the  object  may  be  varied,  and  thus  a  corresponding  approach  to  or  receding 
from  the  screen  may  be  allowed,  which  will  produce  an  increase  or  lessening 
of  the  magnitude  of  the  visible  picture  on  the  wall. 

Some  public  lectures  on  astronomy  and  other  branches  of  natural  history, 
have  had  the  drawings  and  paintings  required  for  the  elucidation  of  their 
subjects  made  in  miniature  upon  glass,  to  be  magnified  afterwards  to  the 
degree  desired,  and  shown  upon  any  part  of  the  lecture-room  by  the  magic 
lantern. 

A  thick  fog  or  smoke  at  night  will  sometimes  reflect  the  images  of  a 
magic  lantern  so  as  to  make  them  distinctly  visible ;  and  there  are  several 
cases  on  record,  where  persons,  wickedly  ingenious-  in  this  way,  have  terri- 
fied ignorant  individuals  almost  to  death,  by  throwing  spectres  from  a  con- 
cealed lantern.  Some  years  ago  a  sentinel  in  St.  James'  Park  was  thus 
persuaded  that  he  had  seen  supernatural  beings  near  him  among  the  trees. 

A  very  charming  illusion  is  produced  by  a  magic  lantern  manoeuvred  on 
one  side  of  a  thin  screen,  while  the  spectators  not  aware  of  the  existence  of 
a  screen,  are  sitting  on  the  other  side.  The  image — let  us  suppose  it  that 
of  a  genius  flying  in  the  air — may  be  first  thrown  upon  the  screen  from  the 
lantern  while  very  near,  and  then  it  will  be  small,  and,  if  desired,  exceed- 
ingly bright  because  the  light  is  much  concentrated.  If  the  exhibitor  then 
gradually  recede  from  the  screen,  adjusting  at  the  same  time  the  distance  of 
the  lens  from  the  picture,  the  image  will  become  progressively  larger,  and 
to  the  spectators  will  appear  to  be  soaring  and  approaching,  until  at  last  the 
expanded  wings  and  limbs  seem  hovering  almost  over  their  heads.  An  end- 
less variety  of  most  ingenious  and  beautiful  exhibitions  of  this  kind  have 
been  made  under  the  name  of  phantasmagoria  or  raising  of  spectres. 

"  The  EYE  itself  is,  in  fact,  but  a  small  camera  obscura."     (Read  the 
Analysis,  page  325.) 

Who  could  at  first  believe  that  in  describing  the  camera  obscura,  as  we 
have  now  done,  we  had  in  reality  been  describing  only  a  large  model  of  that 
most  interesting  of  the  objects  of  creation,  the  living  eye  itself,  the  inlet  of 
man's  knowledge, — or  what  may  be  called  the  visible  dwelling  of  the  soul — 
that  from  which  the  fire  of  passion  darts,  through  which  the  languor  of  ex- 
haustion is  perceived,  and  in  which  life  and  thought  seem  concentrated !  Yet 
the  eye  is  nothing  but  a  simple  camera  obscura  :  formed  of  the  parts  des- 
cribed above  as  essential  to  the  camera  obscura :  but  in  its  simplicity  so 
perfect  that  they  who  delight  to  find  around  them  tangible  evidences  of  the 
existence  of  an  all- wise  and  good  Creator,  point  to  this  in  the  midst  of  thou- 
sands, as  one  of  the  most  undeniable  and  triumphant  proofs.  We  shall  now 
describe  the  eye  and  its  actions  :  and  keeping  present  to  us  the  idea  of  the 
camera  obscura,  as  already  treated  of,  we  shall  find  that  the  use  of  the  various 
parts  will  be  declared  by  merely  enumerating  them.  This  paragraph  should 
be  perused  while  the  reader  has  the  opportunity  of  observing  either  his  own 
eye  reflected  in  a  mirror,  or  the  eye  of  some  companion  near  him. 

The  human  eye,  then  is  a  globular  chamber  of  the  size  of  a  large  walnut, 
having  for  its  outer  wall  a  very  tough  membrane  called,  from  its  hardness, 


THE    EYE  —  STRUCTURE. 


347 


the  sclerotic  coat,  in  the  front  of  which  there  is  one  round  opening  or  win- 
dow, named,  because  of  its  horny  texture,  the  cornea.  The  chamber  is 
lined  with  a  finer  membrane  or  web,  the  choroid  (having  relation  to  colour,) 
which,  to  insure  the  internal  darkness  of  the  place,  *is  covered  with  a  black 
paint,  the  pigmentum  niarum.  This  lining  is  bordered  at  the  edge  of  the 
round  window  by  a  folded  drapery,  the  ciliary  processes,  hidden  from  with- 
out by  being  behind  the  curious  contractile  window-curtain  the  iris,  (so  named 
for  its  rainbow  variety  of  colour  in  different  persons,)  through  the  central 
opening  of  which,  called  the  pupil j  the  light  enters.  Immediately  behind 
the  pupil  is  suspended,  by  attachments  among  the  ciliary  processes,  the  crys- 
talline lens,  a  double  convex,  most  transparent  body  of  considerable  hard- 
ness, which  so  influences  the  light  passing  through  it  from  external  objects, 
as  to  form  perfect  images  of  these  objects,  in  the  way  already  described,  on 
the  back  wall  of  the  eye,  over  which  the  optic  nerve,  there  called  the  retina,  is 
spread  as  a  second  lining.  The  eye  is  maintained  in  its  globular  condition 
by  a  watery  liquid  which  distends  its  external  coverings,  and  which,  in  the 
space  before  the  lens,  or  the  anterior  chamber  of  the  eye,  being  perfectly 
limpid,  is  called  the  aquous  humour,  and  in  the  remainder  or  larger  poste- 
rior chamber,  being  enclosed  in  a  pellucid  spongy  structure,  so  as  to  acquire 
somewhat  of  the  appearance  of  melted  glass,  is  called  the  vitreous  humour. 

The  annexed  figure  represents  an  eye  of  the  common  dimensions,  supposed 
to  be  cut  through  its  middle,  from  above  downwards,  so  as  to  show  the  edges 
of  the  coats,  &c.  C  is  the  outer  or  sclerotic  coat,  known  popularly,  where 
most  exposed  in  front,  as  the  white  of  the  eye.  A  is  the  transparent  cornea 
joined  to  the  edge  of  the  round  opening  of  the  sclerotic;  it  is  more  bulging 
than  the  sclerotic,  or  forms  a  portion  of  a  smaller  sphere  than  the  general 
eyeball,  so  that,  while  it  may  be  truly  called  ^.'bow-window,  it,  or  rather  the 
convex  surface  of  its  contained  water,  is  also  a  powerful  lens  for  acting  on  the 
pencils  of  entering  light.  At  B,  the  similarity  all  round  the  edge  of  the  cor- 
nea, is  attached  to  the  window-curtain  or  iris,  shown  here  edgeways  immersed 
in  the  aqueous  humour,  and  hanging  inwards  from  above  and  below  towards 
its  central  opening  or  pupil,  through  which  the  rays  of  light  are  passing  to 
the  lens.  The  iris  has  in  its  structure  two  sets  of  fibres,  the  circular  and  the 
radiating,  which  cross  and  act  in  opposition  to  each  other ; — when  the  circular 
fibres  contract,  the  pupil  is  lessened,  when  the  radiating  contract,  it  is  en- 
larged ;  and  the  changes  happen  according  to  the  intensity  of  light  and  the 
state  of  sensibility  of  the  retina,  as  may  at  any  time  be  proved  by  closing  the 
eyelids  for  a  moment  to  make  the  pupil  dilate,  and  then  opening  them  to- 
wards a  strong  light,  to  make  it  contract.  Behind  the  pupil  is  seen  the  lens  D 

Fig.  148. 


348  LIGHT. 

with  its  circumference  attached  to  the  ciliary  processes  E  :  it  is  more  convex 
behind  than  before.  The  disease  of  the  eye  called  cataract  (from  the  Greek 
word  implying  obstruction,*)  is  the  circumstance  of  the  lens  become  opaque, 
and  the  cure  is  to  extract  the  lens  entirely,  or  to  depress  it  to  the  bottom  of 
the  eye,  and  then  to  substitute  for  it  externally  a  powerful  artifical  lens  or 
spectacle-glass.  The  three  lines  marking  here  the  boundary  of  the  eye  stand 
for  its  three  coats  as  they  have  been  called,  the  strong  sclerotic,  and  the  double 
lining  of  the  choroid  and  retina.  The  figure  of  a  cross  is  represented  upon 
the  retina  as  formed  by  the  light  entering  from  the  cross  without  (which  cross 
has  to  appear  here  small  aud  near,  although  supposed  to  be  large  and  dis- 
tant.) The  image  of  the  cross  is  inverted,  as  explained  for  the  camera  ob- 
scura :  but  we  shall  learn  below  that  the  perception  of  an  object  may  be 
equally  distinct  in  whatever  position  the  image  fall  on  the  retina.  It  has  been 
explained  above,  that  a  lens  can  form  a  perfect  image  of  considerable  ex- 
tent only  on  a  concave  surface, — and  the  retina  is  such  a  surface.  The 
present  diagram  further  explains  what  is  meant  by  the  anterior  and  poste- 
rior chambers  of  the  eye,  namely,  the  compartments  which  are  before  and 
behind  the  crystalline  lens  D. 

The  nature  of  the  eye  as  a  camera  ,obscura  is  beautifully  exhibited  by 
taking  the  eye  of  a  recently  killed  bullock,  and  after  carefully  cutting  away 
the  back  part  of  the  two  outer  coats,  by  going  with  it  to  a  dark  place  and 
directing  the  pupil  towards  any  brightly  illuminated  objects ;  there  may 
then  be  seen  though  the  semi-transparent  retina,  left  as  a  screen  at  the 
back  of  the  eye,  a  minute  but  perfect  picture  of  all  such  objects — a  picture, 
therefore,  formed  on  the  back  of  the  little  apartment  or  camera  obscura  by 
the  agency  of  the  convex  cornea  and  lens  in  front.  The  picture  is  inverted, 
for  reasons  explained  above. 

Understanding  from  all  this,  that  when  a  man  is  said  to  be  looking  at  an 
object,  his  mind  is  in  truth  only  taking  cognizance  of  the  picture  or  impres- 
sion made  on  his  retina,  it  excites  admiration  in  us  to  think  of  the  exquisite 
delicacy  of  texture  and  of  sensibility  which  the  retina  must  possess,  that  there 
may  be  the  perfect  perception  which  really  occurs  of  even  the  separate  parts 
of  the  minute  images  there  formed.  A  whole  printed  sheet  of  newspaper, 
for  instance,  may  be  portrayed  on  the  retina  on  less  space  than  the  surface 
of  a  finger-nail,  and  yet  not  only  shall  every  word  and  letter  be  separately 
perceivable,  but  in  the  centre  of  the  picture  at  least,  even  an  imperfection  of 
a  single  letter.  Or,  more  wonderful  still,  when  at  night  an  eye  is  turned  up 
to  the  blue  vault  of  heaven,  there  is  portrayed  on  the  little  concave  of  the 
retina  the  boundless  concave  of  the  sky,  with  every  object  in  its  just  propor- 
tions. There  a  moon  in  beautiful  miniature  may  be  sailing  among  white- 
edged  clouds,  and  surrounded  by  a  thousand  twinkling  stars,  all  in  just  pro- 
portion, so  that  to  an  animalcule  within  and  near  the  pupil,  the  retina  might 
appear  another  starry  firmanent  decked  in  its  glory.  If  the  image  in  the 
human  eye  be  thus  minute,  what  must  they  be  in  the  little  eye  of  a  wren,  or 
of  other  animals  smaller  still !  How  wonderful  are  the  works  of  nature  ! 

Because  the  images  formed  on  the  retina  are  always  inverted  as  respects 
the  true  position  of  the  objects  producing  them — just  as  happens  in  a  simple 
camera  obscura — persons  have  wondered  that  things  should  appear  upright, 
or  in  their  true  situations.  The  explanation  is  not  difficult.  It  is  known 
that  a  man  with  wry  neck  judges  as  correctly  of  the  position  of  the  objects 
around  him  as  any  other  person — never  deeming  them  to  be  inclined  or 
crooked,  because  their  images  are  inclined  in  relation  to  the  natural  perpen- 
dicularity of  his  retina;  and  that  a  bed-ridden  person,  obliged  to  keep  his  head 


THE    EYE  —  DOUBLE    SIGHT.  349 

upon  his  pillow,  soon  acquires  the  faculty  of  the  person  with  wry  neck  ;  and 
that  an  affected  girl  inclining  her  head  while  trying  her  various  attitudes, 
learns  from  much  practice,  to  judge  of  the  manoeuvres  of  a  beau  as  conveni- 
ently in  that  way  as  in  any  other ;  and  that  boys  who  at  play  bend  themselves 
down  to  look  backwards  through  their  legs,  although  a  little  puzzled  at  first, 
because  the  usual  position  of  the  images  on  the  retina  is  reversed,  soon  see 
as  well  in  that  way  as  in  any  other.  It  appears,  therefore,  that  while  the 
mind  studies  the  form,  colour,  &c.,  of  external  objects  in  their  images  pro- 
jected on  the  retina,  it  judges  of  their  position,  not  by  the  accidental  position 
of  the  images  on  the  retina,  but  by  the  direction  in  which  the  light  comes 
from  the  object  towards  the  eye — no  more  deeming  an  object  to  be  placed 
low  because  its  image  is  low  in  the  eye,  than  a  man  in  a  room  into  which 
a  sunbeam  enters  by  a  hole  in  the  window-shutter,  deems  the  sun  low  be- 
cause its  image  is  on  the  floor.  A  candle  carried  past  a  key-hole,  throws  its 
light  on  the  opposite  wall,  so  as  to  cause  the  luminous  spot  there  to  move  in 
a  direction  the  opposite  of  that  in  which  the  candle  is  carried;  but  a  child  is 
very  young,  indeed  who  has  not  learned  to  judge  at  once  of  the  true  motion 
of  the  candle  by  the  contrary  apparent  motion  of  the  image.  A  boatman, 
who,  being  accustomed  to  his  oar,  can  direct  its  point  against  any  object  with 
great  certainty,  has  long  ceased  to  reflect,  that  to  move  the  point  of  the  oar 
in  some  one  direction,  his  hand  must  move  in  the  contrary  direction.  Now 
the  seeing  things  upright  by  images  which  are  inverted,  is  a  phenomena  akin 
to  those  which  we  have  here  reviewed. 

Another  question  somewhat  allied  to  the  last  is,  why,  as  we  have  two  eyes, 
and  an  image  of  any  object  placed  before  them  is  formed  in  each — why  the 
object  does  not  appear  to  us  to  be  double.  In  answer  to  this,  again,  we  shall 
only  state  the  simple  facts  of  the  case.  As  in  two  chess-boards  there  are 
corresponding  squares,  so  in  the  two  eyes  there  must  be  corresponding  points, 
and  when  on  those  points  a  similar  impression  is  made  at  the  same  time,  the 
sensation  or  vision  is  single;  but  if  the  impression  be  made  on  points  which 
do  not  correspond,  owing  to  some  disturbance  of  the  natural  position  of  the 
eyes,  the  vision  becomes  double.  Healthy  eyes  are  so  wonderfully  associated, 
that  from  earliest  infancy  they  constantly  move  in  perfect  unison.  By  slightly 
pressing  a  finger  on  the  ball  of  either  eye,  so  as  to  prevents  its  following  the 
motion  of  the  other,  there  is  immediately  produced  the  double  vision ;  and 
tumours  about  the  eye  often  have  the  same  effect.  Person&who  squint  have 
always  double  vision  :  but  they  acquire  the  power  of  attending  to  the  sensa- 
tion in  one  eye  at  a  time.  Animals  which  have  the  eyes  placed  on  opposite 
sides  of  the  head,  so  that  the  two  can  never  be  directed  to  the  same  point, 
must  have  in  a  more  remarkable  degree  the  faculty  of  thus  attending  to  one 
eye  at  a  time. 

The  corresponding  points  in  the  two  eyes  are  equidistant  and  in  similar 
directions  from  the  centres  of  the  retinae,  which  centres  are  called  the  points 
of  distinct  vision,  and  at  them  the  imaginary  lines  named  the  axes  of  the  eye 
terminate — but  it  is  worthy  of  remark  that  these  points,  in  being  both  to 
the  right  or  both  to  the  left  of  the  centres,  must  be  one  of  them  on  the  inside 
of  the  centre  as  regards  the  nose,  and  the  other  on  the  outside — that  is  to 
say,  a  point  of  the  left  eye  between  the  centre  and  nose,  has  its  correspond- 
ing point  in  the  right  eye  between  the  centre  and  the  cheek — and  from  this 
fact  arises  consequences  meriting  attention.  When  the  two  eyes  were  directed 
to  any  object,  their  axes  meet  at  it,  and  the  centres  of  the  two  retinae  are  oppo- 
site to  it,  and  all  the  other  points  of  the  eyes  have  perfect  mutual  correspond- 
ence as  regards  that  object,  giving  the  sensation  of  single  vision ;  but  the 


350  LIGHT. 

images  formed  at  the  same  time,  of  an  object  nearer  to  or  farther  from  the 
eye  than  the  first  supposed,  cannot  fall  on  corresponding  points  for  an  object 
nearer  than  where  the  axes  meet  would  have,  both  its  images  on  the  outsides 
of  the  centres,  and  an  object  more  distant  would  have  both  its  images  on  the 
insides  of  the  centres,  and  in  either  case  the  vision  would  be  double.  Thus 
if  a  person  hold  up  one  thumb  before  his  nose,  and  the  other  in  the  same 
direction,  but  farther  off,  by  then  looking  at  the  nearest,  the  more  distant 
will  appear  double,  and  by  lo'oking  at  the  more  distant,  the  nearest  will 
appear  double. 

The  reason  for  applying  the  term  "  point  of  distinct  vision"  to  the  centre 
of  the  retina,  is  felt  at  once  by  looking  at  a  printed  page,  and  observing  that 
only  the  one  letter  to  which  the  axes  of  the  eye  is  directed,  is  distinctly  seen; 
and,  consequently,  that  although  the  whole  page  be  .depicted  on  the  retina 
at  once,  the  eye,  in  reading  has  to  direct  its  centre  successively  to  every  part. 

On  examining  a  dead  eye,  the  point  of  distinct  vision  is  distinguishable 
from  the  retina  around  by  being  more  transparent.  It  might  have  been 
expected  that  this  point  would  have  been  where  the  optic  nerve  enters  the 
eye  :  but,  in  fact,  the  optic  nerve  enters  considerably  nearer  to  the  nose  than 
the  point  of  distinct  vision  is ;  and  singularly,  where  it  enters,  the  part  is 
altogether  blind  or  insensible.  Had  the  two  optic  nerves,  therefore,  entered, 
at  corresponding  points  of  the  retina,  (in  the  sense  explained  above,)  there 
would  have  appeared  a  black  spot  on  every  object  opposite  to  the  insensible 
points;  but  as  the  case  really  stands,  the  part  of  any  object  from  which  the 
light  passes  to  the  insensible  or  blind  part  of  one  eye  must  be  opposite  to  a 
sensible  part  of  the  other.  The  existence  of  the  blind  spot,  where  the  nerve 
of  the  eye  enters,  is  discoverable  by  placing  in  a  row  three  objects — wafers, 
for  instance— across  a  table,  with  intervals  of  about  two  inches  between  them, 
and  then  looking  with  one  eye,  (the  other  being  shut)  from  a  distance  of  about 
eight  inches,  at  the  wafer  which  is  on  the  side  of  the  nose; — the  middle 
wafer  will  be  invisible,  although  the  eye  will  see  that  on  each  side  of  it ;  and 
if  the  eye  be  then  directed  to  the  middle  wafer,  the  external  one  will  disap- 
pear. Another  proof  is  obtained  by  shutting  one  eye  and  looking  with  the 
other  at  the  points  of  two  fingers  held  together  before  it; — if  one  of  the  fin- 
gers be  then  gradually  moved  away  laterally,  its  point  when  at  a  certain  dis- 
tance from  the  other  will  disappear,  but  will  be  seen  again  when  its  distance 
is  still  increased. 

It  appearing,  from  the  explanations  now  given,  that  there  canot  be  perfect 
sight  unless  where  a  perfect  image  is  formed  on  the  retina,  and  the  truth 
having  been  formerly  explained,  that  images  behind  any  lens  will  be  at  dif- 
ferent distances  from  it,  according  to  the  various  distances  of  the  objects  in 
front,  that  is  to  say,  according  as  the  pencils  of  light  which  fall  upon  it  have 
more  or  less  of  divergence  in  them,  it  follows,  that  the  eye  in  being  able,  as 
it  is,  to  see  distinctly  objects  at  different  distances,  (the  nearest  is  about  five 
inches,)  possesses  a  power  of  altering  the  relation  of  its  parts  to  accommodate 
itself  to  the  circumstances.  We  do  not  yet  perfectly  know  whether  it  does 
this  by  lengthening  or  changing  the  form  of  the  ball  through  the  action  of 
the  surrounding  muscles,  or  by  changing  the  place  or  the  form  of  the  lens, 
but  that  one  or  more  of  these  events  occurs  there  can  be  no  doubt. 

Among  the  eyes  of  the  myriads  of  mankind,  however,  it  happens  that  all 
do  not  originally  possess  these  powers  exactly  in  the  requisite  degree,  and 
that  many  lose  them,  as  life  advances,  from  a  natural  and  usual  decay. 

Persons  are  called  short-sighted  whose  eyes  from  too  great  convexity  of 
the  cornea  or  lens,  have  so  strong  a  bending  or  converging  power,  that  the 


THE    EYE  —  SHORT    SIGHT  —  LONG    SIGHT. 


351 


rays  of  light  entering  them  are  brought 

to  a  focus  before  reaching  the  retina  —  Fig.  149. 

at  a,  for  instance,  instead  of  at  b  :  so 

that  the  rays,  by  spreading  again  beyond 

the  focus,  produce  on   the  retina  that 

sort  of  indistinct  image  which  is  seen  in 

the  camera  obscura,  of  which  the  screen 

is   too    distant  from   the   lens.      This 

defect  of  sight  obliges  the   individual 

when  using  the  naked  eye  to  hold  ob- 

jects very  near  it,  that  the  consequent  greater  divergence  of  the  rays  may 

be  proportioned  to  the  unusual  refracting  power  of  the  eye  —  or  the  person 

may  find  a  remedy  in  placing  concave  lenses  between  the  object  and  the 

eyes,  which  lenses,  by  rendering  light  from  objects  at  a  usual  distance 

more  divergent,  (as  explained  page  340,)  cause  the  perfect  images  in  the 

eye  to  be  formed  farther  from  the  lens,  and  thereby  on  the  retina  itself. 

Without  concave  spectacles  —  as  the  lenses  are  called  when  fixed  together  in 

a  frame  —  persons  with  the  defect  now  under  consideration  cannot  see  dis- 

tinctly any  object  that  is  distant,  for  the  rays,  coming  nearly  parallel,  are 

quickly  gathered  to  a  focus.     This  defect  often  diminishes  with  years,  and 

the   person  who  in  youth  needed  spectacles,  in  old  age  sees  well  without 

them. 

There  is  an  opposite  defect  of  deficient  converging  power  in  the  eye, 
dependent  on  a  too  great  flatness  of  the  cornea  or  lens,  and  which  is  much 
more  common  than  the  last-mentioned  defect  ;  indeed,  the  great  majority  of 
persons  after  middle  age  sooner  or  later  begin  to  experience  it.  In  this  case, 
the  rays  of  light  are  not  yet  collected  into  a  focus  when  they  reach  the 
retina  :  they  would  only  meet  at  b, 

for  instance,  instead  of  as  they  should  Fig.  150. 

do  ate,  and  hence  the  image  is  indis- 
tinct, in  the  same*  manner  as  in  a 
camera  obscura,  when  the  screen  is 
held  too  near  the  lens.  Persons 
suifering  this  defect  cannot,  when 
using  the  naked  eye,  see  distinctly 
to  it,  because  the  gathering  or  converging  power  of  the  eye  cannot  conquer 
the  great  divergence  of  rays  coming  from  a  near  point  ;  and  hence  such  per- 
sons always  remove  objects  under  consideration  to  a  considerable  distance, 
often  to  that  of  arm's  length,  so  as  to  receive  from  them  only  the  rays 
nearly  parallel.  These  persons  in  contradistinction  to  the  last  described, 
are  called  long  -sighted  persons  }  and  after  middle  age,  most  persons  become 
more  or  less  long-sighted.  Their  defect  is  remedied  by  the  common  convex 
spectacles,  which  do  part  of  t*he  converging  work,  so  to  express  ourselves, 
before  the  light  enters  the  eye,  leaving  undone  only  that  which  the  eye  can 
easily  accomplish.  As  this  defect,  like  the  last,  is  met  with  in  all  degrees, 
spectacles  must  be  chosen  accordingly.  Certain  curvatures  or  strengths  of 
these  have  been  particularized  and  numbered  as  naturally  belong  to  dif- 
ferent ages  or  periods  of  life,  but  each  person  should  choose  under  the 
direction  of  an  experienced  judge,  until  that  strength  be  found  which  ena- 
bles him  to  read,  without  any  straining  of  the  eyes,  at  the  common  dis- 
tance of  from  twelve  to  eighteen  inches.  We  cannot  apply  the  mind  to  this 
part  of  our  subject  without  feeling  admiration  at  what  science  has  accom- 
plished for  man  in  assisting  and  restoring  his  sight.  Now  that  in  civilized 


352  LIGHT. 

society,  the  common  employments  and  enjoyments  of  life  require  a  visual 
power  capable  of  distinguishing  such  minute  objects  as  written  or  printed 
characters,  to  deprive  old  men  of  their  spectacles,  would  be  to  condemn 
many  of  them  to  useless  inactivity  and  a  listless  blank  of  mind  for  the 
remainder  of  their  lives. 

An  eye  much  accustomed  to  examine  near  and  minute  objects,  often  loses 
something  of  its  pliancy,  and  becomes  defective  when  tried  at  a  distant 
thing,  as  the  watchmaker's  eye,  the  engraver's,  &c.  On  the  other  hand, 
the  old  seamen's,  which  has  so  often  and  uninterruptedly  been  bent  on  the 
distant  horizon,  straining  to  catch  the  view  of  an  expected  sail,  or  of  land, 
has  a  power-  of  discovering  distant  things  which  is  wonderful ;  but  it  often 
experiences  deficiency  in  regard  to  near  things. 

A  man  who  uses  his  eyes  under  water  sees  very  indistinctly,  because  the 
difference  of  density  between  water  and  the  eye  not  being  so  great  as  be- 
tween air  and  the  eye,  the  bending  or  refraction  of  light  entering  from  the 
water  is  not  so  great  as  of  light  entering  from  air,  and  the  internal  structure 
of  the  human  eye  being  adapted  to  the  greater  refraction,  perfect  images  are 
not  formed  on  the  retina.  A  man  to  see  well  under  water,  therefore,  requires 
to  aid  the  usual  power  of  his  eyes  by  strong  convex  spectacles.  It  is  to 
meet  the  necessity  now  explained,  that  the  lens  of  a  fish's  eye  is  extremely 
convex,  or  almost  round,  as  is  everyday  seen  in  the  white  round  bead  which 
issues  from  the  eye  of  a  boiled  fish — that  little  globe  being  the  crystalline 
lens  of  the  fish  coagulated  or  hardened  like  the  white  of  an  egg  during 
cooking. 

There  are  many  important  considerations  connected  with  the  sensibility  of 
the  retina,  which  regard  rather  the  laws  of  life  than  of  light,  but  we  must 
here  glance  at  a  few  of  them. 

Any  impression  of  light  made  upon  the  retina  lasts  for  about  the  sixth 
of  a  second.  Hence  when  the  burning  end  of  a  stick  is  made  to  sweep 
rapidly  across  the  view,  its  path  appears  to  the  eye  a  Irne  of  light :  ^nd  if 
it  be  made  to  revolve  in  a  circle  six  times  in  a  second,  as  when  moved  by  the 
hand  or  fixed  to  a  turning  wheel,  that  circle  will  appear  to  the  eye  a  com- 
plete ring  of  fire.  The  polished  end  of  an  elastic  wire,  of  which  the  other 
end  is  fixed  in  a  block  of  wood,  when  caused  to  vibrate,  similarly  forms  a 
line  or  a  curve  of  light.  A  harp-string,  while  vibrating  as  it  sounds,  ap- 
pears like  a  flat  transparent  riband.  Lightning  or  other  meteor  darting 
across  the  sky,  although  in  fact  but  a  moving  luminous  point,  is  generally 
thought  of  as  a  long  line  of  light :  the  term  forked-lightning  has  reference 
to  this  prejudice.  The  same  remark  applies  in  a  degree  to  a  sky-rocket  in 
its  rapid  ascent.  Two  or  more  colours  painted  separately  on  the  rim  of  a 
wheel  which  is  made  to  turn  rapidly,  appear  to  a  spectator  to  be  as  com- 
pletely united  as  if  they  were  really  mixed  ; — it  has  been  already  explained 
how  patches  of  all  the  colours  of  the  rainbow,  when  mixed  in  this  way,  form 
white  light.  If  on  one  side  of  a  card  a  little  bird  be  painted,  and  on  a 
corresponding  part  of  the  other  side  a  cage,  then,  on  making  the  card  turn 
rapidly  by  twisting  between  the  fingers  and  thumbs  two  threads  fixed  to  its 
opposite  edges,  the  bird  and  cage  will  be  seen  at  once,  and  the  bird  will 
appear  to  be  imprisoned  in  the  cage  ; — or,  if  a  pensive  Juliet,  sitting  in  her 
bower,  occupy  one  side  of  the  card,  and  a  longing  Romeo  the  other,  by  the 
magic  turn  of  the  threads  the  lovers  may  instantly  be  brought  together. 
Dr.  Paris  displayed  taste  and  an  amiable  ingenuity  in  designing  this  toy 
with  great  variety  of  subjects. 


THE  EYE — JUDGING  BY  APPEARANCES.     353 

A  certain  intensity  of  light  is  necessary  to  distinct  vision,  but  the  degree 
varies  with  the  previous  state  of  the  organ.  A  person  passing  from  the  bright 
day  into  a  shaded  room,  might  for  a  time  fancy  himself  in  totftl  darkness,  and 
to  persons  sitting  in  the  room,  and  become  so  accustomed  to  the  less  light  as 
to  see  well  with  it,  he  might  appear  to  be  almost  blind.  The  dawn  of  morning 
after  the  darkness  of  night  appears  much  brighter  than  an  equal  degree  of 
light  in  the  evening.  When,  as  the  night  falls,  our  lamps  or  candles  are  first 
introduced,  the  glare  is  often  for  a  time  offensive  to  the  eye;  and  a  similar 
feeling  but  still  stronger  is  experienced,  when  in  the  morning,  bed-room, 
window  shutters  or  close  drawn  curtains  are  suddenly  opened  After  the 
repose  of  night,  the  sensibility  of  the  eye,  when  first  opened,  is  often  such 
that  the  globules  of  blood  moving  in  the  capillary  vessels  of  the  retina  pro- 
duce the  impression  there  of  little  balls  of  light  pursuing  one  another  along 
the  tortuous  vessels.  To  a  prisoner  after  long  confinement  in  a  dark  dun- 
geon, the  light  of  the  sun  is  almost  insupportable.  And  a  dungeon,  which  to 
common  eyes  is  utterly  dark,  still  to  its  long-held  inmate  has  ceased  to  be 
so ; — thtre  are  various  instances  in  the  records  of  the  barbarous  ages  of 
prisoners  confined  for  years  in  darkness,  deemed  absolute,  but  who  after  a, 
time  could  see  in  it,  and  make  entertaining  companions  of  the  mice  and 
spiders  which  frequented  their  cells.  The  darkness  of  a  total  eclipse  after 
bright  sunshine,  appears  deeper  than  that  of  midnight,  because  of  the  sudden 
contrast.  The  long  polar  night  of  months  ceases  to  appear  very  dark  to  the 
polar  inhabitants  If  an  eye  be  directed  for  a  time  to  a  black  wafer  laid  on  a 
sheet  of  white  paper,  and  be  then  turned  to  another  part  of  the  sheet,  a  por- 
tion of  the  paper  at  that  other  part,  of  the  size  of  the  wafer,  will  appear 
brilliantly  illuminated  }  for  the  ordinary  degree  of  light  from  it  appears  intense 
to  the  part  of  the  retina  lately  receiving  almost  none.  An  eye  directed  long 
and  intensely  upon  any  minute  object — as  when  a  sailor  watches  a  speck  in 
the  distant  horizon,  supposed  to  be  a  ship,  or  when  a  hunter  on  the  brown 
heath  keeps  his  eye,  fixed  on  a  bird  nearly  of  the  colour  of  the  heath  or 
when  an  astronomer  gazes  long  at  a  little  star — has  the  sensibility  of  its 
centre  at  last  exhausted,  and  ceases  to  perceive  the  object ;  but  on  directing 
the  axis  of  the  eye  a  little  to  one  side  of  the  object,  so  that  an  image  may 
be  formed  only  near  ike  centre,  the  object  may  be  again  perceived,  and  the 
centre  in  the  mean  time  enjoying  repose,  will  recover  its  power. 

But  the  most  extraordinary  fact  connected  with  the  sensibility  of  the  retina 
is,  that  if  part  of  it  be  strongly  exercised  for  a  time  by  looking  at  some  bright- 
coloured  object,  on  the  eye  being  then  turned  away  or  altogether  shut,  an 
impression  of  spectrum  will  remain  of  the  same  form  as  the  object  lately  con- 
templated, but  of  a  perfectly  different  colour.  Thus  if  an  eye  be  directed  for 
a  time  to  a  red  wafer  laid  on  white  paper,  and  be  then  shut  or  turned  to  an- 
other part  of  the  paper,  a  beautifully  bright  green  wafer  will  be  seen ;  and 
vice  versa,  a  green  wafer  will  produce  a  red  spectrum  :  an  orange  wafer  will 
similarly  produce  a  blue  spectrum  ;  a  yellow  one  a  violet  spectrum,  &c.,  and 
a. cluster  of  wafers  will  produce  a  similar  cluster  of  opposite  colours.  Then 
if  the  hand  be  held  over  the  closed  eye  lids  to  prevent  entirely  the  approach 
of  light  to  them,  the  spectrum  of  bright  objects  will  appear  luminous  sur- 
rounded by  a  dark  ground,  and  when  the  hand  is  again  removed,  the  contrary 
will  be  true.  Again,  if  the  eye  be  in  a  degree  fatigued  by  looking  at  the  set- 
ting sun,  or  even  at  a  window  with  a  bright  sky  beyond  it,  or  at  any  very 
bright  object,  on  then  shutting  it,  the  lately  contemplated  forms  will  be  per- 
ceived, first  of  one  vivid  colour,  and  then  of  another,  until  perhaps  all  the 
primary  colours  have  passed  in  review.  These  extraordinary  facts  prove 

23 


854  LIGHT. 

that  the  sensations  of  1'gtt  and  colour,  although  excitable  by  light  are  also 
producible  without  it.  UThis  truth  gave  occasion  to  Darwin's  ingenious 
theory,  that  th^  sensation  of  any  particular  colour,  of-  red,  for  instance,  is 
dependent  upon  a  certain  state  of  contraction  of  the  minute  fibres  of  the 
retina, — and  that  the  fibres,  when  fatigued  in  that  condition,  seek  relief  when 
at  liberty,  by  throwing  themselves  into  an  opposite  state, — as  a  man  whose 
back  is  fatigued  by  bending  forward,  relieves  himself  not  by  merely  standing 
erect,  but  by  bending  the  spine  backwards — which  new  condition  in  the  eye, 
whether  produced  by  light  or  by  any  other  cause,  gives  the  sensation  of 
green.  He  applied  his  explanation  similarly  to  all  other  cases  of  colour. 
It  is  remarkable  that  the  colours  which  thus  appear  opposite  to  each  other  in 
kind  are  those  which,  when  the  solar  spectrum  produced  by  a  prism,  as 
described  a  few  pages  back,  is  painted  round  a  wheel  or  circle,  are  opposite 
to  each  other  in  place. 

There  are  persons  who,  although  having  distinct  perceptions  of  form,  and 
of  light  and  shade,  have  not  the  power  of  distinguishing  colours.  It  is  com- 
mon for  such  persons  to  deem  pink  and  pea-green  (naturally  opposites)  the 
same  colour,  and  therefore,  not  to  distinguish  difference  of  colour  in  a  red 
berry  and  the  leaves  around  it.  A  man  with  this  defect  trusting  to  his  own 
judgment,  has,  without  knowing  it,  dressed  himself  like  a  parrot. 

'  The  mind  judges  of  external  objects  l>y  the  relative  size,  brightness,  colour, 
&c.,  of  the  minute  hut  perfect  images  or  pictures  of  them  formed  at  the 
back  of  the  eye  on  the  expansion  of  nerve  called  the  retina  ;  and  the  art  of 
painting  is  successful  in  proportion  as  it  produces  on  a  larger  scale  such 
a  picture,  which,  when  afterwards  held  before  the  eye  to  reproduce  itself  in 
miniature  upon  the  retina,  may  excite  the  same  impression  as  on  the  origi- 
nal object."  (Read  the  Analysis,  page  325.) 

"We  now  understand  how  an  admirable  miniature  resemblance  of  the  objects 
before  us  is  produced  upon  the  retina  of  the  eye,  by  the  light  from  them  re- 
fracted in  passing  through  the  different  parts  of  the  eye ;  but  after  all,  this  is 
only  a  picture,  and  the  inquiry  remains — which  many  persons  would  suppose 
so  simple  as  to  be  trifling,  but  which  is  in  reality  most  curious  and  important 
— how  we  are  thereby  enabled  to  judge  of  the  magnitudes,  distances,  and 
other  particulars  respecting  the  things  examined  ?  Here  it  will  be  found,  to 
the  surprise  of  persons  first  entering  upon  the  subject,  that  we  learn  the 
meaning  of  a  scene  or  pictorial  signs  only  gradually,  as  we  do  of  any  other 
system  of  signs,  and  that  a  person  whose  eyes,  although  perfect,  had  been 
kept  covered  from  infancy  up  to  maturity,  would  no  more  "  see,"  in  the 
complete  sense  of  the  word,  that  is,  understood,  any  sense  or  prospect  on 
which  he  first  opened  his  eyes,  so  as  to  have  a  perfect  picture  of  it  on  his 
retina,  than  a  child  understands  or  can  read  a  printed  page,  when  he  first 
looks  into  a  book.  Most  interesting  information  has  been  obtained  on  this 
subject,  by  observing  the  facts  where  blindness  from  birth  has,  by  surgical 
operation,  been  suddenly  cured  in  persons  arrived  at  maturity. 

If  a  man  were  placed  from  infancy  in  an  apartment  fitted  up  as  a  camera 
obscura,  and  had  no  means  of  becoming  acquainted  with  external  nature,  but 
by  watching  the  images  appearing  on  the  screen,  he  could  learn  scarcely 
any  thing  of  the  universe  around  him  ;  but  if  after  a  time  he  were  allowed  to 
walk  out,  and  to  examine  by  the  touch  and  by  the  measurement  the  different 
objects  whose  images  he  was  in  the  habit  of  viewing,  and  to  ascertain  what 
size,  shape  and  distance  of  an  object  corresponded  with  a  certain  magnitude 


THE    EYE — VISUAL    ANGLE.  355 

form,  position,  and  brightness  of  image,  the  transient  imagery  might  at  last 
be  to  him  a  very  clear  indication  of  the  real  particulars  :  making  him  in 
imagination  present  to  the  objects,  almost  as  if  he  went  and  examined  them 
with  his  hands.  In  the  same  manner,  in  a  degree,  the  mind  may  be  con- 
sidered as  stationed  in  or  about  the  little  camera  obscura  of  the  eye  whence 
it  cannot  itself  escape  to  examine  external  nature;  but  must  learn  the  mean- 
ing of  the  images  formed  on  the  retina,  by  commanding  the  services  of  the 
bodily  limbs  or  members,  and  the  other  organs  of  sense. — The  judging  of 
things  by  sight,  then,  is  merely  the  interpreting  one  set  of  signs,  as  judging 
by  sounds  or  language  is  interpreting  another,  and  judging  by  hieroglyphics 
or  any  written  character  is  interpreting  a  third.  The  common  visual  signs 
on  the  retina,  however,  are  of  all  signs  the  most  readily  learned  or  under- 
stood, from  having  certain  fixed  relations  in  form,  magnitude  and  position 
to  the  things  signified ;  while  words,  hieroglyphics,  and  written  characters, 
are  quite  arbitrary,  and  have  no  such  relations. 

Bodies  differ  and  are  distinguished  among  themselves  chiefly  by  their 
comparative  dimensions,  that  is,  their  form  and  magnitude,  or  shape  and 
size  :  and  to  ascertain  these  and  the  relative  distances  and  positions,  are  the 
great  objects  which  by  the  eye  the  mind  seeks  to  accomplish.  Now  it  effects 
its  ends  by  considering  collectively. 

1st.  The  tpace  and  place  occupied  by  objects  in  the  field  of  view,  mea- 
sured by  what  is  called  the  visual  angle. 

2d.    The  intensity  ofliyht,  shade,  and  colour. 

3d.    The  divergence  of  the  rays  of  light. 

4th.   The  convergence  of  the  axes  of  the  eyes. 

We  shall  treat  of  these  particulars  separately  in  the  order  now  stated. 

~Lst.   The  space  and  place  occupied  in  the  field  of  view,  measured  by  iae 

visual  angle. 

The  term  field  of  view  is  used  to  designate  that  open  or  visible  space  be- 
fore the  eyes,  in  which  objects  are  seen :  and  it  may  mean  either  the  small 
field  visible  in  one  position  of  the  eyes,  or  that  which  is  perceived  on  direct- 
Fig.  150. 


356  LIGHT. 

ing  them  all  round.  If  a  man  as  at  e,  were  surrounded  by  a  large  globe 
or  sphere  of  glass  as  a,  through  which  his  eye  at  the  centre  might  view  the 
several  objects  around  occupying  certain  situations  and  certain  proportions 
of  the  circumference,  and  if  the  sphere  Ijad  any  equal  divisions  or  degrees 
marked  upon  it  all  round,  he  would  be  able  at  once  to  say  exactly  what 
portion  of  his  sphere  or  field  of  view  was  shadowed  or  occupied  by  any 
single  object,  as  the  cross  here  shown  at  i,  and  thus  to  describe  very  intelli- 
gibly either  for  his  own  recollection,  or  to  inform  others,  its  relative  magni- 
tude and  situation  as  then  appearing  to  him, — just  as  he  might  say,  on 
looking  at  a  tree  in  the  garden  through  a  common  window  which  is  a  por- 
tion of  the  field  of  view  really  divided  by  the  cross  bars,  whether  he  saw  the 
whole  tree  through  one  pane  or  through  several,  and  through  which  pane  or 
panes  he  saw  it.  It  may  be  remarked  farther,  that  whether  the  supposed 
sphere  of  glass  were  large  or  small,  viz.,  were  as  b  or  c,  the  part  of  its  surface 
apparently  occupied  by  any  object  either  beyond  or  within  it,  would  bear  the 
same  proportion  to  the  whole  surface ; — if  a  d  were  a  tenth  of  the  small  circle 
or  globe,  c  g  would  be  a  tenth  of  a  larger.  Now  as  men  have  found  it  con- 
venient to  consider  a  circle  (and  every  circle)  as  divisible  into  360  degrees, 
(which  are  smaller,  therefore,  in  a  small  than  in  a  larger  circle,  although  in 
each  having  the  same  relation  to  the  whole,)  the  ready  mode  of  comparing 
the  apparent  magnitude  of  objects  is  to  say  how  many  of  these  degrees  of  the 
field  of  view  each  object  occupies  :  and  this  is  really  what  is  meant  by  tne 
apparent  size  of  an  object.  And  because  the  most  convenient  way  of  mea- 
suring a  portion  of  a  circle,  of  which  the  whole  is  not  seen,  is  to  measure  by 
a  fit  instrument  the  angle  or  corner  formed  at  its  centre  by  lines  drawn  from 
the  extremities  of  the  portion, — as  here  the  angle  at  e  formed  by  the  lines  c  e 
and  g  e,  the  object  is  said  either  to  occupy  a  certain  number  of  degrees  of  the 
circumference  of  the  circle,  or  to  subtend  an  angle  of  the  same  number  of 
degrees  at  its  centre,  and  this  angle  is  called  the  visual  angle,  the  subject  of 
our  present  disquisition. 

The  visual  angle,  then,  in  regard  to  any  object,  is  that  included  between 
the  lines  or  rays,  as  a  u  and  d  i}  which,  from  the  extreme  points  of  the  object, 

Fig.  151. 


as  a  d}  meet  and  cross  in  the  lens  of  the  eye,  and  go  afterwards  to  form  the 
extremes  of  the  image  on  the  retina,  and,  as  formerly  explained,  the  angle  is 
the  same  on  each  side  of  the  lens,  viz ,  towards  the  object  or  towards  the 
image. 

Now  if  all  bodies  were  at  the  same  distance  from  the  eye,  the  magnitude 
of  their  images  formed  on  the  retina,  or  in  other  words,  of  the  visual  angles 
subtended  by  them,  would  be  an  exact  measure  of  their  comparative  real 
magnitudes,  as  is  seen  in  i  u,  the  image  of  the  great  cross  a  d,  and  in  i  o  the 
image  of  the  small  cross  b  d :  but  it  is  evident  here,  that  the  cross  c  e,  which 


THE    EYE — APPARENT    SIZE    OF    OBJECTS.          357 

is  twice  as  large  as  b  d,  makes,  because  twice  as  far  off,  an  image  of  only  the 
same  size  as  b  d,  and  an  image  therefore  only  half  as  large  as  that  of  a  cross 
a  d  equal  in  size  with  itself  :  and  the  same  rule  of  proportion  holds  for  all 
other  comparative  distances — at  a  hundred  times  the  distance,  an  object 
appearing  only  the  hundreth  part  as  tall,  and  so  forth.  To  judge,  therefore 
by  the  eye,  of  the  true  size  of  an  object,  we  must  know  its  distance  as  well 
as  its  apparent  size  or  visual  angle. 

Many  familiar  facts  receive  their  explanation  from  the  law  of  the  visual 
angle  or  apparent  size  being  less  always  in  proportion  as  the  distance  of 
an  object  is  greater. 

A  man  (instead  of  the  cross  here  shown)  ate?,  standing  near  the  outside  of 
a  window,  as  b  c  (here  represented  edgeways)  may  to  the  eye  of  a  spectator 
within  the  window  at  h,  subtend  the  same  visual  angle,  or  appear  as  tall  as 
the  window,  the  light  from  the  man's  head  passing  through  the  top  of  the 
window,  and  that  from  his 

feet   passing  through  the  Fig.  152. 

bottom ;  but  if  the  man 
then  moves  away  from,  the 
window,  the  eye  of  the 
spectator  will  be  able  to  see 
his  whole  body  through  a 
s'rnaller  and  a  smaller  ex- 
tent of  the  window, — as 
through  half  its  height  or  a  c,  when  he  is  twice  as  distant,  or  at/,  and  through 
the  third  or  o  c,  when  he  shall  be  three  times  as  distant,  or  at  g,  and  so  forth, 
for  any  other  distance ;  so  that  soon  a  small  figure  of  a  man  cut  in  paper,  if 
applied  upon  the  glass,  would  exactly  cover  the  part  of  it  through  which  the 
light  from  him  entered  the  spectator's  eye,  and  would  then,  by  completely 
hiding  him  from  view,  be  an  exact  measure  of  his  apparent  size  :  and  at  last 
a  fly  passing  over  the  pane  might  equally  hide  him,  and  the  fly  then  would 
subtend  a  larger  visual  angle  than  he,  that  is  to  say,  would  be  forming  on  the 
retina  a  larger  image  than  the  man.  Thus  it  often  happens  in  reality,  that 
a  person  sitting  near  a  window,  and  intent  upon  some  object  of  study  or  of 
conversation,  mistakes  a  fly  on  the  glass  for  a  man  at  a  distance;  or,  on  the 
contrary,  a  man  for  a  fly.  It  is  ascertained  that  the  eye,  with  an  ordinary 
degree  of  light,  can  see  an  object  which  in  the  field  of  view  occupies  only  the 
sixtieth  of  a  degree  (or  one  minute.)  This  space  is  about  the  100th  of  any 
inch  in  a  circle  of  twelve  inches  in  diameter,  the  eye  being  supposed  in  the 
centre  of  the  circle.  Now  a  body  smaller  than  this  at  six  inches  from  the 
eye,  or  any  thing,  however  large,  placed  so  far  from  the  eye  as  to  occupy  in 
the  field  of  view  less  space  than  this,  is  invisible  to  ordinary  sight.  At  four 
miles  off,  a  man  becomes  thus  invisible.  A  pin-head  near  will  hide  a  house 
on  a  distant  hill — nay,  will  hide  even  the  planet  Jupiter,  although  1,000 
times  bigger  than  this  earth. 

In  accordance  with  the  principle  now  explained,  a  marine  telescope  has 
been  constructed,  in  which  the  field  of  view  is  divided  by  fine  cross  wires, 
or  otherwise,  so  that  the  person  using  it  can  say  at  once  how  much  of  its 
field  any  object  occupies.  When  ships  are  in  chase,  it  is  common,  by  this 
instrument,  or  some  other  which  will  detect  a  change  of  visual  angle,  or 
apparent  size,  to  view  the  fleeing  or  pursuing  ship ;  and  if  the  apparent 
size  be  observed  to  increase,  the  conclusion  follows  that  the  ships  are 


358  LIGHT. 

nearing  each  other ;  if,  on  the  contrary,  the  size  diminishes,  the  chased  ship 
is  escaping. 

By  applying  this  rule,  whenever  the  real  size  of  a  distant  object  is  known, 
the  distance  is  ascertainable,  and,  vice  versa,  where  the  distance  is  exactly 
known,  the  size  is  deterrainable  : — for  it  is  evident  that  if  a  body,  as  a  ship, 
known  to  be  100  feet  tall,  occupy  or  subtend  in  the  field  of  vision  the  360th 
part  of  a  whole  circle,  or  one  degree,  the  whole  circle  must  be  in  circum- 
ference 360  times  100  (hundred)  feet,  or  36,000;  and  the  diameter  of  any 
circle  being  nearly  one-third  of  its  circumference,  while,  in  the  case  Supposed, 
the  distance  of  the  ship  is  the  half-diameter,  we  learn  that  distance.  Again, 
if  we  know  the  distance  of  a  ship  or  other  object  to  be  a  mile,  and  if  we  then 
find  the  visual  angle  subtended  by  the  object  to  be  the  1,000th  part  of  a 
circle,  we  know  its  true  size  to  be  the  1,000th  part  of  a  circle,  of  which  the 
half-diameter  or  radius  is  one  mile.  It  is  by  applying  this  rule  in  a  manner 
to  be  afterwards  explained,  that  we  determine  the  size  of  the  heavenly 
bodies. 

We  now  perceive  that  if  the  rays  of  light  coming  to  the  eye  through  a 
plate  of  glass,  from  objects  seen  beyond  it,  could  leave  marks  in  the  glass, 
at  the  points  where  they  passed,  and  marks  capable  of  giving  out  the  same 
kind  of  light  as  the  objects,  there  would  be  formed  upon  the  glass  such  a 
representation  or  picture  of  the  objects  formed  or  viewed  through  it,  that 
•when  held  before  the  eye,  it  would  form  on  the  retina,  the  image  or  images 
the  same  in  almost  all  respects  as  the  objects  themselves;  for  from  the  dif- 
ferent points  of  the  glass,  light  could  dart  to  the  eye  of  the  same  kinds  and 
in  the  very  same  directions  as  that  originally  coming  from  the  objects. 
Now  the  art  of  painting  seeks  so  to  dispose  lights,  shades  and  colours  on 
any  plane  surface,  as  to  produce  the  sort  of  representation  of  objects  here 
contemplated,  while  the  picture-frame  has  to  recall  the  window-frame,  or 
edge  of  the  plate  of  glass  through  which  the  true  scene  is  supposed  to  be 
viewed.  It  is  admirable  how  perfectly  this  art  now  accomplishes  its  ends ; 
and  although  there  are  still  trifling  differences  between  the  effect  upon  the 
eye,  of  the  picture  and  of  the  realities — which  peculiarities  we  shall  consider 
presently,  and  how  they  may  be  combated  so  as  to  render  the  illusion 
almost  perfect, — it  is  not  one  of  them,  as  might  be  supposed  from  the  small 
extent  of  the  canvass,  that  the  picture  appears  to  the  retina  smaller  than  the 
objects  themselves.  Few  people,  before  studying  this  subject,  are  aware 
that  in  a  good  picture  the  size  of  the  figures  is  always  made  exactly  such, 
that  at  the  distance  from  the  eye  at  which  the  picture  is  meant  to  be 
viewed,  they  produce  on  the  retina  the  very  same  size  of  image  as  would 
be  produced  by  the  realities  seen  under  the  aspect  represented  in  the  picture. 
To  become  sensible  of  this,  let  a  person  look  through  a  window-pane,  with 
the  eye  at  the  distance  of  eight  inches  from  it,  and  let  him  trace  with  a 
sharp  point  upon  the  glass,  previously  coated  with  gum,  the  outline  of  the 
scene  beyond — perhaps  a  street  or  square, — he  will  find  that  the  outline  of 
a  man  seen  there  at  the  distance  of  twenty  paces,  and  appearing  perfectly  to 
coincide  with  the  boundaries  of  the  person,  so  that,  if  opaque,  it  would  just 
hide  the  person,  will  be  scarcely  half  an  inch  tall,  while  the  figure  of  the 
man  a  few  hundred  paces  off,  will  appear  so  small,  that  the  minuter  fractures 
could  not  be  distinguished,  even  if  they  could  be  drawn. 

Now  as  a  person  who  reads  the  description  of  an  elephant,  does  not 
deem  the  animal  larger  or  smaller  because  of  the  size  of  letter  used  in  the 
printing,  or  in  the  size  of  the  accompanying  engraved  representation;  and 
as  a  man  in  a  picture  gallery  viewing  minatures  and  larger  portraits,  does 


THE    EYE — APPARENT    SIZE    OF    OBJECTS.          359 

not  conceive  of  the  originals  according  to  the  size  of  the  representations  : 
and  as  a  man  viewing  a  well-executed  picture  of  a  Grecian  temple,  never 
dreams,  unless  his  attention  be  particularly  directed  to  the  fact,  that  upon 
the  canvass  the  distant  pillars  of  the  rows  are  much  smaller  than  the  near 
ones  ;  but  in  all  such  cases  the  mind  merely  uses  the  signs  to  help  it  to  con- 
ceive of  the  things  according  to  previous  knowledge,  or  to  other  principles 
of  judging  : — so  in  any  common  case  of  seeing,  the  mind  takes  little  account 
of  the  apparent  size  of  objects,  but  passes  instantly  from  the  types  to  the 
realities,  which  are,  generally,  more  or  less  known,  and  it  soon  ceases  to  be 
aware  that  the  apparent  size  of  the  same  object  ever  changes.  Few  persons, 
for  instance,  are  aware  that  when  two  friends  shake  hands,  each  appears 
to  the  mere  eye  of  the  other  ten  times  taller  than  when  he  has  walked  tea 
paces  away;  or  that  a  chair  at  one  end  of  a  room  appears  to  a  person  sitting 
at  the  other,  only  half  as  large  as  a  chair  in  the  middle  of  the  room,  but 
such  are  the  facts;  and  they  may  be  immediately  proved  by  holding  a 
common  eye-giass  or  ring  at  a  certain  distance  from  the  eye,  and  then 
looking  through  it  at  any  similar  objects  placed  at  different  distances  ;  then, 
while  of  a  chair  standing  near,  only  a  small  part  will  be  visible  through  the 
ring — of  a  distant  chair  the  whole  may  be  seen  ;  and  so  of  any  other  case. 
At  five  miles  distance,  the  fleets  which  met  on  the  great  day  of  Trafalgar 
might  have  been  seen  through  a  marriage-ring  as  the  picture-frame.  There 
are  occasions,  however,  where  the  usual  collateral  helps  to  the  immediate 
recognition  of  objects  being  wanting,  the  observer's  attention  is  strongly 
aroused  to  the  facts  of  their  diminutive  appearance  produced  by  distance ; 
for  instance,  when  a  man  on  a  long  sea-voyage  first  approaches  a  land  of 
which  the  features  are  in  a  degree  new  to  him  ;  as  when  an  Englishman 
arriving  in  India,  scarcely  believes  that  the  little  specks  which  he  sees  scat- 
tered along  the  shore  are  commodious  dwellings,  or  that  what  seem  to  him 
only  luxuriant  herbs  or  bushes,  are  magnificent  palm-trees. 

For  the  same  reason  that  a  distant  body  to  the  mere  eye  appears  diminutive, 
namely,  the  smallness  of  the  visual  angle  subtended  by  it,  so  does  a  distant 
motion  to  the  mere  eye  appear  slow.  A  carriage  dashing  past  a  pedestrian 
in  the  street,  may  surprise  him  by  its  speed;  but  if  viewed  at  the  same  time 
by  a  spectator  at  the  top  of  a  lofty  tower  near,  it  seems  to  be  but  crawling 
along  the  pavement.  A  ship  driven  before  a  tempest,  seems  to  a  sailor  on 
board  almost  to  fly  through  the  white  foam  which  surrounds-tar ;  but  if  then 
observed  by  a  spectator  on  shore,  as  an  object  on  the  distant  horizon,  she  is 
scarcely  perceived  to  change  her  place.  A  balloon  high  in  the  air,  and  borne 
aiong  on  the  wings  of  the  wind  at  the  rate  of  seventy  or  eighty  miles  an  hour, 
may  still  for  a  time  leave  a  spectator  on  earth  doubtful  as  to  whether  it  be  in 
motion,  or  in  what  direction'it  moves.  The  moon  in  her  orbit  wheels  round 
the  earth  at  the  astonishing  rate  of  about  2,000  miles  an  hour,  yet,  owing 
to  her  distance  from  it,  her  motion  is  not  visible  to  the  naked"  eye  of  the 
inhabitants  of  the  earth,  except  by  comparing  her  place  at  considerable 
intervals.  In  respect  to  bodies  still  more  distant  than  the  moon,  the  truth 
at  present  under  consideration  is  still  more  striking. 

Having  now  explained  how  the  apparent  transverse  measures  or  breadth  of 
bodies  and  of  space,  in  other  words,  the  visual  angle  subtended  by  them, 
is  affected  by  their  distance  from  the  eye,  we  proceed  to  show  how  it  is 
affected  also  by  their  po&ition. 

A  globe  at  a  certain  distance  from  the  eye,  however  turned,  preserves  the 
same  appearance  in  the  field  of  view,  and  its  outline  traced  upon  a  plate  of 


360  LIGHT. 

glass  held  across  between  it  and  the  eye,  is,  like  its  direct  shadow  upon  a 
wall,  always  a  circle ;  but  an  egg  which,  held  in  one  position,  produces  a 
circular  outline  or  image,  when  held  in  another,  produces  an  image  nearly 
oval.  A  wheel  when  viewed  sideways  appears  a  perfect  circle,  when  viewed 
edgeways  it  appears  a  broad  straight  band  or  line,  and  in  any  intermediate 
position  it  appears  oval.  The  apparent  form,  then,  of  a  body,  is  only  a  hint 
to  the  mind  from  which,  by  former  experience  or  instruction,  it  guesses  at 
the  true  form.  If  a  man  had  never  seen  an  egg  but  endways,  he  never 
could  have  known  that  it  was  not  a  sphere. 

If  any  long,  straight  object,  as  a  beam,  be  placed  with  one  of  its  ends 
directly  to  the  eye,  that  end  only  can  be  seen,  and  according  to  the  case, 
must  appear  a  square  or  circle  of  the  diameter  of  the  beam  ;  if  it  then  be 
placed  with  its  side  directly  to  the  eye,  its  whole  length  will  be  seen  •  and  if 
placed  in  any  intermediate  position,  it  will  appear  more  or  less  shortened  '} 
— in  all  cases,  its  outline  on  the  retina  being  similar  to  that  of  its  shadow 
on  a  wall  directly  behind  the  person.  A  man  has  advanced  on  a  spear 
pointed  directly  to  his  eye  without  seeing,  or  on  the  end  of  a  bar  of  iron 
carried  on  the  shoulder  of  a  porter  in  the  street.  A  common  telescope  held 
with  its  end  to  the  eye,  appears  a  perfect  circle,  if  then  inclined  a  little,  it 
seems  to  jut  out  on  one  side,  and  as  the  inclination  is  increased,  it  juts  out 
more  and  more,  until  it  displays  its  whole  length.  A  great  ship  of  war 
whose  stern  is  towards  a  spectator,  appears  a  rounded  building  with  its 
rows  of  windows  like  those  of  a  peaceful  habitation  j  but  as  it  turns,  it 
gradually  reveals  the  long  batteries  of  bristling  cannon.  A  straight  row  of 
a  thousand  similar  objects,  as  of  soldiers  in  rank,  pillars,  trees,  &<?.,  may 
appear  to  a  person  at  the  extremity  as  only  one  object  of  the  kind,  the 
nearest  individual  completely  hiding  all  the  others ;  but  if  viewed  from  the 
side  and  at  a  certain  distance,  the  individuals  may  be  counted. 

The  appearance  now  treated  of  is  called  foreshortening,  and  is  to  be  noted 
wherever  surfaces  or  lines  are  not  placed  so  as  directly  to  face  the  spectator. 
Perhaps  the  most  important  case  of  foreshortening  is  when  the  eye  looks 
more  or  less  obliquely  along  an  extensive  plane  surface,  the  general  surface 
of  the  earth,  for  instance,  or  of  the  sea,  by  estimating  aright  the  foreshortening 
of  which,  we  judge  of  the  distance  or  situation  of  the  objects  placed  upon  it. 
It  will  be  readily  perceived,  that  in  all  such  cases  the  more  distant  portions 
of  the  surface  are  progressively  more  foreshortened  than  the  nearer; — for  a 
man  standing  at  a  on  the  plane  as  a  b,  and  with  his  eye  at  c,  looking  down 

before  him,  seesapor  i^n  of 

Fig.   153.  the  surface  a  <7  almost  direct- 

ly, or  with  a  little  foreshort- 
ening, and  an  extent,  as  a  d, 
equal  to  the  height  of  the 
eye,  will  subtend  in  his  eye, 
an  angle  of  45°,  or  half  a 
right  angle,  viz.,  the  angle 
a  c  d,  and  therefore  rather 
more  than  half  of  all  that  can 
be  subtended  by  a  straight  line  or  space  from  his  feet  to  the  horizon,  how- 
ever distant ;  the  next  equal  spaces,  viz.,  d  f,  will  subtend  an  angle  of  only 
18°,  viz.,  d  c.f,  the  next  of  8°,  viz.,  f  eg,  and  so  on;  #nd  as  he  carries  his 
view  more  and  more  forward,  the  surface  becomes  to  it  more  and  more  oblique, 
until  at  last  the  light  coming  seems  more  to  skim  along  the  level  than  to  rise. 
This  explains  why  a  person  having  a  side  view  of  a  row  of  separate  objects, 


THE    EYE — FORESHORTENING.  861 

as  of  men  in  line,  trees,  pillars,  &c.,  may  see  through  or  between  the  nearest 
of  them,  but  towards  the  extremes  sees  them  as  if  standing  in  closest  possible 
array,  or  as  if  forming  a  continued  surface.  The  same  remark  explains  why 
masses  of  cloud  scattered  uniformly  over  the  sky,  may  allow  a  spectator  to 
see  wide  intervals  of  the  blue  heaven  over  head,  while  all  around  there  is  a 
dense  cloudy  mass  appearing  to  rest  on  the  horizon. 

If  a  man  standing  on  a  hill  look  down  upon  a  field  or  plain  which  is  well 
known  to  him,  and  if  he  see  some  objects  near  its  side,  and  some  near  its 
middle,  and  some  near  its  distant  border,  he  knows  at  once  how  far  they  are 
from  him  and  from  one  another.  Similarly,  when  viewing  the  ocean  from  a 
lofty  cliff,  and  seeing  ships  scattered  over  its  face,  he  may  judge  correctly  of 
their  distance,  for  he  can  see  only  a  certain  extent  of  ocean  which  becomes 
to  him  as  a  known  field.  The  man  stationed  at  the  flag-staff  on  the  High 
Knowl  of  St.  Helena,  looks  down  upon  the  circular  field  of  the  Atlantic  a 
hundred  miles  broad,  and  can  tell  the  distance  of  a  sail  in  sight  to  within 
a  mile  or  two.  Now,  although  the  ground-plan  of  an  extensive  landscape 
may  not  be  so  level  as  the  face  of  the  ocean,  there  is  still  an  approximation, 
which  very  considerably  assists  a  spectator's  judgment  of  distances. 

Painters  are  not  only  careful  to  foreshorten,  according  to  the  proportion 
explained  above  all,  the  objects  which  they  portray,  but  they  often  avail 
themselves  of  the  principles  to  produce  most  striking  effects.  For  instance, 
Martin,  in  many  of  his  beautiful  designs,  by  judicious  foreshortening,  has 
exhibited  miles  in  extent  of  gorgeous  architecture  and  of  armed  men,  on  a 
space  of  canvass  that  would  seem  scarcely  more  than  sufficient  to  receive  a 
very  few  figures ;  he  has  made  a  single  magnificent  pillar  or  accoutred  warrior 
in  the  foreground,  become  the  type  which  first  fills  the  mind  with  admiration, 
and  then  sends  it  along  the  retiring  lines  of  beautiful  perspective,  where 
every  tip  or  edge  renews  the  first  impression.  A  man  lying  on  a  high  table 
or  bed,  with  his1  feet  towards  the  spectator,  is  foreshortened  into  a  roundish 
heap,  of  which  the  soles  of  the  feet  hide  the  greater  part.  This  is  the  de- 
scription of  ths  painting  which  has  been  called  the  "  Miraculous  Entomb- 
ment," and  it  is  because  an  unreflecting  spectator  moving  sideways  with  the 
expectation  of  seeing  more  of  the  body,  still  sees  only  the  soles  of  the  feet, 
and  may  suppose  the  body  turned  round  so  as  to  keep  the  feet  towards  him, 
that  the  painting  has  received  its  appellation.  For  nearly  the  same  reason, 
the  eye  of  a  common  full-faced  portrait  may  seem  to  follow  a  spectator  to 
whatever  part  of  the  room  he  goes, — for  by  moving  to  one  side  he  cannot 
see  the  side  of  the  eye-balls.  It  is  related  of  a  murderer,  that  he  was  im- 
pelled to  commit  suicide  by  observing  that  the  eyes  of  the  portrait  of  his 
victim  were  always  fixed  upon  him.  A  rifleman  portrayed  as  if  taking  aim 
directly  in  front  of  the  picture,  will  appear  to  every  spectator  in  the  room 
to  be  pointing  at  him  especially.  To  terrify  young  ladies,  a  little  arch  Cupid 
has  been  similarly  represented  with  his  arrow  pointed  directly  at  them,  and 
just  ready  to  let  it  slip  from  his  bended  bow. 

As  the  painter,  availing  himself  of  a  knowledge  of  the  principles  now  ex- 
plained, by  which  the  eye  usually  judges  of  size  and  distance,  may  produce 
on  his  canvass  the  most  charming  illusions,  so  may  the  tasteful  landlord,  in 
his  ornamental  gardens  and  pleasure-grounds,  by  working  his  levels  into 
artificial  undulation  of  hill  and  dale,  and  clothing  these  with  tree  and  edifice 
of  magnitudes  to  correspond — make  the  eye  of  a  spectator  luxuriate  in  the 
contemplation  of  the  supposed  extensive  plains,  lofty  mountains,  widespread 
lakes,  and  distant  padogas — all  within  the  narrow  space  of  an  acre  or  two; 
thus,  by  other  means,  producing  on  the  retina  the  same  impressions  as  Claude 
Poussin,  or  Wilson  has  done  by  the  finest  pictures. 


362 


LIGHT. 


When  any  objects  or  mass  of  objects  is  foreshortened,  by  one  part  being 
farther  from  the  eye  than  another,  that  part  appears  also  in  a  proportion 
smaller  than  the  other.  For  example,  in  a  straight  row  of  similar  houses, 
pillars,  trees,  &c.,  (see  the  next  cut,)  those  nearest  to  the  eye  will,  on  a  glass 
held  before  the  eye  to  receive  their  images,  from  the  largest  images,  and  there 
will  be  a  gradual  diminution  from  the  largest  to  the  least,  so  that  lines  drawn 
upon  the  glass  along  the  tops  and  bottoms  of  the  images  would  tend  to  a 
point,  called  for  a  reason  to  be  explained  below,  the  va?rishwg  point.  Thus 
a  person  looking  from  a  window  along  a  straight  street,  must,  to  see  the 
chimney  of  the  nearest  house,  look  through  the  top  of  the  window,  and  to  see 
the  street  door  must  look  through  the  bottom  ;  but  the  most  distant  house, 
both  top  and  bottom,  may  be  concealed  from  view  by  a  little  spot  upon  the 
glass  at  the  height  of  the  eye.  This  remarkable  tapering  of  foreshortened 
objects  may  of  course  be  strikingly  observed  on  looking  at  any  correctly, 
made  drawing  or  engraving  meant  to  represent  a  retiring  row  of  similar  ob- 
jects;— such  drawing  being  in  truth  an  attempt  to  realize  by  art,  on  the 
surface  of  a  sheet  of  paper,  the  appearance  of  the  objects  as  seen  through  a 
window  or  aperture  the  size  of  the  paper. 

The  art  which  gives  rules  for  tracing  objects  on  a  plane  surface,  as  they 
would  appear  to  an  eye  looking  at  them  through  that  surface,  if  transparent, 
with  their  various  degrees ;  first,  of  apparent  diminution  on  account  of  dis- 
tance ;  and,  secondly,  of  foreshortening  on  account  of  the  obliquity  of  view, 
is  called,  from  the  Latin  word  signifying  to  look  through  the  art  of  perspec- 
tive. It  regards  entirely  the  two  particulars  now  mentioned ;  and  notwith- 
standing the  terror  with  which,  in  the  imaginations  of  many  young  painters, 
the  study  of  it  is  clothed,  by  reason  of  the  mathematical  difficulties  with  which 
it  has  usually  been  mixed  up,  it  is  in  itself  exceedingly  simple.  We  hope 
that  a  person  capable  of  ordinary  attention,  will,  after  what  we  have  already 
said,  and  after  the  few  additional  remarks  which  we  have  still  to  make  on  the 
appearances  of  nature,  be  able  readily  to  understand  the  great  rules  of  per- 
spective. Although,  without  a  knowledge  of  these  rules,  a  quick  eye  soon 
enables  its  possessor  to  sketch  from  nature  with  much  truth ;  and  although 
the  two  instruments,  the  camera  obscura,  already  described,  and  camera  lu- 
cida,  to  be  described  in  a  future  page,  give  almost  mathematical  accuracy  to 
drawings  from  nature,  without  requiring  other  skill  in  the  draughtsman  than 

to  trace  and  make  permanent, 

Fig.  154.  with  ink  or  pencil,  the  lines  of 

light  which  he  sees  on  the 
paper ;  still  the  subject  is  so 
interesting  to  all  who  attempt 
to  sketch,  and  indeed  to  all 
who  wish  to  look  intelligently 
either  at  nature  or  at  the 
works  of  art,  that  none  who 
have  the  opportunity  of  study- 
ing it  should  neglect  it. 

Supposing  a  straight  row  of 
similar  objects,  as  of  the  stone 
blocks  or  pillars  represented 
here  from  a  or  b  to  S,  to  be 
viewed  by  a  person  stationed 

near  the  side  and  end  of  the  row,  as  over  the  point  O,  then,  because,  as 
already  explained,  objects  to  the  eye  appear  smaller  in  exact  proportion  to 


THE    EYE — PROSPECTIVE.  363 

their  increased  distance  from  it,  the  second  block,  if  twice  as  far  off  as  the 
first,  would  appear  only  half  as  large  ;  the  third  if  three  times  as  far,  would 
be  only  one-third  as  large,  and  so  on  to  any  extent  and  for  any  other  pro- 
portions ;  and  if  the  1,000th  of  any  other  block,  owing  to  its  distance,  sub- 
tended to  the  eye  an  angle  less  than  the  sixtieth  of  a  degree  of  the  field  of 
view,  it  would  be  altogether  invisible  (as  explained  at  page  333,)  even  if 
nothing  intervened  between  it  and  the  eye.  Then,  where  the  row  ceased  to 
be  visible  from  the  minuteness  of  the  parts,  or  from  the  fact  of  the  nearer 
objects  concealing  the  more  remote,  it  might  be  said  to  have  reached  its 
vanishing  point.  When  a  student  of  perspective  has  learned  all  that  regards 
the  vanishing  point  in  relation  to  a  line,  and  the  corresponding  vanishing 
line,  in  relation  to  a  surface,  he  has  learned  half  of  his  art.  The  above  cut 
considered  as  the  representation  of  a  street  running  directly  south  to  S, 
sketched  from  a  window  looking  along  its  centre,  will  serve  as  a  useful 
illustration. 

It  is  important,  first,  to  remark,  that  in  any  case  of  a  straight  line,  or^, 
row  of  objects  thus  vanishing  from  sight,  as  here  the  line  or  row  a  $,  in 
whatever  direction  it  points,  whether  east,  west,  north,  or  south,  &c.,  in  that 
direction,  too,  will  its  remote  or  vanishing  extremity  appear  to  be  from  the 
eye.  In  this  sketch  the  row  a  S  is  supposed  to  run  directly  south ;  and 
although  the  eye  to  see  the  near  end  of  it,  would  have  to  look  towards  the 
left  hand,  or  in  a  degree  east,  still  every  successive  pillar  would  be  more  and 
more  nearly  south,  and  the  point  in  the  heavens,  or  in  the  picture,  or  in  a 
transparent  plane  before  the  eye,  where  the  line  would  vanish,  would  be  so 
nearly  south  from  the  eye,  and  not  to  the  east,  because  the  pillars  happened 
to  the  east  of  the  individual,  that  no  ordinary  measure  would  detect  the 
little  want  of  correspondence ;  then  similarly,  if  there  were  more  rows  of 
objects,  as  of  pillars,  houses,  trees,  &"c.,  parallel  to  the  first,  but  considerably 
apart  from  each,  as  the  lines  here  a  S,  b  S,  d  S,  &c.,  still  all  would  vanish 
or  seem  to  terminate  in  the  very  same  point  of  the  field  of  view.  The  rea- 
son of  this  is  easily  understood.  Let  us  suppose  a  line  drawn  directly  south 
from  the  eye  to  the  point  S,  between  the*  parallel  lines  of  pillars,  houses, 
and  trees,  a  S,  b  S,  d  S,  &c.,  also  pointing  directly  south ;  and  let  us  sup- 
pose the  two  rows  of  pillars  to  be  100  feet  apart,  then  evidently  for  the 
same  reason  as  the  space  between  the  top  and  bottom  of  the  pillars,  that  is 
to  say,  their  height  becomes  apparently  less  and  less  as  their  distance  from 
the  eye  increases,  so  will  the  space  between  each  pillar  and  its  opposite,  or 
between  it  and  the  point  corresponding  to  it  in  the  visual  ray  along  which 
the  eye  looks,  become  apparently  less,  and  therefore  the  lines  *of  pillars 
really  100  feet  apart  from  each  other,  and  50  feet  from  the  visual  ray,  will, 
at  a  certain  distance  from  the  eye,  (viz.,  where  a  space  of  50  or  100  feet  is 
apparently  reduced  to  a  point,)  appear  to  join,  and  the  three  lines  will  ap- 
pear to  meet  in  that  point,  beyond  which  none  of  them  can  be  visible,  and 
which  is  therefore  the  vanishing  point  of  all.  The  conception  of  this  truth 
may  be  facilitated  by  supposing  a  planet  to  be  visible  to  the  exact  point  of 
the  heavens  at  the  moment  of  observation ;  then,  if  the  three  parallel  lines 
were  continued  on  to  the  planet,  and  were  visible  all  the  way,  they  would 
arrive  there  with  the  interval  between  them  just  as  when  they  left  the 
earth';  but  as  a  planet,  although  thousands  of  miles  in  diameter,  owing  to  its 
distance  from  the  earth,  appears  on  earth  only  as  a  point,  much  more  would 
two  lines  only  100  feet  apart  be  there  undistinguishable  in  place  by  human 
sight.  And  what  is  true  of  space  of  100  feet  between  parallel  lines,  is 
equally  true  of  a  mile  or  of  thousands  of  miles.  As  a  general  ru!e,  there- 


364 


LIGHT. 


Fig.  155. 


fore,  it  holds,  that  all  lines  in  nature  parallel  to  each  other,  when  represented 
in  perspective,  teed  towards  an  end  in  the  same  vanishing  point;  and  that 
point  is  the  situation  where  the  line  terminates,  along  which  the  eye  looks 
when  directed  parallel  to  any  one  of  the  real  lines.  And  this  is  true  only  of 
lines  all  in  the  same  level  or  horizontal  plane,  viz.,  such  as  might  lie  along 
the  surface  of  the  sea,  but  also  of  lines  placed  one  above  another,  as  those 
running  along  the  tops  and  bottoms  of  the  pillars  here,  or  along  the  walls, 
roofs,  and  windows  of  the  houses,  or  along  the  roots  and  summits  of  the 
trees,  and  indeed  of  all  lines  in  whatever  situation,  provided  they  are  paral- 
lel to  the  visual  ray,  and  therefore  to  one  another.  And  the  truth  holds 
equally  with  respect  to  lines  which  do  not  vanish  at  the  "  point  of  sight,"  or 
centre  of  the  picture,  as  with  respect  to  those  which  do.  When  it  is  ascer- 
tained, therefore,  that  a  line  or  boundary  of  any  natural  or  artificial  object 
has  a  certain  inclination  to  the  axis  of  the  picture,  or  to  what  we  have  de- 
scribed as  the  principal  visual  ray,  then  also  is  it  known  that  all  parallels 
t$)  that  line  have  their  vanishing  point  in  the  same  spot  of  the  field  of  view, 
and  a  line  supposed  to  be  drawn  from  the  eye  to  the  heavens,  or  really 
drawn  from  the  eye  to  the  picture  in  that  direction,  marks  upon  the  picture 
the  true  vanishing  point. 

It  will  be  understood  why,  in  a  long  arched  tunnel  or  a  cathedral,  with 
many  longitudinal  lines  on  its  floor,  walls,  roof,  &c.,  all  such  lines  seen  by 
an  eye  looking  along  from  one  end,  appear  to  converge  to  a  point  at  the 
other,  like  the  radii  of  a  spider's  web;  and  why  similarly  in  the  represen- 
tation of  a  common  room,  viewed  from  one  end,  all  the  lines  of  the  corners, 

tops  and  bottoms  of  windows, 
floor,  stripes,  on  a  carpet,  cor- 
ners of  tables,  &c.,  being  paral- 
lel to  each  other,  tend  to  the 
same  vanishing  point  as  V. 
The  appearance  of  the  lines  in 
the  floor  of  this  room  may  re- 
call that  of  the  furrows  in  a 
ploughed  field  as  seen  from 
one  end,  when  they  appear 
like  the  ribs  of  a  fan  spread 
out  towards  the  spectator. 
The  same  considerations  will 
explain  the  phenomenon  often 
to  be  observed,  of  two  little 
clouds  seen  near  each  other, 
and  almost  motionless  for  a 

time  in  the  distant  sky,  but  which  on  approaching  the  spectator  with  the 
wind,  appear  gradually  to  separate,  and  in  a  corresponding  degree  to  enlarge, 
until  one  of  them  sweeps  past  considerably  to  the  right  hand,  and  the  other 
considerably  to  the  left;  after  this,  they  lessen  and  approximate  as  they  be- 
fore enlarged  and  separated,  and  at  last  beyond  the  spectator,  appear  as 
small  and  as  near  as  when  first  observed.  Clouds  being  so  mutable  and  un- 
certain in  their  forms,  persons  have  been  led  to  deem  all  apparent  changes 
in  them,  of  form,  size  and  place,  to  be  real  changes,  and  not,  as  they  gene- 
rally are,  mere  optical  or  perspective  illusion. 

By  far  the  most  important  vanishing  point  in  common  scenes  is  the  middle 
of  the  horizon  or  level  line,  and  in  a  picture  properly  placed,  it  is  at  the  exact 
height  of  the  eye.  It  is  marked  S  in  the  figure  before  the  last,  and  V  in  the 


THE     E  Y  E — P  ERSPECTIVE.  365 

last  figure.  Because  in  houses,  the  roofs,  foundations,  floors,  windows,  &c., 
are  all  horizontal,  the  vanishing  points  of  their  lines  must  be  somewhere  in 
the  horizon,  and  if  the  spectator  be  in  the  middle  of  a  street  or  of  a  building, 
and  be  looking  in  the  direction  of  its  walls,  their  vanishing  point  will  be  in 
the  centre  of  the  scene  or  picture ;  if  he  be  elsewhere,  it  will  be  to  one  side. 
In  holding  up  a  picture-frame,  through  which  to  view  a  scene  suitable  for  a 
picture,  it  would  be  found  most  generally  befitting  to  raise  it  until  the  line  of 
the  horizon  appeared  to  cross  it  at  about  one-third  from  the  bottom  : — this 
fact  becomes  the  reason  of  the  rule  in  painting,  so  to  place  the  horizontal  line. 
In  beginning  a  picture,  this  line  is  usually  the  first  line  drawn  on  the  canvas, 
marking  the  place  of  the  vanishing  points  of  all  level  line  and  surfaces.  And 
the  eye  of  the  spectator  is  supposed  to  be  placed  before  the  middle  of  it  and 
generally  about  as  far  from  the  picture  as  the  picture  is  itself  long,  such  being 
the  extent  of  view  which  the  eye  at  one  time  most  conviently  commands. 

Understanding  now  that  the  apparent  or  perspective  direction  of  all  lines 
in  a  scene  is  towards  their  vanishing  points  as  above  discovered,  and  the  rule 
having  been  given  for  determining  these  points  in  a  drawing,  we  proceed 
to  show  how  much  of  a  line  drawn  to  any  vanishing  point  belongs  to  the 
known  magnitude  of  any  object  through  which  it  passes ;  in  other  words, 
how  much  an  object  is  in  perspective  foreshortened  in  consequence  of  its 
distance  and  obliquity  of  position. 

If  we  suppose  A  S  P  to  represent  a  plate  of  glass  seen  edgeways,  and  that 
towards  the  point  S  in  it,  an 

eye  is    looking   from    the  Fig.   156. 

point  D,  evidently   then,  a  ij\ 

line  from  P  continued  in  the 
direction  of  R  and  beyond 
until  vanished  from  sight, 


would  have  as  its  perspec- 
tive image  or  representation 

on  the  glass  a  line  reaching — =-p  ^""T^"""?  ^^^3^1? 

from   P   to   S, — S    being, 

moreover,  the  point  ofsiyht  here,  and  the  pictorial  vanishing  point  of  the 
line.  Now  to  divide  the  representative  line  P  S  so  as  to  correspond  with 
any  given  portions  of  the  original  line  P  R,  &c.,  it  would  only  be  necessary 
to  draw  other  lines  from  the  place  of  the  eye  I)  to  cut  or  touch  the  original  line 
in  the  situations  desired,  and  these  lines  would  cut  the  perspective  line  S  P 
as  required  :  for  instance,  the  portion  of  true  line  a  b  would  be  represented 
by  that  portion  of  the  image-line  S  P  included  between  the  two  lines  a  D, 
and  b  D,  and  so  of  any  other  portions.  There  are  figures  drawn  on  many 
mathematical  scales  by  which  such  problems  as  this  can  be  at  once  opproxi- 
matively  solved ;  and  it  would  be  possible,  by  trigonometrical  calculation,  to 
solve  them  exactly  in  all  cases  :  but  the  most  generally  convenient  mode  in 
practice  is  to  sketch  on  the  intended  drawing  (as  that  of  which  the  bounda- 
ries are  given  in  the  next  eut)the  kind  of  measure  shown  above,  by  setting 
off  from  the  point  of  sight  S,  a  distance  on  the  horizontal  line,  as  at  I),  equal 
to  the  distance  of  the  eye  from  the  picture,  and  then  by  oblique  lines  from  D 
drawn  upon  the  baseline  P  R,  to  cut  the  perpendicular  line  P  S  in  the  situa- 
tions desired — as  is  seen  in  the  last  figure,  which  differs  from  fig.  161 
chiefly  in  having  the  point  of  distance  marked  be/ore  its  point  of  sight,  in- 
stead of,  as  here,  laterally.  And  the  line  P  S  being  always  cut  by  the  oblique 
line  from  D  in  proportion  to  the  length  of  base-line  between  P  and  the  ex- 
tremity of  the  oblique  line,  a  horizontal  line  drawn  through  any  point  in  if, 


866 


LIGHT 


cuts  in  corresponding  proportions  all  the  other  lines  which  have  their  vanishing 
points  in  the  horizontal  line  at  S,  for  instance  a  S,  b  S,  &c.  Thus,  to  draw 
in  perspective,  on  the  surface  above  represented  and  prepared,  a  chess-board 
or  board  of  squares,  it  is  necessary  to  set  off  the  breadth  of  the  board  on  the 


Fig.  157. 


I) 


base-line  to  the  right  and  left  of  P,  viz.,  at  b  and  a,  and  then  to  draw  to  the 
point  of  sight  as  a  vanishing  point,  the  lines  a  S  and  b  S,  part  of  which  lines 
will,  therefore,  represent  the  sides  of  the  board,  and  then  to  draw  the  diago- 
nal b  I),  which,  for  the  reason  above  stated,  will  cut  the  lines  P  S  and  a  S  in 
proportion  to  the  length  of  base-line  to  the  right  of  their  extremities ;  a  efb, 
therefore,  is  a  square  seen  in  perspective,  and  any  number  of  smaller  in- 
cluded squares  are  made  by  drawing  lines  from  the  vanishing  point  to  equal 
divisions  on  the  base,  and  making  cross  lines  where  the  diagonal  cuts  these. 
Much  of  the  delight  which  the  art  of  painting  is  calculated  to  afford  is  lost 
to  the  world,  because  persons  in  general  know  not  how  to  look  at  a  picture. 
Unless  a  spectator  places  himself  where  he  can  see  the  objects  in  true  per- 
spective, so  that  he  may  fancy  himself  looking  at  them  through  a  window  or 
opening,  every  thing  must  appear  to  him  false  and  distorted.  The  eye 
should  be  opposite  the  point  of  sight  of  the  picture,  and,  therefore,  on  a  level 
with  the  line  of  the  horizon,  and  it  should  be  at  the  required  distance,  which 
is  generally  at  least  as  great  as  the  length  of  the  picture.  But  blame  not 
unfrequently  rests  also  with  the  artist,  from  his  having  neglected  the  study  of 
perspective.  It  is  very  common,  for  instance,  to  see  miniature  resemblances 
of  architectural  structures  so  foreshortened  and  tapered,  that  the  eye,  to  see 
them  in  true  perspective,  would  require  to  be  within  an  inch  of  the  paper ; 
whence  at  the  usual  distance  of  ten  or  twelve  inches  they  are  seen  as  hideous 
distortions.  The  specimens,  in  the  few  preceding  pages,  necessarily  exem- 
plify in  a  degree  this  error,  because  the  point  of  distance  had  to  be  marked 
where  there  was  but  a  small  page.  These  figures,  therefore,  by  any  person 
studying  the  subject  particularly,  should  be  drawn  on  a  scale  so  much  larger 
as  to  allow  the  eye  really  to  view  them  at  the  distance  supposed. 

A  means  of  judging  of  the  dimensions  of  the  bodies  by  the  visual  angle,  but 
which  depends  neither  on  the  absolute  size  of  the  image,  nor  on  the  fore- 
shortening of  the  ground  plane  on  which  the  body  stands,  is  to  use  known 
objects  in  view  as  measures  for  others  near  them  which  are  unknown. 

If  any  person  of  our  acquaintance  be  standing  at  some  distance  from  us 
near  another  person  who  is  a  stranger,  we  know  how  tall  the  stranger  is  by 
taking  the  acquaintance  as  a  measure. 

In  pictorial  representations  of  objects  little  familiar  as  to  many  people  are 
the  Egyptian  pyramids,  the  bodies  of  the  whale,  the  elephant,  the  camel,  &c., 
human  beings  may  be  represented  around  them  to  serve  as  measures  for  the 


THE    EYE — PERSPECTIVE.  367 

less  known  object.  The  Colossus  of  Rhodes  seen  from  afar,  might  to  a 
stranger  have  appeared  but  an  ordinary  statue  of  a  man,  but  the  exact  mag- 
nitude would  have  been  known  as  soon  as  a  ship  of  known  dimensions  were 
seen  sailing  into  port  between  his  gigantic  limbs. 

When  an  unpracticed  eye  is  first  directed  from  a  distance  to  a  great  ship  of 
war,  it  will  on  many  accounts  dwell  upon  the  object  with  wonder  and  admi- 
ration ;  but  it  may  not  judge  truly  of  the  enormous  magnitude  until  it  be 
near  enough  to  perceive  the  sailors  climbing  on  the  rigging,  and  appearing 
there,  by  comparison,  as  flies  or  little  birds  appear  among  the  branches  of  a 
majestic  tree. 

By  having  a  measure  of  this  kind  presented  to  us,  the  magnitude  and  eleva- 
tion of  great  edifices  are  rendered  more  obvious.  The  magnificient  pile  of 
St.  Paul's  in  London  becomes  still  more  striking,  when  we  discover  visitors 
looking  from  the  balconies  near  the  summit-cross.  They  appear  so  minute 
among  the  surrounding  huge  masses  that  a  person  is  for  a  while  disposed  to 
doubt  whether  they  be  men  :  but  the  fact  once  ascertained,  the  grandeur  of 
the  temple  is  most  impressive. 

Manny  persons  cannot  distinguish  between  the  little  pilot  balloon  (some- 
times dispatched  before  the  great  one  to  show  the  direction  of  the  wind)  and 
the  great  balloon  itself,  until  with  the  last  they  perceive  the  aeronauts  as 
little  black  points  suspended  under  the  globular  clouds. 

Strangers  visiting  Switzerland,  on  first  entering  the  valleys  there,  are  often 
much  deceived  as  to  their  extent.  Being  familiar  generally  with  more  lowly 
hills  and  shorter  valleys  at  home,  which,  however,  from  being  near  to  the 
eyes,  form  bulky  images,  and  having  at  first  no  other  measure,  they  almost 
universally  underrate  the  Alpine  dimensions : — they  will  wonder,  for  instance, 
in  the  valley  of  Chamouny,  that  they  should  be  travelling  swiftly  for  hours 
without  reaching  the  end,  where  on  entering  they  did  not  anticipate  a  drive 
of  more  than  half  an  honr. 

The  author  once  sailed  through  the  Canary  Islands,  and  passed  in  view 
of  the  far-famed  Peak  of  Teneriffe.  It  had  been  in  sight  during  the  afternoon 
of  the  preceding  day,  at  a  distance  of  more  than  100  miles,  disappointing 
general  expectation  by  appearing  then  only  as  an  ordinary  distant  hill  rising 
out  of  the  ocean,  but  next  morning,  when  the  ship  had  arrived  within  about 
twenty  miles  of  it,  and  while  another  ship  of  the  fleet,  holding  her  course  six 
miles  nearer  to  the  land,  served  as  a  measure,  it  stood  displayed  as  one  of 
the  most  stupendous  single  objects  which  on  earth,  and  at  one  view,  human 
vision  can  command.  The  ship  in  question,  whose  side,  showing  its  tiers 
of  cannon,  equalled  in  extent  the  front  of  ten  large  houses  in  a  street,  and 
whose  masts  shot  up  like  lofty  steeples,  still  appeared  but  as  a  speck  rising 
from  the  sea,  when  compared  with  the  huge  prominence  beyond  it,  towering 
sublimely  to  heaven,  and  around  which  the  masses  of  cloud,  although  as  lofty 
as  those  which  sail  over  the  fields  of  Britain,  seemed  still  to  be  hanging  low 
on  its  sides.  Teneriffe  alone  of  very  high  mountains,  rises  directly  and 
steeply  out  of  the  bosom  of  the  ocean,  to  an  elevation  of  13,000  feet,  and  as 
an  object  of  contemplation,  therefore  is  more  impressive  than  even  the  still 
loftier  summits  of  Chiraborazo  or  the  Himalayas,  which  rise  from  elevated 
plains,  and  in  the  midst  of  surrounding  hills. 


308  LIGHT. 

It  is  because  objects  which  are  nearly  on  a  level  with  us,  as  contrasted  with 
such  as  are  either  much  above  or  much  below,  are  in  general  more  nume- 
rously surrounded  by  other  known  objects  which  serve  as  measures  of 
comparison,  that  we  judge  so  much  more  correctly  of  the  size  and  distance 
of  those  near  our  level  than  of  others. 

A  man  walking  like  ourselves  on  the  sea-shore  or  other  level,  is  at  once 
fully  recognized ;  and  probably  it  may  not  occur  to  us,  that  he  appears  smaller 
on  account  of  the  distance  ;  but  if  the  same  man  be  seen  afterwards  at  an 
equal  distance  above  us,  collecting  the  sea-fowl's  eggs  on  the  face  of  a  cliff, 
or  below  us,  gathering  shells  on  the  beach  when  we  ourselves  have  reached 
the  height,  he  appears  no  bigger  than  a  crow  ;  yet  in  all  the  cases  he  is  where 
the  same  bulk  forms  the  same  magnitude  of  image  on  the  retina. 

Even  on  a  horizontal  plane,  if  the  general  surface  be  bare  and  uniform, 
single  distant  objects  appear  very  diminutive.  This  is  true,  for  instance,  of 
a  man  seen  apart  from  his  caravan,  while  journeying  across  a  s-iudy  desert  j 
bul  a  man  viewed  at  an  equal  distance,  in  the  midst  of  a  cultivated  land- 
scape, appears  of  his  natural  size.  The  same  is  true  of  a  boat  or  ship  seen 
out  on  the  high  sea,  as  constrasted  with  the  like  viewed  in  a  harbor  where 
other  known  objects  are  near  them. 

We  may  now  understand  why  the  sun  and  moon,  at  rising  or  setting, 
appear  to  us  much  larger  than  when  they  have  attained  meridian  height — 
although,  if  we  examine  them  by  any  measure  of  the  visual  angle,  as  simply 
by  looking  at  them  through  the  same  ring  or  tube,  we  find  that  there  is 
scarcely  a  difference ;  and  singularly  we  find  the  difference  to  be,  that  the 
orbs  appear  horizontally  even  less  than  when  seen  on  the  meridian,  owing 
to  our  being  then  about  4,000  miles  more  distant  from  them.  The  sun 
and  moon  as  they  appear  from  this  earth  are  nearly  of  the  same  size, 
each  occupying  in  the  field  of  view  about  the  half  of  a  degree,  or  as  much  as 
is  occupied  by  a  circle  of  a  foot  in  diameter  when  held  125  feet  from  the 
eye — which  circle,  therefore,  at  that  distance,  and  at  any  time,  would  just 
hide  either  of  them.  Now  when  a  man  sees  the  rising  moon  apparently 
filling  up  the  end  of  a  street,  which  he  knows  to  be  100  feet  wide,  he  very 
naturally  believes  that  the  moon  then  subtends  a  greater  angle  than  usual, 
until  the  reflection  occurs  to  him,  that  he  is  using  as  a  measure,  a  street  known 
indeed  to  be  100  feet  wide,  but  of  which  the  part  concerned,  owing  to  its 
distance,  occupies  in  his  eye  a  very  small  space.  The  width  of  the  street 
near  him  may  occupy  60°  of  his  field  of  view,  and  he  might  see  from  between 
the  houses  many  broad  constellation  instead  of  the  moon  only,  but  the  width 
of  the  street  far  off  may  not  occupy,  in  the  same  field  of  view,  the  twentieth 
part  of  a  degree,  and  the  moon,  which  always  occupies  half  a  degree  will 
there  appear  comparatively  large.  The  kind  of  illusion  now  spoken  of  is 
yet  more  remarkable  when  the  moon  is  seen  rising  near  still  larger  known 
objects,  for  instance,  beyond  a  town,  or  a  hill  which  then  appears  within  the 
luminous  circle.  Any  person  who  from  the  river-side  terraces  of  Greenwich 
has  observed  the  sun  setting  beyond  London,  with  St.  Paul's  Cathedral 
included  in  the  glorious  picture,  will  recollect  a  most  interesting  example  of 
our  present  subject. — That  our  ocular  judgment  of  the  size  of  the  sun  or 
moon  is  thus  influenced  'by  the  presence  or  absence  of  objects  of  comparison, 
and  not  by  the  place  of  the  bodies  in  the  sky,  is  proved  by  the  fact,  that  a 
person  viewing  these  bodies  at  any  eleva-tion  from  the.  bottom  of  some  of  the 
Swiss  valleys,  where  he  might  almost  suppose  himself  placed  at  the  centre 
of  the  earth,  and  looking  abroad  along  an  endless  extent  of  precipices — if  he 


THE    EYE — INTENSITY    OF    LIGHT.  369 

can  closely  compare  them  with  certain  known  magnitudes  of  ridge  or  forest 
bounding  his  view,  sees  them  as  large  as  they  appear  from  other  situations 
when  rising  beyond  a  low  horizon.  Another  proof  is  aiforded  by  the  case 
of  a  balloon  at  a  great  elevation  seen  crossing  the  disc  of  the  sun  or  moon, 
and  then  appearing,  however  large  in  reality,  as  an  absolute  speck  within 
the  vast  luminous  area.  In  a  future  paragraph  it  will  be  explained,  that 
another  circumstance  contributes  to  cause  the  sun  and  moon,  when  low,  to 
appear  larger  than  when  high,  namely,  their  apparent  dimness,  owing  to  the 
obstruction  of  their  light  in  traversing  the  low  dense  atmosphere. 

It  may  be  remarked  here,  that  the  visual  estimate  formed  of  the  great 
size  of  the  sun  and  moon  when  they  are  seen  on  the  horizon,  is  not  an 
illusion,  as  is  popularly  supposed,  but  an  approximation  to  truth,  still  pro- 
digiously short  of  the  reality.  When  we  see  a  distant  tree,  or  a  house,  or 
a  hill,  apparently  within  the  circumference  of  one  of  these  orbs,  it  is  really 
true  that  the  orb  is  larger  than  the  tree,  or  house,  or  hill,  just  as  another 
more  distant  hill  would  be  larger  than  nearer  objects  similarly  surrounded 
by  its  outline;  but  the  celestial  body  is  so  much  larger,  that  even  if  the 
whole  British  Isles  could  be  lifted  away  from  the  earth,  and  suspended  near 
the  moon,  as  a  map  in  the  sky,  they  would  hide  from  a  spectator  on  earth 
but  a  small  part  of  the  disc  of  the  moon. 

Having  now  shown  that  the  visual  angle  or  apparent  size  can  be  a  measure 
of  the  distance  of  any  object  only  when  the  true  size  also  is  known,  or 
of  the  trjue  size  when  the  distance  is  known,  we  proceed  to  examine  other 
means  which  the  eye  commands  for  guessing  at  distances. 

2d.     Intensify  of  liylit,  shade  and  colour.     (See  the  Analysis,  pages  325 

and  355.) 

It  has  already  been  explained  that  light,  like  every  other  influence,  radi- 
ating from  a  centre,  becomes  rapidly  weaker  as  the  distance  from  the  centre 
increases,  being,  for  instance,  only  one-fourth  part  as  intense,  at  double 
distance,  and  in  a  corresponding  proportion  for  other  distances  ;  while  it  is 
still  farther  weakened  by  the  obstacle  of  any  transparent  medium  through 
which  it  passes.  Now  the  eye  soon  becomes  sufficiently  familiar  with  these 
truths,  to  judge  from  them,  with  considerable  accuracy,  of  -  the  comparative 
distances  of  objects. 

The  fine  Gothic  pile  of  "Westminster  Abbey  may  break  upon  the  view  in 
some  situations  where  nearer  edifices,  and  perhaps  some  minor  imitations  of 
its  beauties,  already  fill  the  eye  with  their  strong  lights,  but  the  misty  or 
less  distinct  outline  of  the  venerable  pile  may  warn  the  approaching  stranger 
of  its  true  magnitude,  and  prepare  him  for  the  enjoyment  which  a  nearer 
inspection  of  its  grandeur  and  perfection  is  to  afford. 

A  small  yacht  or  pleasure-boat  may  be  built  according  to  the  same  model 
or  with  the  same  comparative  dimensions  as  a  first-rate  vessel  of  war,  and 
may  be  in  view  from  the  shore  at  the  same  time,  only  so  much  nearer  than 
the  ship,  that  both  shall  form  images  of  the  same  magnitude  on  the  retina  of 
a  spectator.  In  such  a  case,  an  unpractised  eye  might  have  difficulty  to 
discriminate,  but  to  an  old  seamen,  the  bright  lights  of  the  little  vessel, 
contrasted  with  the  softer  or  more  misty  appearance  of  the  larger,  would 
declare  the  truth  at  once.  A  haziness  occurring  in  the  atmosphere  between 
the  little  vessel  and  the  eye  might  considerably  favour  the  illusion. 

In  a  fleet  of  ships,  if  the  sun's  direct  rays  fall  upon  one  here  and  there 

24 


370  LIGHT. 

through  openings  among  the  clouds,  while  the  others  remain  in  shade,  the 
former,  in  appearance,  starts  towards  the  spectator.  In  like  manner,  the 
mountains  of  an  unknown  coast,  if  the  sunshine  fall  upon  them,  appear 
comparatively  near,  but  if  the  clouds  again  intervene,  they  recede  and  mock 
the  awakened  hope  of  the  approaching  mariner. 

A  conflagration  at  night,  however  distant,  appears  to  spectators,  generally, 
as  if  very  near,  and  inexperienced  persons  often  run  towards  it  with  the 
hope  of  soon  arriving,  but  find,  after  miles  travelled,  that  they  have  made 
but  a  little  part  of  the  way. 

A  person  ignorant  of  astronomy  deems  the  heavenly  bodies  so  much 
nearer  to  the  earth  than  they  are,  merely  because  of  their  being  so  bright 
or  luminous.  The  evening  star,  for  instance,  seen  in  a  clear  sky,  over  some 
distant  hill-top,  appears  as  if  a  dweller  on  the  hill  might  almost  reach  it — 
for  the  most  intense  artificial  light  which  could  be  placed  on  the  height 
would  be  dim  in  comparison  with  the  beauteous  star;  yet  to  a  dweller  on 
the  hill  it  appears  just  as  distant  as  to  one  on  a  plain;  nay,  at  a  thousand 
miles  farther  west,  and,  therefore,  nearer,  the  appearance  would  still  be 
nearly  the  same. 

The  concave  of  the  starry  heavens  appears  flattened  above,  or  as  if  its 
zenith  were  nearer  to  the  earth  than  its  sides  or  horizon,  because  the  light 
from  above  having  to  pass  through  only  the  depth  or  thickness  of  the  atmo- 
sphere, is  little  obstructed,  while  of  that  which  darts  towards  any  place 
horizontally  through  hundreds  of  miles  of  dense  vapour-loaded  air,  only  a 
small  part  arrives. 

The  sun  and  moon  appear  larger  at  rising  and  setting  than  when  midway 
in  heaven,  partly,  as  already  explained,  because  they  can  then  be  easily  com- 
pared with  other  objects,  of  which  the  size  is  known,  but  partly,  also,  because 
"of  the  much  less  light  arriving  from  them  in  the  former  situation,  while 
their  diameters  are  nearly  the  same. 

A  fog  or  mist  is  said  to  magnify  objects  seen  through  it.  The  truth  is, 
that  because  it  diminishes  the  intensity  of  the  light  from,  them,  it  makes 
them  appear  farther  distant  without  lessening  the  visual  angles  subtended 
by  them ;  and  because  an  object  at  two  miles,  subtending  the  same  angle 
as  an  object  at  one  mile  is  twice  as  large,  the  conclusion  is  drawn  that  the  dim 
object  is  large.  Thus,  a  person  in  a  fog  may  believe  that  he  is  approaching 
a  great  tree,  fifty  yards  distant,  when  the  next  step  throws  him  into  the  bush 
which  had  deceived  him  — Two  friends  meeting  in  a  fog  have  often  mutu- 
ally mistaken  each  other  for'  persons  of  much  greater  stature. — A  row  of 
fox-glove  flowers  on  a  neighbouring  bank  has  been  mistaken  for  a  company 
of  scarlet-clad  soldiers  on  the  more  distant  face  of  the  hill. — There  arc,  for 
similar  reasons,  frequent  misjudgings  in  late  twilight  and  early  dawn. — The 
purpose  of  a  thin  gauze  screen  interposed  between  the  spectators  in  a  theatre 
and  some  person  or  object  meant  to  appear  distant,  is  intelligible  on  the  same 
principle  :  a  boy  near,  so  screened,  appears  to  be  a  man  at  a  distance. — The 
art  of  the  painter  uses  sombre  colours  when  his  object  is  to  produce  in  his 
picture  the  effect  of  distance.  On  the  alarming  occasion  of  a  very  dense 
fog  coming  on  at  sea,  where  the  ships  of  a  fleet  are  near  to  each  other, 
without  wind,  and  where  there  is  considerable  swell  or  rolling  of  the  sea, 
much  damage  is  often  done  :  but  it  is  to  be  remarked  in  such  a  case,  that  the 
size  of  each  ship  approaching  to  the  shock,  is  always,  in  the  apprehension 
of  the  crew  of  the  other  ship,  exaggerated. 

The  celebrated  Spectre  of  the  Bracken,  among  the  Hartz  Mountains,  is  a 
good  illustration  of  our  present  subject.  On  a  certain  ridge,  just  at  sunrise 


THE    EYE — INTENSITY    OF    LIGHT.  371 

a  gigantic  figure  of  a  man  had  often  been  observed  walking,  and  extraordi- 
nary stories  were  related  of  him.  About  the  year  1800  a  French  philosopher 
and  a  friend  went  to  watch  the  apparition ;  but  for  many  mornings  they 
paraded  on  the  opposite  ridge  in  vain.  At  last,  however,  the  monster  was 
seen,  but  he  was  not  alone ;  he  had  a  companion,  and  singularly  he  and  his 
companion  aped  all  the  motions  and  atitudes  of  the  observer  and  his  com- 
panion ;  in  fact  the  spectres  were  merely  shadows  of  the  observers,  formed 
by  the  horizontal  rays  of  the  rising  sun  falling  on  the  morning  fog  which 
hovered  over  the  valley  between  the  ridges ;  and  because  the  shadows  were 
very  faint,  the  figures  were  deemed  distant,  seeming  men  walking  on  the 
opposite  ridge,  and  because  a  comparatively  small  figure  seen  near,  but  sup- 
posed distant,  appears  of  gigantic  dimensions,  these  shadows  were  accounted 
giants. 

While  the  comparative  intensities  of  light  coming  from  bodies  considered 
as  wholes,  or  from  their  sides  similarly  exposed  to  the  source  of  light — fur- 
nish an  indication  of  their  different  distances  from  the  observer,  the  compara- 
tive intensities  from  their  sides  dissimilarly  or  unequally  exposed  to  the 
source  of  light,  and  which,  therefore,  reflect  light  to  the  eye,  or  are  illumi- 
nated in  different  degrees,  furnish  an  indication  of  the  forms  and  attitudes 
of  the  bodies.  In  observing,  for  instance,  a  white  house  exposed  to  the  sun, 
it  is  seen  that  the  side  directly  receiving  the  rays  is  highly  illuminated  or 
bright,  while  the  other  sides  are  less  so,  and  are  said  to  be  in  the  shade — a 
shade  which  is  more  or  less  deep  in  proportion  as  there  are  few  or  many 
sources  of  reflected  light  near  it.  The  different  faces  or  walls  of  such  a  house 
are  to  the  sense  of  the  observer,  as  strongly  distinguished  from  each  other, 
by  the  mere  difference  of  shade,  as  if  they  were  of  different  colours,  or  as  if 
they  were  examined  by  the  touch,  or  by  walking  round  them.  If  the  object 
were  a  ball  instead  of  a  square  house,  there  would  still  be  as  great  differences 
of  shade  in  the  half  not  receiving  direct  rays,  but  the  parts,  instead  of  forming 
abrupt  contrasts  like  the  walls  of  a  house,  would  appear  to  melt  into  each 
other,  marking  the  beautiful  round  contour  of  the  object.  The  consideration 
of  all  such  cases  forms  the  subject  of  chiaro-oscuro,  so  interesting  to  the 
pajnter. 

Had  there  not  been  in  nature  the  provision  of  light  and  shade  now  des- 
cribed, the  sense  of  sight  would  have  been  of  comparatively  little  use,  and  a 
mass  of.things  in  the  light,  if  of  the  same  colour,  would  have  been  as  little 
distinguishable  from  one  another  by  a  person  looking  directly  at  them,  as 
things  forming  a  mass, or  shadow  are  distinguishable  by  a  person  looking  at 
the  shadow.  It  is  this  provision,  therefore,  which  enables  us,  independently 
of  colour,  to  distinguish  the  profile  or  outlines.of  different  bodies  placed  near 
to.  each  other,  and  to  distinguish  in  the  same  body  the  protuberent  or  other 
form  of  the  surfaces  which  is  towards  the  observer.  But  for  this,  it  would 
have  been  impossible  to  distinguish,  for  instance,  between  a  white  wall  when 
naked  and  when  having  various  white  objects  placed  before  it  j  and  it  would 
have  been  impossible  to  distinguish  between  the  rounded  figures,  if  similarly 
coloured,  of  a  flat  circle,  a  sphere  and  a  cone,  all  directly  opposed  to  the  eye ; 
but  in  reality,  by  some  difference  of  shade,  the  white  objects  are  distin- 
guished from  the  wall,  and  in  the  three  geometrical  figures,  the  uniformly 
bright  surface  of  the  circle,  the  soft  rounded  shadowing  of  the  sphere  and 
the  shade  coming  to  a  point  on  the  cone,  at  once  declare  the  true  forms.  But 
for  the  shadowed  parts,  the  facade  of  a  white  palace  of  varied  architecture 
would  have  been  an  unmeaning  sheet  of  lights :  the  lights,  however,  and 
shadows  produced  by  the  juttings  and  recesses,  mark  the  variety  of  surface 


372  LIGHT. 

most  completely;  and  the  round  pillar  is  distinguished  from  the  square,  and 
every  pediment,  and  capital,  and  architectural  ornament,  stands  out  plea- 
singly conspicuous.  But  for  light  and  shade  again,  the  "  human  face  divine." 
would  have  been  an  unmeaning  patch  of  flesh,  for  there  are  few  lines  in  it 
but  those  made  by  different  exposures  to  the  lights,  and  yet  its  every 
prominence  and  depression,  and  every  momentary  change,  are  so  truly  indi- 
cated to  the  eye  that  it  becomes  full  of  meaning  or  expression.  How  well 
mere  light  and  shade  serve  to  convey  what  the  eye  has  to  learn  of  a  scene 
or  object,  may  be  perceived  by  examining  any  of  the  admirable  engravings 
which  now  abound,  and  which,  although  made  up  entirely  of  degrees  of 
shade,  or  of  black  and  white,  are  scarcely  inferior  in  expression  to  finished 
paintings. 

The  student  of  painting  soon  learns  that  the  lines  called  outlines,  by  which 
he  first  sketches  subjects,  do  not  exist  at  all  in  nature,  and  have  to  be  again 
effaced  in  his  finished  work  :  for  they  only  mark  the  place  where  lights  and 
shades  happen  to  meet.  Much  may  be  conveyed  to  the  mind,  however,  by 
a  mere  outline,  and  particularly  of  lines  if  different  breadth  and  thickness 
are  used  to  mark  the  situation  of  the  fainter  and  deeper  shadows. 

The  subject  of  chiaro-oscuro  is  not  so  simpl«  as,  from  the  fact  of  the  sun 
being  the  great  source  of  light,  might  at  first  be  supposed;  for  although  this 
be  true,  still  every  body  which  reflects  the  sun's  light  becomes  a  new  source 
to  those  about  it,  and  the  shading  of  a  picture  must  have  reference  to  all  such 
sources,  and  to  the  colours  of  the  body  itself,  and  of  the  neighbouring  bodies. 

In  looking -at  an  extended  landscape,  it  is  seen  that  the  near  objects  con- 
sidered as  wholes,  are  comparatively  bright,  that  their  shadows  are  strongly 
marked,  and  that  their  peculiar  colours  are  everywhere  easily  distinguishable 
— as  of  flowers,  fruit,  foliage,  &c.,  but  of  objects  farther  off,  the  colours,  with 
increasing  distance  become  dim,  the  lights  and  shadows  melt  into  each  other 
or  are  confused,  and  the  illumination  altogether  becomes  so  faint  that  the 
eye  at  last  sees  only  an  extent  of  distant  blue  mountain  or  plain — appearing 
bluish,  partly  because  the  transparent  air  through  which  the  light  must  pass 
has  a  blue  tinge,  and  partly  because  the  quantity  of  light  which  can  arrive 
through  the  great  extent  of  air  is  insufficient  to  exhibit  the  detail.  TJie 
ridge  called  Blue  Mountains  in  Australia,  another  of  the  same  name  in 
America,  and  many  others  elsewhere,  are  not  really  blue,  for  they  possess 
all  the  diversity  of  scenery  which  their  climates  can  give,  but  to  the  eye 
which  first  discovered  them,  bent  on  them  generally  from  a  distance  :  they 
all  at  first  appeared  blue,  and  they  have  retained  the  name. 

In  a  good  picture,  where,  upon  canvass  stretched  on  a  frame,  the  artist  has 
disposed  the  lights,  shades  and  colours  in  the  very  situations  and  with  the 
intensities  which  they  would  have  had  on  coming  from  the  real  scene  to  the 
eyes,  through  a  plate  of  glass  filling  up  the  frame,  all  that  we  have  now 
been  saying  is  strictly  exemplified.  In  the  foreground,  the  objects  are  large 
and  bright,  but  as  the  scene  is  supposed  to  be  gradually  more  remote,  the 
size  and  brightness  correspondingly  diminish,  until,  at  last,  there  is  only  a 
dim  mixture  of  bluish  or  grayish  masses  forming  the  horizon  and  sky. 

A  child  during  what  may  be  called  the  education  of  the  sense  of  sight, 
has  a  strong  perception  of  the  vast  differences  of  appearance  which  things 
assume  according  to  their  accidental  distance  from  the  eye,  their  position, 
their  exposure  to  light,  &o. ;  for  many  of  these  differences,  being  at  first 
calculated  to  deceive  the  young  judgment,  have,  from  time  to  time,  been 
noted  by  him  with  surprise.  Thus,  a  boy  when  he  first  discovers  that  a  ship 
which  at  the  quay,  with  her  white  sails  spread  out;  concealed  from  him 


THE    EYE — PERSPECTIVE.  373 

half  the  heavens,  is  in  an  hour  or  two  afterwards,  seen  by  him  on  the  dis- 
tant horizon  as  a  dark  speck  hardly  big  enough  to  hide  one  star,  has  his 
attention  strongly  awakened,  and  he  feels  surprised;  or,  again,  when  he 
discovers  that  the  faint  blue  unchanging  mass  which  he  had  always  observed 
bounding  in  one  direction,  the  view  from  the  house  of  his  infancy,  is  a  dis- 
tant mountain  side,  thickly  inhabited  and  corvered  with  fields  and  gardens, 
where  in  succession,  all  the  bright  colours  of  the  different  seasons  predo- 
minate— he  is  equally  struck.  But  as  soon  as  experience  has  enabled  him 
to  interpret  readily  and  correctly,  the  visual  signs  under  every  variety  of 
circumstance,  his  attention  passes  so  instantly  from  them  to  the  realities — 
which  alone  are  interesting  to  him — just  as  it  might  pass  from  the  paper  and 
printing  of  a  newspaper  to  the  important  intelligence  communicated  by  them 
— that  he  very  soon  ceases  Jo  be  aware  that  the  sign,  which,  in  every  case, 
similarly  suggests  the  object,  is  not,  also,  in  every  case  similar  to  itself,  and 
the  very  same  true  and  complete  representation  of  the  reality.  The  prejudice 
that  the  sign  is  of  this  nature  becomes  quickly  so  strong,  that  even  a  diffi- 
cult effort  has  been  made  by  a  grown  person  again  to  attend  to  the  mere 
appearances,  in  any  scene  of  which  the  realities  are  known. 

This  attempt  to  analyze  mere  appearances,  and  which,  in  one  sense,  is  an 
attempt  to  unlearn  something,  or  to  retrograde,  is  called,  as  already:"stated, 
the  study  of  perspective.  When  it  regards  the  apparent  reduction  of  size  and, 
the  foreshortening  of  bodies  under  various  circumstances,  it  is  called  linear 
perspective — when  it  regards  the  fading  of  light  and  the  modifying  of  colour 
it  is  called  aerial  perspective.  As  the  art  of  painting  depends  entirely  upon 
the  understanding  of  these  two  departments,  the  gradual  progress  which  it 
has  made  in  different  countries  is  a  measure  of  the  degree  in  which  the  com- 
mon prejudice  that  things  appear  as  they  are  has,  in  them,  been  overcome. 
Where  this  prejudice  exists,  any  untaught  person  conceives  a  good  painting 
to  be  merely  a  miniature  representation  drawn  according  to  a  certain  re- 
duced scale, — as  of  arf  inch  to  a  yard, — and  in  which  all  the  demensions  of 
things  are  to  be  measured  as  simply  as  in  the  reality — while  the  colour,  as 
to  vividness,  &c.,  should  perfectly  agree  with  the  originals.  This  statement 
is  remarkably  illustrated  by  the  facts,  that  children  in  their  rude  atempts  to 
paint,  always  aim  at  realizing  the  notion  of  the  art  above  detailed,  and  that 
such  has  been  the  first  stage  of  painting  in  every  country.  In  Europe  now, 
owing  to  the  labours  of  men  of  genius,  art  in  painting  may  be  said  almost 
to  rival  nature,  producing  scenes  as  lovely  as  the  finest  of  nature's  scenes, 
and  scarcely  distinguishable  from  them  :  but  in  other  countries,  as  in  China 
and  India,  among  the  native  artists,  the  first  stage  of  the  art  is  still  in  exist- 
ence. In  a  Chinese  picture,  owing  to  the  absence  of  perspective  propor- 
tions, an  extensive  subject  is  only  a  collection  of  portaits  of  men  and  things 
drawn  all  on  the  same  scale,  and  placed  near  one  another,  and  where  all  the 
colours  are  as  vividly  shown  as  if  the  objects  were  only  a  few  feet  from  the 
eye ;  there,  the  figures  at  the  bottom  or  fore-ground  are  supposed  to  repre- 
sent the  objects  nearest  to  the  spectator,  while  the  figures  higher  up  are  sup- 
posed to  be  of  more  remote  objects,  all  appearing  as  they  might  be  seen  in 
succession  by  a  person  who  had  the  power  of  flying  over  the  country.  This 
kind  of  picture  or  representation,  although  not  natural,  if  all  viewed  at  once, 
may  communicate  more  information  than  a  single  common  painting,  for  it  is 
equivalent  to  many  such.  In  Europe,  lately,  the  principle  has  been  again 
usefully  acted  upon  for  certain  purposes,  as  for  representing  on  one  long 
sheet  or  on  a  succession  of  sheets,  connected  in  a  suitable  manner,  the  banks 
of  a  river  or  a  line  of  a  road.  The  banks  of  the  Rhine  particularly  have  thus 


374  LIGHT. 

been  admirably  portrayed,  so  that,  the  spectator  directing  his  eye  along  the 
paper  feels  almost  as  if  carried  in  a  balloon  to  view  in  detail  the  whole  of 
the  real  and  enchanting  scenery.  The  principle  might,  perhaps,  with  ad- 
vantage, be  acted  upon  still  more  extensively — for  instance,  to  produce, 
instead  of  common  maps  or  charts  of  countries,  true  bird's-eye  views,  over 
which  the  eye,  moving  from  place  to  place,  and  at  every  new  point  of  sight, 
would  see  a  certain  portion  of  the  country,  as  a  bird  or  aeronaut  would,  the 
sketch  being  supposed  to  be  taken  from  that  certain  elevation  deemed  most 
suitable  for  the  ends  in  view. 

3d  Divergence  of  the  rays  ofliglit.     (See  Analysis,"page  355.) 

This  is  the  next  circumstance  to  be  mentioned  by  which  the  eye  judges 
of  distance.  Supposing  the  line  E  F  to  mark  t(je  place  and  breadth  of  the 
pupil  of  the  eye,  the  light  entering  from  an  object  at  a  which  is  near  (it  is 

here  placed  nearer  than  an  object 

Fig.  1-58.  could  be  seen  in  reality,)  is  very 

divergent,  or  is  spreading  with  a 
large  angle  j  from  b  the  pencil  of 
rays  is  less  divergent,  or  opens 
with  a  smaller  angle  ;  from  c  it  is 
less  divergent  still,  and  so  on. 
Now  the  eye,  to  form  an  image 

on  its  retina  requires  to  exert  a  bending  power  exactly  proportioned  to  the 
divergence  of  the  received  rays  :  and  it  appears  to  have  a  sense  of  the  effort 
made,  which  becomes  to  the  person  a  kind  of  measure  of  the  distance  of  the 
object.  This  divergence  of  the  rays  entering  the  eye,  is  the  chief  circum- 
stance in  which  the  most  perfect  painting  must  still  differ  in  its  effect  upon 
the  eye  from  a  natural  scene — for,  first,  in  the  natural  scene,  the  objects  are 
generally  more  distant  than  their  representation  can  be ;  and  secondly,  while, 
in  nature,  every  object,  according  to  its  distance,  is  sending  rays  which  reach 
the  eye  with  different  divergence,  and  which  rays,  therefore,  can  produce 
distinct  images  on  the  retina  at  any  one  time,  only  of  the  objects  which  are 
at  the  same  distance  from  the  eye,  the  rays  from  a  picture,  which  is  a 
single  plane  surface,  come  from  every  part  with  the  same  divergence,  and 
the  eye  must  feel  a  disappointment  in  not  having  to  accommodate  its  power 
of  bending,  to  the  different  distances  attempted  to  be  portrayed  on  the  can- 
vass. It  might  be  expected  that  this  kind  of  disappointment  would  be  more 
felt  on  looking  at  a  common  picture  placed  a  few  feet  from  the  eye,  than  at 
the  sort  of  picture  called  panorama,  which  is  on  a  larger  scale  and  propor- 
tionately more  distant,  but  such  is  not  the  case  :  and  the  reason  seems  to  be 
that  in  the  former  the  illusion  is  not  intended  to  be  complete,  the  fact  of  its 
being  but  a  picture  not  being  at  all  concealed,  and  the  eye  is  therefore  at 
once  told  to  expect  a  difference  of  feeling  ; — but  in  the  panorama,  the  whole 
circumstances  are  arranged  to  deceive  the  eye  entirely,  if  possible,  and  to 
make  it  believe  that  the  images  on  the  retina  are  formed  by  light  from  the 
objects  themselves, — then  to  the  eye,  really  deceived  in  all  other  particulars, 
the  non-accordance  with  nature  in  this  one  is  strongly,  and,  by  some  persons, 
even  painfully  felt,  so  as  on  their  first  entering  the  place  to  cause  headache 
or  giddiness. — The  illusion  and  consequently  the  pleasure  from  viewing  any 
picture  may  be  made  more  complete  by  the  spectator  using  lenses  or  spec- 
tacles, such  that  the  focal  distance  shall  be  equal  to  the  distance  of  the  paint- 
ing from  the  eye;  because  such  lenses,  as  was  formerly  explained,  would 
render  all  the  rays  entering  the  eye  nearly  parallel,  and  therefore  very  nearly 
such  as  would  arrive  from  objects  at  a  considerable  distance. 


THE    EYE — PERSPECTIVE.  375 

4:th.     Convergence  of  the  axes  of  the  eyes.     (See  the  Analysis,  page  355.) 

This  is  the  last  circumstance  to  be  mentioned,  by  which  a  person  through 
the  eye,  judges  of  the  distance  of  objects.  In  consequence  of  there  being 
in  the  two  eyes  corresponding  parts  which  must  be  similarly  affected  by  any 
object,  that  the  person  may  have  a  single  vision  of  it,  as  was  explained  in  a 
former  page,  the  axes  of  both  eyes  must  point  to  the  object,  and  if  it  happen 
to  be  very  near,  they  will  meet  and  cross  each  other  so  near  the  face  as  to 
produce  the  appearance  of  squinting, — seen  when  a  person  trying  to  look  r.t 
the  point  of  his  nose, — but  if  the  object  be  more  distant,  the  obliquity  will  be 
less,  until  at  last  the  eyes  directed  to  a  thing  at  a  very  great  distance,  will 
have  their  axes  almost  parallel.  The  last  figure  may  serve  also  to  explain 
this  subject.  Supposing  E  and  F  to  mark  the  place  of  the  two  eyes  if  the 
object  looked  at  be  near  them,  as  at  a,  they  must  be  very  much  turned 
inwards,  that  their  axes  may  meet  at  a ;  if  it  be  at  b,  they  will  be  less 
turned,  if  at  c  less  still,  and  so  forth. 

When  the  eyes  are  not  directed  to  any  thing  in  particular,  the  axes  gene- 
rally become  parallel,  or  as  if  they  were  pointed  to  a  very  distant  object ;  and 
because  this  happens  generally  when  persons  are  reflecting  on  things  which 
are  absent  and  seen  only  by  the  mind's  eye,  it  is  an  expression  of  counten- 
ance held  to  mark  contemplation  or  thoughtfulness. 

The  direction  of  the  visual  axis  is  another  particular,  like  the  divergence 
of  light  as  to  which  a  mere  picture,  can  never  produce  upon  the  eye  pre- 
cisely the  effect  of  the^  objects  themselves.  To  see  a  picture,  the  axes  must 
meet  at  it,  and  generally,  therefore,  at  a  few  feet  from  the  eye  ;  while  to  see 
the  objects  of  nature,  they  often  do  not  meet  nearer  than  at  miles.  By  a 
glass,  however,  as  will  be  explained  a  little  farther  on,  it  is  possible  to  correct 
also  this  defect,  and  to  render  the  optical  illusion,  as  regards  still  objects, 
absolutely  complete. 

When  a  picture  has  to  represent  objects  supposed  far  from  the  eye,  the 
farther  the  picture  itself  is  placed  from  the  eye  supposing  the  figures  to  be 
made  proportionately  large,  the  more  nearly  perfect  will  the  illusion  become, 
because  the  divergence  of  rays  and  convergence  of  the  axes  (the  two  circum- 
stances in  which  the  effect  of  a  mere  picture  on  the  eye  must  always  differ 
from  the  effect  of  a  real  scene)  will  be  in  proportion  more  nearly  natural. 
This  explains  in  part  why  the  picture  called  panorama  (from  Greek  words 
signifying  a  view  of  the  whole}  is  an  exhibition  so  charming  ;  for  usually  the 
pain  ting  is  far  removed  from  the  eye,  and  is  drawn  on  a  proportionately  large 
scale,  and  the  eyes  feel  that  the  light  comes  from  a  considerable  distance,  and 
that  their  axes  do  not  need  to  converge  very  much ;  and  when  in  such  a  case, 
the  first  impression  of  the  want  of  absolute  conformity  to  nature  has  passed 
away,  the  illusion  becomes  nearly  complete.  But  a  not  less  important  pecu- 
liarity in  the  panorama  is,  that  instead  of  being  a  painting  on  a  plane  surface 
like  common  pictures,  and  embracing  only  a  small  part  of  the  field  of  view, 
it  is  on  a  curved  surface  which  entirely  surrounds  the  spectator,. and  on  which 
all  the  objects  visible  in  the  various  directions  from  the  supposed  place  are 
seen  in  the  very  situations  which  in  nature  they  hold ;  and  the  spectator  is 
enabled  to  conceive  much  more  distinctly  of  each  particular  by  seeing  it  in 
relation  to  others  around.  Few  persons  can  forget  the  impression  made  on 
them  by  the  first  panorama  which  they  may  have  seen ;  and  after  increased 
maturity  of  judgment,  they  will  discover  still  more  and  stronger  reasons  for 
admiring  this  almost  miraculous  mode  of  instantly  transporting  them  to  any 
distance,  beyond  seas  and  other  dangers,  to  contemplate  at  their  ease  the 


876  LIGHT. 

most  interesting  scenes  on  earth,  represented  under  the  most  favourable  cir- 
cumstances of  position,  light  and  weather.  Hence  few  persons  of  good  taste 
neglect  the  opportunity  now,  in  most  great  towns  so  frequently  offered,  of 
obtaining  at  little  cost  so  high  a  gratification. 

To  correct  the  slight  remaining  optical  defects  of  a  common  panorama,  a 
large  lens  may  be  used,  of  which  the  focal  distance  is  equal  to  the  distance 
of  the  picture  from  the  eye.  This  has  the  effect  of  diminishing  the  diver- 
gence of  the  rays  until  it  becomes  exactly  that  which  belongs  to  the  supposed 
remoteness  of  the  objects,  and  it  also  bends  the  light  so  that  the  axes  of  the 
eyes  may  be  nearly  parallel.  The  author  has  found  a  convenient  mode  of 
using  the  lens  for  such  a  purpose  to  be  to  cut  out  two  round  pieces  from 
opposite  sides  of  it,  and  to  form  thenuinto  a  pair  of  spectacles : — from  one 
lens  three  pairs  may  be  formed.  Panorama  exhibitors  should  keep  such 
lenses  or  spectacles  for  the  use  of  visitors. 

The  effect  of  the  magnitude  and  distance  of  the  ordinary  large  panoramic 
views  might,  with  the  assistance  of  proper  glasses,  be  had  from  even  the 
smallest  picture  on  engraved  representation  embracing  the  same  field ;  and  it 
is  remarkable  that  some  enterprising  person  has  not  undertaken  to  publish 
sets  of  interesting  views  fitted  to  be  used  in  that  way.  A  common  panorama, 
occupying  a  circular  wall  of  150  feet  in  circumference  and  twenty  feet  high, 
may  be  reduced — and  still  retaining  the  same  truth  of  proportions,  to  appear 
on  a  piece  of  paper  five  feet  long  and  eight  inches  high  or  broad ;  and  if 
this  were  set  up  in  a  suitable  frame,  like  a  wall,  round  the  head  of  a  spectator, 
while  its  edges  were  concealed  by  drapery  or  otherwise,  and  the  eye  could 
only  view  it  through  fit  glasses  placed  in  its  centre  and  made  to  turn  round 
so  as  to  command  the  whole,  it  could  not  by  any  ordinary  spectator  be  dis- 
tinguished from  the  large  panorama.  With  the  art  of  lithography,  now  so 
well  adapted  for  producing  soft  representations  of  scenery,  the  expense  of 
such  views  might  be  very  moderate,  allowing  them  to  form  a  common  part  of 
library  furniture.  When  we  reflect  upon  the  expansion  of  mind  obtained  by 
travelling,  and  that  not  a  few  of  the  advantages  would  follow  a  familiarity 
with  a  good  selection  of  panoramic  views,  it  is  not  perhaps  too  much  to  sup- 
pose that  courses  of  instruction  in  geography,  history,  &c.,  may  before  long 
be  illustrated  by  this  most  interesting  mode  of  aiding  the  conception  and 
memory. 

Common  paintings  and  prints  may  be  conside%d  as  detached  parts  of  a 
panoramic  representation,  showing  as  much  of  that  general  field  of  view 
which  always  surrounds  a  spectator,  as  can  be  seen  by  the  eye  kept  in  one 
place,  and  looking  through  a  window  or  other  opening  of  moderate  size. 
The  pleasure  from  contemplating  these  is  much  increased  by  using  a  lens  or 
such  spectacles  as  above  described.     There  is  in  the 
Fig.  159.  shops  such  lens,  with  the  title  of  optical  pillar  ma- 

chine, or  diagonal  mirror,  fitted  up  so  that  the  print 
to  be  viewed  is  laid  upon  a  table  beyond  the  stand  of 
the  lens,  and  its  reflection  in  a  mirror  supported  diago- 
nally over  it,  is  viewed  through  the  lens.  The  illu- 
sion is  rendered  more  complete  in  such  a  case  by 
having  a  box,  as  a  b,  on  the  bottom  of  which  the  paint- 
ing is  laid,  and  at  the  top  of  which  the  lens  and  mirror, 
fixed  in  a  smaller  box  at  a,  are  made  to  slide  up  and 
down  to  allow  of  a  ready  adjustment  of  the  focal  dis- 
tance. This  box  used  in  a  reverse  way  becomes  a 
perfect  camera  obscura.  The  common  show-stalls  seen  in  the  streets  are 


THE   EYE — PA  IN  TIN  a   REPRESENTING  MOTION.     377 

boxes  made  somewhat  on  this  principle,  but  without  the  mirror ;  and 
although  the  drawings  or  prints  in  them  are  generally  very  course,  they  are 
not  uninteresting.  To  children  whose  eyes  are  not  yet  very  critical,  some 
of  the  show-boxes  afford  an  exceeding  great  treat. 

A  still  more  perfect  contrivance  of  the  same  kind  has  been  exhibited  for 
some  time  in  London  and  Paris  under  the  title  of  Casmoramo,  (from  Greek 
words  signifiying  views  of  the  world,  because  of  the  great  variety  of  views.) 
Pictures  of  moderate  size  are  placed  beyond  what  have  the  appearance  of 
common  windows,  but  of  which  the  panes  are  really  large  convex  lenses 
fitted  to  correct  the  errors  of  appearance  which  the  nearness  of  the  pictures 
would  else  produce.  Then,  by  adding  various  subordinate  contrivances, 
calculated  to  aid  and  heighten  the  effects,  even  shrewd  judges  have  been  led 
to  suppose  the  small  pictures  behind  the  glasses  to  be  very  large  pictures, 
while  all  others  have  let  their  eyes  dwell  upon  them  with  admiration,  as 
magical  realizations  of  the  natural  scenes  and  objects. "  Because  this  contri- 
vance is  cheap  and  simple,  many  persons  affect  to  despise  it ;  but  they  do  not 
thereby  show  their  wisdom :  for  to  have  made  so  perfect  a  representation 
of  objects,  is  one  of  the  noblest  triumphs  of  art,  whether  we  regard  the 
pictures  as  drawn  in  true  perspective  and  colouring,  or  the  lenses  which 
assist  the  eye  in  examining  them. 

It  has  already  been  stated  that  the  effect  of  looking  through  such  glasses 
at  near  pictures,  is  obtainable,  in  a  considerable  degree,  without  a  glass,  by 
having  the  pictures  very  large,  and  placing  them  at  a  corresponding  distance. 
The  rule  of  proportion  in  such  a  case  is,  that  a  picture  of  one  foot  square  at 
one  foot  distance  from  the  eye,  appears  as  large  as  a  picture  of  60  feet  square 
at  60  feet  distance.  The  exhibition  called  the  Diorama  is  merely  a  large 
painting  prepared  in  accordance  with  the  principle  now  explained.  In  prin- 
ciple it  has  no  advantage  over  the  cosmoramo  or  the  show-box,  to  compensate 
for  the  greater  expense  incurred,  but  that  many  persons  may  stand  before  it 
at  the  same  time,  all  very  near  the  true  point  of  sight,  and  deriving  the 
pleasure  of  sympathy  in  their  admiration  of  it,  while  a  slight  motion  of  the 
spectator  does  not  make  his  eye  lose  the  right  point  of  the  view. 

A  round  building  of  prodigious  magnitude  has  lately  been  erected  in  the 
Regent's  Park  in  London,  on  the  walls  of  which  is  painted  a  representation 
of  London  and  the  country  around,  as  seen  from  the  cross  on  the  top  of  St. 
Paul's  Cathedral.  The  real  scene  is  unquestionably  one  of4he  most  extra- 
ordinary which  the  world  affords,  and  this  representation  of  it  combines  the 
several  advantages  of — the  circular  view  of  the  panorama — the  size  and  dis- 
tance of  the  great  diorama — and  that  from  the  details  being  so  minutely 
painted,  distant  objects  may  be  examined  by  a  telescope  or  opera-glass. 

From  what  has  now  been  said,  it  may  be  understood,  that  for  the  purpose 
of  representing  still-nature,  or  mere  momentary  states  of  moving  objects,  a 
picture  truly  drawn,  truly  coloured,  and  which  is  either  very  large  to  correct 
the  divergence  of  light  and  convergence  of  visual  axes,  or  if  small,  is  viewed 
through  a  glass,  would  affect  the  retina  exactly  as  the  realities.  But  the 
desideratum  still  remained  of  being  able  to  paint  motion.  Now  this,  too, 
has  been  recently  attempted,  and  in  many  cases  with  singular  success,  chiefly 
by  making  the  picture  transparent,  and  throwing  lights  and  shadows  upon  it 
from  behind.  In  the  exhibition  of  the  diorama  and  cosmorama  there  have 
been  thus  represented  with  admirable  truth  and  beauty  such  phenomena  as — 
the  sunbeams  occasionally  interrupted  by  passing  clouds,  and  occasionally 
gilding  the  varied  scene :  perhaps  darting  through  the  windows  of  a  venerable 
cathedral  and  illuminating  the  interesting  objects  in  its  interior — ^the  rising 


378  LIGHT. 

and  disappearing  of  mist  over  a  landscape — running  water,  as,  for  instance, 
the  cascades  among  the  sublime  precipices  of  Mount  St.  Grothod,  in  Switzer- 
land ; — and  one  of  the  most  striking  scenes  of  all,  a  great  fire  or  conflagra- 
tion. In  the  cosmoramo  of  Regent  Street,  the  great  fire  of  Edinburgh  was 
admirably  represented ;  first,  that  noble  city  was  seen  sleeping  in  darkness 
as  the  fire  began,  then  the  conflagration  grew  and  lighted  up  the  sky,  and  at 
short  intervals,  as  the  wind  increased,  or  as  roofs  fell  in,  there  were  bursts 
of  flame  towering  to  heaven,  and  vividly  illuminating  every  wall  or  spire  which 
caught  the  direct  light — then  the  clouds  of  smoke  were  seen  rising  in  rapid 
succession  and  sailing  northward  upon  the  wind,  until  they  disappeared  in 
the  womb  of  distant  darkness.  So  naturally  was  all  this  represented,  that 
no  stranger  can  have  viewed  the  appalling  scene  with  indifference,  while  on 
those  who  knew  the  city,  the  effect  can  scarcely  have  been  weaker,  than  if 
they  had  witnessed  the  reality.  The  mechanism  for  producing  such  effects 
is  very  simple;  but  spectators,  that  they  may  fully  enjoy  them,  need  not 
particularly  inquire  about  it. 

It  is  remarkable,  when  the  imagination  is  once  excited  by  some  beautiful 
or  striking  view,  how  readily  any  visual  hint  produces  clear  and  strong 
impressions.  One  day  in  the  cosmorama,  a  school-boy  visitor  exclaimed 
with  fearful  delight  that  he  saw  a  monstrous  tiger  coming  from  its  den  among 
the  rocks; — it  was  a  kitten  belonging  to  the  attendant,  which  by  accident 
had  strayed  among  the  paintings.  And  another  young  spectator  was  heard 
calling  that  he  saw  a  horse  galloping  up  the  mountain  side; — it  was  a 
minute  fly  crawling  slowly  along  the  canvas.  There  is,  in  this  department, 
a  very  fine  field  yet  open  to  the  exercise  of  ingenuity,  for  the  contemplation 
of  pictures  representing  motion  or  progressive  events,  may  be  made  the 
occasion  of  mental  excitement  the  most  varied  and  intense.  For  instance, 
there  are  few  scenes  on  earth  calculated  to  awaken  more  interesting  reflec- 
tions on  the  condition  of  human  nature  than  that  beheld  by  a  person  who 
sails  along  the  river  Thames  from  London  to  the  sea,  a  distance  of  about 
sixty  miles,  through  the  wonders  which  on  every  side  there  crowd  on  the 
sight — the  fores'ts  of  masts  from  all  parts  of  the  world — the  glorious  monu- 
ments of  industry,  of  philanthropy,  of  science — the  endless  indications  of 
the  riches,  the  high  civilization,  and  progressive  happiness  of  the  people. 
Now  this  scene  was  lately  in  one  of  our  theatres,  strikingly  portrayed  by 
what  was  called  a  moving  panorama  of  the  southern  bank  of  the  Thames. 
It  was  a  very  long  painting,  of  which  a  part  only  was  seen  at  a  time  gliding 
slowly  across  the  stage,  and  the  impression  made  on  the  spectators  was  that 
they  themselves  were  sailing  down  the  river  in  a  steamboat,  and  viewing  the 
fixed  realities.  In  the  same  manner  might  be  most  interestingly  represented 
the  whole  coast  of  Britain,  or  any  other  coast,  or  any  line  or  road,  or  even  a 
line  of  balloon  flight.  There  was  another  moving  panorama  exhibited  about 
the  same  time  at  Spring  Garden,  aiming  at  an  effect  of  still  greater  difficulty, 
viz.,  to  depict  a  course  of  human  life  ;  and  the  history  chosen  was  that  of  the 
latter  part  of  Bonaparte's  career.  Scenes  representing  the  principal  events, 
were,  in  succession,  made  to  glide  across  the  field  of  view,  and  were  so 
designed  that  the  real  motion  of 'the  picture  gave  to  the  spectator  the  feeling 
that  the  events  were  then  in  progress ;  and  with  the  accompaniments  of  clear 
narration  and  suitable  music,  they  produced  on  those  who  viewed  them  the 
most  complete  illusion.  The  story  began  by  recalling  the  blow  struck  at 
Bonaparte's  ambition  in  the  battle  of  Trafalgar;  and  to  mark  how  completely, 
by  representations  of  various  movements  and  situations  of  the  battle,  the  spec- 
tators were  in  imagination  made  present  to  it,  the  author  may  mention  that 


T  H  E  E  Y  E — P  A  I  N  T  I  N  G  REPRESENTING  MOTION.    379 

on  the  occasion  of  his  visiting  the  exhibition,  a  young  man  seeing  a  party  of 
British  represented  as  preparing  to  board  an  enemy's  ship,  started  from  his 
seat  with  a  hurrah,  and  seemed  quite  confounded  when  he  discovered  that 
he  was  not  really  in  the  battle.  To  the  views  of  Trafalgar  succeeded  many 
others,  similarly  introduced  and  explained,  in  each  of  which  the  hero  himself 
appeared  :  there  were  his  defeat,  at  Waterloo — his  subsequent  flight — his 
delivery  of  himself  to  the  British  admiral — his  appearing  at  the  gangway  of 
the  Bellerophon  to  thousands  of  spectators  in  boats  around,  while  in  Plymouh 
harbour,  previous  to  his  departure  for  ever  from  the  shores  of  Europe — his 
house  and  habits  during  his  exile,  with  various  .picturesque  views  of  St. 
Helena ; — and  last  of  all,  that  solemn  procession,  in  which  the  bier  with  his 
lifeless  corpse  was  moving  slowly  on  its  way  to  the  grave  under  the  willow- 
tree.  The  exhibition  now  spoken  of  might  have  been  made  better  in  all 
respects,  yet  in  its  mediocrity  it  served  to  prove  how  admirably  adapted  such 
unions  of  painting,  music,  and  narration  are  to  affect  the  mind,  and  there- 
fore to  become  the  means  of  conveying  most  impressive  lessons  of  historical 
fact  and  moral  principle. 

Painting,  whether  employed  to  portray  scenes  of  entirely  still-nature,  or 
scenes  involving  some  kind  of  motion  as  above  described,  has  still,  as  its 
great  aim  or  end,  merely  to  represent  interesting  subjects,  and  to  give  to,  the 
spectator  as  much  as  possible  that  clear  conception  of  them  which  is  obtained 
by  ocular  examination  of  realities;  and  thus,  as  a  system  of  visual  signs  of 
thought,  it  becomes,  like  language,  which  is  a  system  of  audible  signs,  a 
means  of  expanding  the  boundaries  of  individual  human  existence  into  wider 
space  and  time,  and  thus  of  elevating  human  nature.  While  it  portrays  only 
strict  matters  of  fact,  whether  of  past  or  present  time,  as  particular  human, 
individuals,  objects  of  natural  history,  the  beautiful  and  magnificent  scenes 
of  nature,  interesting  events  which  the  artist  has  the  means  of  faithfully 
representing,  &c.,  it  may  be  called  truly  historical  painting,  embodying  the 
materials  of  true  history,  both  natural  and  civil,  and  then  it  is  of  singular 
value.  But  even  when  applied  to  other  purposes,  it  may  be  fraught  with 
delight ;  and  just  as  language,  of  which  the  grand  object  or  use  is  to  express 
strict  truths,  has  still  been  admirably  employed  in  giving  a  permanent  exist- 
ence to  a  variety  of  fictions,  from  the  wildest  fables  and  rhapsodies  to  the 
historical  plays  and  novels  of  modern  times,  as  those  of  Shakspeare  and  of 
Scott — which  plays  and  novels,  although  not  furnishing  true  portraits  of 
individual  human  nature,  are  yet  most  correct  portraits  of  general  human 
nature — so  may  painting  be  employed  in  embodying  fictions  adapted  to  its 
peculiar  powers,  and  it  may  do  so  in  a  manner  to  prove  the  artist  endowed 
with  the  highest  degree  of  human  genius.  It  should  always  be  recollected, 
however,  that  what  is  usually  dignified  with  the  name  of  historical  painting, 
really  bears  to  historical  truths  only  the  kind  of  relation  which  novels  and 
plays  bear  to  it,  and  often  approaches  even  less  nearly  to  the  truth ;  for  it 
pretends  to  relate  a  thousand  minute  circumstances  which  no  history  has 
preserved,  and  which,  therefore,  only  the  imagination  of  the  artist  can  supply. 
Thus  when  a  painter,  knowing  that  Lucretia  stabbed  herself  in  the  presence 
•of  her  father  and  others,  after  the  crime  of  Tarquin,  exhibits  a  woman  dying, 
and  a  certain  number  of  persons  around  her  in  horror  and  astonishment,  he 
no  more  represents  the  real  Lucretia  and  her  friends  than  he  represents  any 
other  particular  young  woman  and  her  friends;  for  he  is  quite  assured  that 
not  one  of  the  figures  in  such  a  picture  is  a  portrait  of  the  individual  whose 
name  it  bears;  his  picture,  therefore,  in  so  far, is  as  untruth  or  fiction,  while 
it  very  probably  has  some  of  the  additional  errors  and  even  absurdities  so 


380  LIGHT. 

common  among  historical  painters,  in  respect  of  national  usage  in  costume, 
religion,  manners,  &c,  and  in  respect  to  the  general  personal  appearance, — 
as  when  a  Rubens,  wishing  to  represent  Sabine  or  other  ladie,s,  gave  them 
the  Dutch  corpulency  deemed  comely  in  his  own  country,  although  it 
strikingly  contrasted  with  the  true  forms  of  Italian  or  Grecian  nymphs. 
From  all  this  it  appears  that  historical  pictures  may  often  be  regarded  as 
portraitures,  not  of  the  realities,  but  of  comedians  acting  scenes  in  historical 
plays  intended  to  represent  the  realities. 

In  dealing  with  the  events  of  ordinary  history,  there  is  no  strong  reason 
why  artists  may  not  please  themselves  and  their  spectators  as  we  have  now 
been  describing ;  but  it  may  admit  of  doubt  whether  similar  liberties  should 
be  allowed  with  respect  to  religion  Yet  any  painting  of  the  last  supper,  for 
instance,  or  of  the  ascension,  is  not  more  true  than  a  theatrical  representation. 
To  judge  of  the  nature  of  such  a  picture  we  have  only  to  suppose  any  of  the 
events  recorded  in  the  New  Testament  to  be  represented  by  a  painter  in 
China  with  the  countenances  seen  on  Chinese  tea-boxes ;  such  a  representa- 
tion would  appear  in  Europe  revoltingly  absurd;  but  the  common  practice 
here  is  only  a  degree  better,  Italian  countenances  being  usually  substituted 
for  the  Jewish;  and  twenty  painters,  undertaking  the  same  subject  gene- 
rally put  different  persons  into  all  the  situations.  Then  it  can  produce  no 
pleasing  impression  on  a  Christian's  mind  to  be  told,  that  an  admired  paint- 
ing of  the  crucifixion  was  made  chiefly  from  the  body  of  an  executed  mur 
derer;  or  that  fora  praised  representation  of  the  triumphal  entry  into  Jeru- 
salem, the  painter  had  deemed  his  own  physiognomy  the  most  befitting  for 
the  principal  figure,  while  he  copied  the  portrait  of  Voltaire  as  a  specimen 
9 '  the  bad  Jews,  of  Newton  as  a  specimen  of  the  good,  and  of  wife,  cousins, 
acquaintances,  and  old  clothes-men,  to  make  up  the  remaining  groups.  With 
the  knowledge  that  such  things  have  often  been,  it  need  not  surprise  that 
many  persons  of  correct  feeling  turn  with  horror  from  all  these  mimicries 
and  falsehoods,  to  seek  their  idea  of  God  and  his  providence  in  the  sublime 
descriptions  of  his  attributes,  which  written  language  conveys  and  which  all 
creation,  in  a  mute  language  not  less  impressive,  so  strongly  confirms.  When 
men  generally  could  not  read,  and  as  a  mass  were  extremely  ignorant,  various 
means  of  fixing  their  attention  upon  religious  subjects  might  be  useful,  and 
therefore  proper,  as  sacred  plays,  certain  processions,  pictures,  &c.,  which 
have  now  in  many  countries  ceased  to  be  either;  but  a  person  of  good  sense 
will  continue  to  regard  with  a  certain  respect  whatever  at  any  time  may  have 
contributed  to  reclaim  portions  of  mankind  from  barbarism  and  wickedness 
to  the  just  appreciation  as  the  divine  charities  of  a  pure  religion. 

There  are  in  painting  other  classes  of  fictions,  which  pretend  to  nothing 
beyond  fiction,  and  which  yet  are  truly  admirable;  such  are  personifications 
of  the  virtues  and  vices,  serving  to  recommend  the  practice  of  the  former, 
and  to  deter  from  that  of  the  latter — almost  all  Hogarth's  works  are  of  this 
character,  and  evince  the  highest  mental  acumen  and  genius  : — then  may  be 
mentioned  the  personifications  of  what  have  been  called  the  elements  and 
powers  of  nature,  including  many  of  the  personages  of  the  heathen  mytho- 
logy— then  other  generalizations  of  the  characteristics  of  human  or  other 
nature,  as  scenes  of  domestic  affection,  of  the  play  of  the  passions,  &c.,  &c.; 
and  because  many  subjects  when  so  sketched,  are  intelligible  to  the  eye  with 
the  suddenness  of  lightning,  where  longest  verbal  description  would  convey 
the  idea  but  imperfectly,  the  art  of  painting,  in  regard  to  them,  possesses  a 
truly  magical  and  inestimable  power. 

As  painting,  whether  employed  to  represent  matters  of  fact  or  of  fiction, 


THE    TELESCOPE.  381 

can  accomplish  its  ends  only  through  the  means  of  drawing  or  linear  per- 
spective, and  of  shading  and  colouring,  or  aerial  perspective,  these  subjects 
require  to  be  studied  by  every  artist  with  great  attention  ;  but  it  is  important 
for  all  to  be  aware  that  the  greatest  mastery  over  these,  which  are  merely  the 
mechanical  parts  of  the  art,  will  go  a  very  short  way  towards  producing  good 
performances,  unless  there  be  present  also  the  genius  to  select  or  to  compose 
subjects  worthy  of  being  represented, — indeed,  will  go  little  farther  to  make 
a  painter  than  the  learning  of  mere  penmanship  goes  to  make  a  historian  or 
a  poet.  This  remark  seems  the  more  necessary,  because  there  is  in  human 
nature  a  disposition  to  value  so -much  the  means  by  which  important  ends  are 
attained,  that  often  the  end  itself  is  forgotten  in'  the  contemplation  of  the 
means — as  when  a  person,  perceiving  that  money  will  procure  all  desirable 
things,  at  last  becomes  the  insane  miser,  and  dies  from  want  of  the  common 
necessaries  of  life  rather  than  touch  his  hoarded  treasures  : — while  among 
painters,  as  among  persons  of  other  occupations,  the  talent  for  the  inferior 
or  more  mechanical  departments  of  the  art,  is  more  common  than  for  the 
higher.  Do  we  not  see  the  subordinate  accomplishments  of  the  painter,  by 
not  a  few,  both  artists  and  pretended  connoisseurs,  supposed  to  be  the  prin- 
pal?  But  this  is  evidently  to  value  the  dress  or  clothing  instead  of  the 
person ;  or  like  the  bibliomaniac,  to  regard  the  type  and  binding  of  books 
more  than  the  subject.  To  prove  how  unessential  what  is  called  high- 
finishing  in  painting,  is  to  the  complete  attainment  of  the  purposes  of  the 
art,  we  may  instance  the  cartoons  of  the  immortal  Raphael,  which  to  the 
mere  mechanic  in  art  appear  almost  daubs,  although  exciting  such  enthu- 
siasm in  the  superior  mind  :  and  many  of  the  mere  sketches  of  genius  are 
to  a  true  taste  more  precious  than  some  of  the  most  labored  pieces  in  our 
galleries.  As  it  is  of  no  importance  to  a  man  who  sees  approaching  the 
friend  of  his  heart,  whether  it  be  by  day-light  or  candle-light,  or  with  the 
source  of  light  above  or  below,  &c.,  provided  there  is  light  enough  for  him 
to  distinguish  clearly  the  friend  of  his  heart,  so  is  it  of  no  importance  how 
any  interesting  subject  is  represented,  provided  the  picture  vividly  excite  a 
true  conception  of  the  subject.  A  painter  will  discover  the  difficulties  which 
a  brother  artist  had  to  surmount  in  representing  an  object  in  some  particular 
predicament,  as  regards  the  light,  &c.,  and  may  estimate  the  talent  accord- 
ingly :  but  the  great  proportion,  even  of  the  most  accomplished  ordinary 
spectators,  will  generally  be  looking  beyond  the  sign  to  the  thing  signified, 
heedless  of  the  artist's  difficulties.  In  consequence,  however,  of  the  preju- 
dice in  favor  of  l(  a  sweet  or  adorable  bit  of  colouring,"  as  it  will  sometimes 
be  called — and  which  in  truth  may  have  the  merit  of  most  natural  colouring, 
there  are  preserved  in  many  galleries  pictures  disgusting  in  almost  all  other 
respects,  as  of  drunken  Dutch  boors,  with  fiery  noses  and  physiognomies 
degrading  to  human  nature,  &c.,  &c.,  on  seeing  which  the  man  of  taste 
deplores  that  the  art  of  painting  should  so  often  have  been  prostituted  by 
clever  men  to  the  vile  purpose  of  representing  things  of  worse  than  no  interest. 

"  When  the  image  formed,  as  above  described,  beyond  a  lens,  is  viewed  in 
the  air  by  an  eye  placed  still  farther  beyond  in  the  same  direction,  the 
arrangement,  according  to  minor  circumstances,  constitutes  either  the 
common  TELESCOPE  or  MICROSCOPE."  Read  the  whole  second  paragraph 
of  the  Analysis,  page  325. 

The  name  of  telescope  (a  compound  Greek  term,  signifying  to  see  far,  as 
microscope  signifies  to  see  what  is  small,)  applies  to  that  wondrous  instru- 
ment of  modern  invention  b^  the  use  of  which  the  intelligent  soul  may  be 


382  LIGHT. 

said,  on  the  beams  of  light  as  its  path,  to  dart  widely  into  space  for  the  pur- 
pose of  contemplating  the  distant  glories  of  creation  ;  or  again,  by  which  it 
can  command  distant  objects  instantly  to  approach,  for  the  purpose  of  conve- 
nient inspection.  In  ancient  times,  a  man',  while  looking  with  admiration 
on  the  bright  face  of  the  moon,  might  have  exclaimed  "  How  pleased  would 
I  be,  had  I  the  power  to  fly  upwards  to  that  celestial  orb,  the  better  to  under- 
stand its  nature  and  beauties ;"  but  he  could  little  have  anticipated  that  the 
day  was  coming  when  human  ingenuity  would  find  means  in  a  great  measure 
to  satisfy  the  wish  : — now  the  telescope  is  this  means,  for  one  which  merely 
doubles  apparent  magnitudes,  shows  the  moon  exactly  as  she  would  appear 
to  a  person  who  had  ascended  towards  her  from  the  earth  a  distance  of  120,000 
miles,  while  one  of  greater  power  produces  effects  correspondingly  great. 
— But  to  examine  the  heavenly  bodies  is  by  no  means  the  only  use  of  the 
telescope,  man  being  often  extremely  interested  to  discover  what  is  passing 
at  a  distance  on  the  surface  of  the  earth  around  him.  Thus,  by  a  telescope, 
the  military  chief  obtains  a  close  view  of  approaching  friends  or  foes,  while 
they  are  yet  concealed  from  the  naked  eye.  in  the  blue  mist  of  distant  moun- 
tain or  plain — and  similarly  'the  sea-captain,  while  persons  around  him  per- 
ceive only  a  little  speck  on  the  far  horizon,  discovers  there  a  ship  of  class  and 
nation  at  once  evident  to  him,  and  with  the  crew  of  which,  by  the  additional 
use  of  signal  flags,  he  is  enabled  readily  to  converse  At  midnight,  a  tele- 
scope directed  to  a  distant  cathedral,  may  so  effectually  call  it  into  the  presence 
of  the  observer,  that  on  the  clock-turret  may  be  watched  the  progress  of  the 
slow  hands  which  tell  of  the  unceasing  lapse  of  time.  A  man,  in  the  midst 
of  a  wide  plain,  or  on  a  lofty  hill-top,  or  far  on  the  face  of  a  lake,  who  might 
suppose  himself  quite  alone  and  unseen,  would  yet,  by  a  telescope,  be 
instantly  placed  under  the  observation  of  whoever  chose  to  watch  him.  And 
the  same  might  happen  to  a  man  within  the  high  walls  of  his  own  garden, 
or  even  within  his  house  near  an  open  window,  if  a  straight  line  could  reach 
him  from  some  station  where  an  observer  was.  Some  remarkable  cases  of 
actions,  imagined  by  the  parties  to  have  been  done  in  perfect  secrecy,  have 
thus  been  brought  to  light. 

Now  the  telescope,  with  its  extraordinary  powers,  exhibits  but  another 
modification  of  the  simple  case  (described  at  page  341,  and  exemplified  in 
the  camera  obscura,  &c.,)  of  an  image  formed  for  visual  inspection  beyond 
a  lens  And  we  shall  here  explain  that  its  powers  depend  altogether  on  the 
two  circumstances,  first  of  its  large  lens  collecting,  for  the  formation  of  the 
image  (subsequently  transferred  to  the  observer's  retina)  a  thousand  times 
or  more  the  quantity  of  light  which  the  naked  pupil  could  receive ;  and 
second,  of  its  forming  by  this  light  an  image,  to  which  the  eye  may  be  brought 
very  near,  so  as  to  examine  it  with  magnifying  glasses  of  any  power. 

To  understand  this  well,  we  must  recall,  that  the  nature  of  the  bending  of 
light  in  passing  through  a  lens  is  such,  that  all  the  rays  reaching  the  lens 

Fig.  160. 


from  any  point  of  a  visible  object  in  front,  and  forming  what  is  called  a  pencil 
of  light — as  that  spreading  from  the  point  A  of  the  cross  here  represented,  to 


THE    TELESCOPE;  383 

the  lens  L — are  collected  in  a  corresponding  point,  as  a,  at  the  focal  dis- 
tance beyond  the  lens,  so  as  always  to  meet  the  central  ray  of  the  pencil 
here,  (the  direct  line  A  a,)and,  therefore,  when  the  light  comes  from  above 
the  centre  of  the  lens,  the  focal  meeting  is  below,  as  shown  here;  and  when 
it  comes  from  below,  the  meeting  is  above;  then  the  same  happening  as  re- 
gards every  visible  point  of  the  object  (the  rays  from  only  the  two  extreme 
points  A  and  B  are  here  represented)  at  corresponding  points  beyond  the 
lens  in  the  space  between  a  and  6,  the  collected  light,  if  received  on  a  white 
screen  placed  there,  as  in  the  camera  obscura,  will  make  apparent  to  an  eye 
in  any  direction  a  beautiful  inverted  image  of  the  object.  Now  in  the  place 
where  the  rays  meet  to  form  this  image,  if  no  screen  be  interposed,  the  rays, 
although  not  lost  or  destroyed,  but  merely  cross  each  other  in  the  air,  with- 
out interference,  nearly  as  they  previously  crossed  in  the  lens,  and  spread 
again  beyond  the  focal  points,  or  towards  c  as  here  shown,  as  they  origi- 
nally spread  from  the  several  points  of  the  object  itself;  an  eye,  therefore, 
placed  anywhere  beyond  c,  must  receive  portions  of  the  pencil  from  every 
point  of  the  image,  and  may  see  the  image  in  the  air  as  it  would  see  an 
object  situated  where  the  image  is,  in  the  focus  of  the  lens.  This  may  be 
observed  at  once  by  holding  a  spectacle-glass  or  any  lens  at  a  proper  distance 
between  an  object  and  the  eye. 

Now  a  telescope  is  merely  a  long  tube,  blackened  within  to  exclude  and 
destroy  useless  light,  and  having  a  large  lens,  called  the  object-glass,  filling 
its  distant  end,  to  gather  the  light  from  the  objects  in  front,  and  with  that 
light  to  form  images  towards  the  other  or  near  end  of  the  tube,  where  the 
eye  may  conveniently  inspect  them.  These  images,  for  a  purpose  to  be  im- 
mediately explained,  are  examined  through  another  lens  called  the  eye-glass, 
which  is  fixed  in  a  small  tube  made  to  slide  backwards  and  forwards  in  the 
large,  so  as  to  admit  of  the  focal  distances  being  adjusted  to  the  power  of 
different  eyes,  &c.  The  accompanying  sketch  shows  the  progress  of  the 


light  from  the  object  A,  through  the  object-glass  L,  to  form  an  image  5  a, 
and  afterwards  to  be  bent  by  the  eye-glass,  1),  so  as  to  enter  the  pupil  of 
the  eye  at  E,  where  the  rays  cross,  to  form  the  last  image  on  the  retina. 

In  the  simple  telescope  with  only  two  lenses,  as  above  represented,  called 
the  astronomical  telescope}  or  the  night- telescope ,  because  chiefly  used  at  night, 
the  image  is  inverted ;  but  this  is  a  circumstance  of  no  importance  in  view- 
ing the  heavenly  bodies;  to  fit  the  instrument,  however,  for  viewing  terres- 
trial objects,  it  is  necessary  to  place  in  the  tube  another  simple  or  compound 
lens,  which  shall  form  a  second  image  from  the  first,  and  by  inverting  a 
second  time,  shall  produce  an  image  equally  upright. 

To  determine  how  much  larger  an  object  will  appear  when  viewed  through 
a  certain  telescope, — for  instance,  through  one  with  an  object-glass  of  three 
feet  focus, — than  when  viewed  by  the  naked  eye,  we  must  recollect  that  the 
image  is  formed  in  the  focus  of  the  object-glass,  or  at  I  a,  in  the  last  figure, 
and  subtends  from  the  centre  of  that  glass  or  lens,  the  same  visual  angle  as 
the  object  itself  (a  fact  explained  at  page  357,)  and  to  an  eye  placed  there 


384  LIGHT. 

would  appear  of  the  same  size  as  the  object,  but  if  the  eye  be  brought  nearer 
to  the  image  than  the  centre  of  the  object-glass,  the  image  will  appear  by  so 
much  taller  and  broader,  £nd  thus,  as  compared  with  the  object,  may  be 
called  so  much  magnified.  Now  as  the  naked  eye  cannot  see  distinctly  an 
object  nearer  to  it  than  at  about  six  inches,  because  of  the  great  divergence 
of  light  from  a  nearer  radiant  point,  the  telescope  in  question,  without  an 
eye-glass,  would  allow  the  eye  to  come  only  six  times  nearer  to  the  image 
than  when  at  the  centre  of  the  object-glass  and  would  only  magnify  the 
diameter  six  times;  but  if  then  an  eye-glass,  as  D,  of  half  an  inch  focus 
were  placed  half  an  inch  from  the  image,  so  as  to  render  the  rays  of  every 
pencil  parallel,  and  therefore  fitted  to  the  powers  of  the  eye,  while  the  dif- 
ferent parcels  would  cross  each  other  a  little  way  beyond  the  glass,  as  shown 
above,  an  eye  placed  to  receive  in  its  pupil  the  crossing  parcels,  would  see 
the  image  as  large  as  if  at  half  an  inch  from  it,  and  therefore  72  times  nearer 
than  if  viewed  from  the  object-glass,  and  therefore  again  as  of  72  times  greater 
diameter.  Now,  as  in  all  cases,  the  image  in  a  telescope  is  in  the  focus  both 
of  the  object-glass  and  eye-glass,  and  is  therefore  nearer  to  the  latter  than  to 
the  former  in  proportion  as  their  focal  distances  differ,  the  magnifying  power 
is  measured  by  that  difference — in  the  case  at  present  supposed,  the  differ- 
ence is  as  72  to  1,  and  72  is  the  magnifying  power  of  the  telescope.  The  rule 
is  generally  thus  expressed,  "  divide  the  focal  distance  of  the  object-glass  by 
that  of  the  eye-glass,  and  the  quotient  is  the  magnifying  power."  It  is  always 
to  be  remembered,  that  if  the  diameter  of  an  object  be  magnified  10  times, 
the  surface  is  magnified  100  times,  and  so  in  proportion  for  other  jiumbers. 

With  such  means  of  aiding  the  sight,  then,  it  is  that  we  discover  the 
mountains  of  our  moon,  and  can  even  measure  their  altitudes;  that  we  can 
see  the  four  beautiful  moons  of  the  planet  Jupiter;  that  we  can  perceive 
marks  and  irregularities  on  the  surfaces  of  the  other  planets,  enabling  us  to 
say  at  what  rate  they  severally  whirl  round  their  axes,  experiencing  the 
phenomena  of  day  and  night; — and  that  we  can  determine  many  other 
interesting  particulars. 

The  discovery  of  the  telescope  is  said  to  have  been  first  made  accidentally 
by  the  children  of  a  Dutch  spectacle-maker,  while  playing  with  their  father's 
work ;  but  it  was  turned  to  no  use  until  Galileo,  led  by  science,  fell  upon  it 
again,  and  with  the  knowledge  of  its  worth,  obtained  from  it  the  most  sub- 
lime results.  The  human  heart  can  rarely  have  throbbed  with  such  delight 
as  Galileo's  when  he  first  directed  his  optic  tube  to  the  heavens,  and  through 
it  contemplated  so  many  glorious  objects  before  unseen  by  human  eye; — as 
the  planet  Venus,  our  beautiful  morning  and  evening  star,  appearing  not  a 
circle,  but  a  crescent  like  our  moon  in  her  quarters — as  the  satellites  of  Ju- 
piter— the  rings  of  Saturn — myriads  of  stars  until  then  invisible  to  man; 
and  in  a  word,  when  he  beheld  the  undoubted  proofs  of  the  true  system  of 
the  universe,  as  his  genius  had  before  conceived  it,  uniting  the  greatest 
simplicity  with  unspeakable  grandeur. 

The  Galilean  telescope  was  simply  a  large  object-glass  to  collect  much 
light,  with  a  small  concave  eye-glass  placed  so  as  to  intercept  the  converg- 
ing rays  before  they  reach  their  focus,  and  to  change  their  convergency  into 
the  parallelism  which  the  eye  could  command.  This  telescope,  although 
magnifying  less  than  that  made  of  two  convex  glasses,  as  above  described, 
still,  from  occasioning  no  loss  of  light  by  the  crossing  of  rays  in  forming  an 
image,  was  of  considerable  power.  The  common  opera-glass  is  a  telescope 
made  on  this  principle. 

It  was  explained  at  page  338,  that  a  ray  of  light,  in  being  bent  or  refracted 


THE    TELESCOPE.  385 

by  transparent  media,  as  by  a  lens,  is  also  divided  into  rays  of  the  different 
colours  seen  in  the  rainbow.  Hence  an  image  formed  behind  a  simple  lens 
has  coloured  edges  or  fringes.  This  fact  rendered  the  images  of  small  ob- 
jects much  magnified,  in  the  first  made  telescopes,  very  indistinct :  and,  but 
for  the  important  discovery  made  by  Dollond,  the  optician,  that  different 
kinds  of  glass  have  dispersive  and  refractive  powers  of  different  relative 
force,  so  that  a  concave  lens  of  a  certain  curve  applied  to  a  convex  lens 
might  completely  counteract  the  dispersion  of  colour  by  the  latter,  while  it 
left  enough  of  the  convergence  of  the  rays  for  the  formation  of  an  image — 
refracting  telescopes  would  have  always  been  very  imperfect.  Dollond 
called  his  telescopes  achromatic,  or  not-colouring.  It  is  very  remarkable, 
that  he  had  the  fortune  to  obtain  some  glass  for  his  purposes,  more  suitable 
than  any  which  has  been  procured  since,  or  which  could  be  made  by  known 
rules,  until  the  late  improvements  in  the  manufacture  suggested  by  the  inge- 
nuity of  Mr.  Farraday.  The  author  of  this  work  carried  abroad  with  him 
a  small  telescope  of  old  Dollond's,  which  often  gave  more  correct  information 
respecting  minute  colored  objects  at  a  distance,  as  signal  flags  at  sea,  than 
much  larger  glasses  of  modern  make. 

The  MICROSCOPE  of  greatest  power  and  with  the  form  called  compound, 
in  its  structure  approaches  very  closely  to  the  telescope,  the  chief  difference 
being,  that  while  in  the  telescope  a  large  distant  object  forms  in  the  focus  of 
the  object-glass  an  image  exactly  as  much  smaller  than  itself  as  the  distance 
of  the  image  from  the  glass  is  less — in  the  microscope  conversely,  a  small 
object  placed  near  the  focus  of  the  object-glass  produces  a  more  distant 
image,  as  much  larger  than  itself  as  the  image  is  more  distant  than  it — and 
in  the  one  case  as  in  the  other,  the  image  is  viewed  through  an  appropriate 
eye-glass.  The  object-glass  in  the  telescope  is  large,  in  the  microscope  it  is 
generally  very  small.  If,  in  the  latter,  an  object-glass  be  used  of  one-eighth 
of  an  inch  focal  distance,  and  the  object  be  so  placed  that  its  image  is  formed 
at  six  inches,  the  image  will  be  of  diameter  48  times  as  great  as  the  object,  or 
will  have  nearly  2;500  times  as  much  surface  :  and  if  that  image  be  viewed 
through  an  eye-glass  of  half  an  inch  focus,  the  image  will  appear  still  twelve 
times  larger,  or  30,000  times  larger  than  the  object. 

A  simple  convex  lens  is  called  a  single  microscope,  and  it  magnifies,  as 
already  explained,  chiefly  by  allowing  the  eye  to  be  brought  much  nearer  to 
the  object  than  the  distance  at  which  the  object  could  be  ssen  without  the 
glass  ;  but  even  where  the  distance  of  the  eye  and  object  is  not  changed,  a 
lens  interposed  will  still  magnify  by  bending  the  light,  as  at  d  and  /',  making 
that  which  comes  to  the  eye  at  e  from 

the  top  of  such  an  object  as  the  little  •        Fig-  162. 

cross  a,  to  appear  to  come  from  b, 
and  that  from  the  bottom  to  come 
from  Cj  thus  magnifying  the  cross 
here  represented  by  the  black  lines 
to  appear  of  the  size  represented  by 
the  dotted  lines.  A  concave  lens 
minifies  for  the  contrary  reason. 

Perhaps  there  is  not  a  greater  treat  for  a  person  who  has  feeling  for  the 
beauties  of  nature,  than  to  explore  with  the  microscope.  While  the  telescope 
lifts  the  mind  to  the  contemplation  of  boundless  space  occupied  by  myriads 
of  suns,  and  exhibits  this  globe  of  ours  as  less,  compared  with  the  universe 
around  it,  than  a  leaf  is  compared  with  a  forest,  or  one  grain  of  sand  com- 
pared with  all  which  lies  on  the  ocean's  shore,  the  microscope  again  excites 

25 


380  LIGHT. 

new  astonishment  by  showing  on  a  leaf,  or  in  a  single  drop  of  some  water  in 
which  the  leaf  has  been  infused,  thousands  of  living  creatures,  and  of  crea- 
tures not  imperfect  because  thus  small,  but  endowed  with  organs  and  parts 
as  complex  and  curious  as  those  of  an  elephant.  And  he  who  admires  the 
curious  structure  of  a  honey-comb,  may  bend  his  eye  through  the  microscope 
upon  the  cut  surface  of  a  willow-branch,  or  of  other  wood,  there  to  see  a 
similar  structure  more  -wonderful  still ;  or  he  may  compare  the  lace  of  a 
fly's  wing  with  the  most  perfect  which  human  art  can  weave;  or  the  beauti- 
ful proportions  and  perfection  of  the  limbs  and  weapons  of  an  insect,  invisi- 
ble, perhaps,  to  the  naked  eye,  with  any  larger  objects  of  the  kind  already 
known  to  him. 

Telescopes  and  microscopes  might  with  propriety  be  both  called  micro- 
scopes, for  often  the  telescopic  object  subtends  to  the  naked  eye  even  a  smaller 
angle  than  the  objects  which  the  microscope  examines.  The  minutest  visible 
insect  at  hand  may  hide  from  the  eye  a  planet  at  a  distance.  The  image  in 
the  telescope,  however,  is  always  much  smaller  than  in  the  microscope  be- 
cause the  rays  from  a  distance  being  nearly  parallel,  must  form  the  image 
nearly  in  the  principal  focus  of  the  object  glass ;  while  for  the  microscope, 
the  rays  from  the  near  object  being  very  divergent  may  be  made  to  form  the 
image  far  beyond  that  focus,  and  therefore  proportionally  larger. 

"  Light  falling  on  very  smooth  or  polished  surfaces,  is  reflected  so  nearly 
in  the  order  in  which  it  falls,  as  in  many  cases  to  appear  to  the  eye  as  if 
coming  directly  from  the  objects  originally  emitting  it, — and  such  sur- 
faces are  called  MIRRORS  ;  the  surface  of  which  is  flat  as  well  as  polished, 
is  called  a  plane  mirror."  (Read  the  Analysis,  page  325.) 

If,  on  a  marble  slab,  or  other  flat  surface,  (represented  here  at  M  R,  with 
the  edge  supposed  towards  the  spectator,)  a  ball 
Fig.  163.  were  projected  from  A  perpendicularly  towards 

n.,  D,  the  ball  would  rebound  directly  back  to  A, 

but  if  projected  obliquely,  as  from  B  to  D,  it 
would  not  return  to  the  first  situation  B,  but  to 
b,  a  corresponding  situation  on  the 'opposite  side 
of  the  perpendicular,  thus  making  the  angle  of 
the  return  or  reflection  equal  to  the  angle  of 
approach  or  incidence;  the  same  would  be  true 
of  a  ball  approaching  obliquely  from  any  other 
point,  as  C,  and  rebounding  to  c.  Now  light 
'is  reflected  from  polished  surfaces  according  to 
the  same  law,  so  that  an  eye  at  A  would  see 
itself  as  if  placed  at  d,  an  eye  at  b  would  see  an 
object  really  at  B,  as  if  it  were  at  e,  and  so  forth. 
Where  the  existence  of  a  mirror  is  not  suspected,  the  objects  reflected  from 
it  are  held  to  be  realities  placed  beyond  where  it  is.  A  wild  animal  will 
attack  its  image  in  a  glass  :  and  the  fable  says  that  a  dog  crossing  a  brook, 
quitted  the  piece  of  meat  in  its  mouth  to  catch  the  tempting  image  which  he 
saw  in  the  water  below.  The  reason  that  an  object  seen  in  a  plane  mirror 
appears  to  be  just  as  far  beyond  the  mirror  as  its  true  distance  on  the  side 
of  the  spectator,  is,  that  the  diverging  rays  of  a  pencil  of  light  have  the 
same  divergence  after  as  before  reflection.  , 

Any  plane  very  smooth  surface  reflects  light  as  now  described,  and  is  a 
mirror;  but  different  substances  send  back  very  different  proportions  of  the 


ID 


MIRRORS.  387 

light  which  falls  on  them.  A  highly  polished  metallic  surface  is  the  best 
mirror,  often  returning  three-fourths  of  the  whole  light.  Hence,  in  reflect- 
ing telescopes,  the  mirrors  are  made  of  polished  metals. 

Our  common  looking-glasses  are  really  metallit  mirrors,  for  it  is  the 
smooth,  clear  surface  of  the  quicksilvered  tin  foil  behind  the  glass  which  re- 
flects the  light,  the  glass  itself  merely  serving  the  purpose  of  preserving  the 
metallic  surface  perfectly  clean  and  flat.  There  is  always  an  imperfection 
in  such  glass  mirrors,  when  used  for  viewing  objects  obliquely,  because  the 
external  surface  of  the  glass  acts  also  as  a  mirror,  although  so  much  more 
feebly  than  the  metal  behind,  and  forms  a  separate  image  not  quite  coincid- 
ing with  the  other,  and  therefore  mixing  with  and  confusing  it. 

The  mirror-power  of  glass  unaided  is  seen  from  the  panes  of  a  plate-glass 
window,  which  make  objects  in  front  very  visible,  although  by  no  means 
with  clearness  comparable  to  that  from  a  metallic  surface.  All  common 
panes  of  glass  in  windows,  or  in  print  frames,  &c.,  reflect  as  much  light  as 
plate-glass,  but  the  reflection  being  irregular  because  the  surface  is  irregular, 
scarcely  attracts  notice. 

The  smooth  surface  of  a  fluid  ^  a  mirror,  which  is,  moreover,  horizontal ; 
and  when  that  surface  is  metallic,  as  of  mercury,  the  mirror  is  most  perfect. 
In  water,  spirits,  oil^or  any  other  liquid,  it  is  also  perfect,  but  feebler. 

The  mirror  of  liquid  quicksilver  is  sometimes  used  by  astronomers  in 
observing  the  apparent  altitudes  of  the  heavenly  bodies,  for  the  image  in  the 
mirror  appearing  exactly  as  much  below  the  horizon  as  the  object  is  really 
above  it,  half  the  distance  between  them  is  the  true  height. 

A  varnished  picture  or  any  japanned  surface,  is  a  mirror:  nay,  even  a 
polished  table  of  mahogany  or  other  wood — as  it  is  well  known  among  play- 
ful children.  The  author,  while  writing  this,  has  before  him  a  table  covered 
with  black  leather,  and  in  that  covering,  as 'a  mirror,  he  clearly  sees  the 
bright  objects  beyond  the  table.  Polished  stones,  as  marble  slabs,  &c.,  reflect 
as  much  as  glass.  Even  a  surface  of  air  may  be  a  mirror,  as  where  a  cold 
and  dense  stratum  happens  to  lie  in  contact  with  a  warmer  and  rarer  stratum. 
In  such  cases,  where  particular  causes  have  unequally  heated  different  levels 
of  the  atmosphere,  the  trees,  islands,  £c.,  happening  to  be  below,  are  reflected 
from  above,  and  appear  as  if  in  the  sky.  This  phenomena  is  called  mirage. 
It  is  often  to  be  observed  over  the  burning  sands  of  Africa,  where  the  air  is 
much  heated ;  and  elsewhere  certain  kinds  of  mists  and  thin  clouds  produce 
a  similar  effect,  causing,  for  instance,  a  ship  to  appear  as  if  suspended  aloft, 
with  keel  uppermost. 

In  certain  cases,  an  object  seen  by  the  light  reflected  from  a  mirror  appears 
reversed,  as  when  the  right  hand  of  a  person  standing  before  a  glass  becomes 
the  type  for  the  left  hand  of  the  image ;  or  when  a  tree,  or  rock,  or  moun- 
tain, seen  in  the  mirror  of  a  lake,  has  its  top  downwards. 

It  is  on  this  account,  that  a  man  painting  his  own  portrait  from  a  mirror, 
is  apt  to  reverse  all  the  accidental  characteristics  of  the  countenance  or  per- 
son, not  the  same  on  both  sides ;  and  if,  as  is  generally  true,  one  eye  be 
higher  than  the  other,  or  the  nose  be  a  little  to  one  side,  a  very  incorrect 
resemblance  will  be  produced.  Hence  also  a  person  whose  countenance  is 
at  all  thus  peculiar  never  sees  himself  in  a  mirror  as  he  appears  to  others; 
and  a  belle  or  beau,  who  has  decided  that  a  curl  is  more  graceful  on  the  left 
temple,  may  unconsciously  leave  it  on  the  right. 

By  an  image,  however  reflected  from  a  first  mirror  to  a  second,  and  from 
that  to  the  eye,  persons  may  see  the  object,  or  themselves,  if  they  choose, 
as  others  see  them.  What  a  pity  that  there  are  not  some  moral  mirrors  to 
answer  an  analogous  purpose  ! 


388  LIGHT. 

A  candle  placed  between  two  parallel  mirrors  fixed  on  opposite  sides  of  a 
room,  makes  visible  in  either  glass  to  a  spectator  near  the  middle  of  the 
room  an  endless  straight  line  of  lights.  If  the  glasses  be  inclined  to  each 
other,  the  lights  will  appear  as  if  placed  in  the  circumference  of  a  circle,  of 
which  the  centre  is  where  the  prolonged  mirrors  would  meet :  this  fact  is 
well  illustrated  in  the  beautiful  toy  called  the  kaleidoscope.  It  is  possible 
to  place  a  few  mirrors  in  such  situations  around  an  apartment,  that  a  man 
entering  it,  may  see  himself  multiplied  into  a  crowd,  and  a  few  ornamental 
pillars  may  produce  the  effect  of  thousands  formed  into  long  colonades  of 
retiring  lines. 

The  sun  or  moon  reflecting  in  a  still  lake,  appear  as  they  do  in  the  sky ; 
but  if  the  surface  of  the  water  become  at  all  ruffled  by  the  breeze,  instead 
of  one  distinct  image,  there  will  be  a  long  line  of  bright  tremulous  reflection. 
The  reason  of  this  appearance  is,  that  every  little  wave,  in  an  extent  perhaps 
of  miles,  has  some  part  of  its  rounded  surface  with  the  direction  or  obliquity 
which  according  to  the  required  relation  of  the  angles  of  incidence  and  reflec- 
tion, fits  it  to  reflect  the  light  to  the  eye,  and  hence  every  wave  in  that  extent 
sends  its  momentary  gleam,  which  is  succeeded  by  others. 

Although  the  external  surface  of  glass  reflects  but  a  small  part  of  the  light 
which  falls  upon  it,  being,  therefore,  a  feeble  mirror,  still  curiously,  if  light 
which  has  entered  a  piece  of  glass,  fall  very  oblique- 
Fig.  164.  ]y  Up0n  the  back  or  internal  surface,  instead  of 
passing  out  there,  it  is  more  perfectly  reflected  than 
it  would  be  by  the  best  metallic  mirror.  This  light 
from  A  entering  a  piece  of  glass  at  B,  is  entirely 
reflected  at  C,  the  back  of  the  piece,  and  escapes 
at  D  towards  E.     The  back  of  a  wedge  of  glass, 
or  common  prism,  thus  becomes  a  perfect  mirror. 
It  is  this  fact  which  enabled  Dr.  Wallaston  to  construct  that  beautiful  little 
instrument  called  by  him  the   Camera  Lucida.     The  two  surfaces  at  the 
back  of  the  small  prism  of  glass  A  become  mir- 
Fig.  165.  rors,  the  first  reflecting  to  the  second,  and  the 
second. to  the  eye  at  E,  the  objects  in  the  land- 
scape before  it,  while  the  eye  also  sees  through 
the  glass  to  the  paper  below  at  B,  and  may  sup- 
pose the  imagery  to  be  feebly  portrayed  on  the 


r 


A\ 


~~  paper:  with  a  pencil  that  appearance  is  made 
permanent,  and  a  correctly-drawn  outline  of  the 
scene  is  at  once  obtained  The  instrument  for 
assisting  draftsmen  is  still  simpler  than  the  ca- 
mera obscura.  Other  modifications  of  the  instru- 
ment have  since  been  contrived. 

The  same  fact  of  the  internal  surface  of  a  transparent  mass  becoming  a 
mirror,  gives  us  the  explanation  of  that  apparition  or  phenomenon  so 
admired  before  it  was  understood,  and  not  less  admired  since — the  rainbow, 
or  arc  in  the  sky,  as  in  France  and  elsewhere  it  is  named — an  object  which 
the  poets  of  nature  have  almost  worshipped  for  its  beauty,  and  which  few  of 
us  can  cease  to  remember  as  one  of  the  delights  of  our  boyish  days,  when 
we  saw  it  stretching  over  the  haunts  of  our  young  pleasures,  and  may  have 
pursued  it  in  the  hope  of  catching  some  of  the  falling  rubies  and  emeralds, 
or  bright  coloured  dew  of  which  it  might  be  composed. 

When  a  partial  shower  of  rain  falls  on  the  side  of  the  landscape  opposite 
to  where  the  sun  is  shining,  there  immediately  appears  in  the  shower  a 


MIRRORS.  389 

variegated  arch,  red  at  its  external  border  or  confine,  and  then  successively 
orange,  yellow,  green,  &c.  (in  the  order  of  the  colours  of  the  prismatic  spectrum 
described  at  page  337,)  towards  its  inner  border.  Its  centre  is  directly  oppo- 
site to  the  sun,  or  at  the  end  of  the  straight  line  supposed  to  be  drawn  from, 
the  sun  through  the  eye  of  the  spectator  towards  the  opposite  horizon ;  and 
being,  therefore,  always  under  the  horizon,  the  bow  is  less  than  a  semicircle. 
The  diameter  of  the  circle  of  which  the  bow  is  a  part,  occupies  nearly  82° 
of  the  field  of  view,  that  is  to  say,  the  bow  always  coincides  with  a  hoop  of 
one  foot  diameter  held  eight  inches  from  the  eye.  There  is  a  second  bow  of 
much  fainter  light  than  the  first,  and  with  the  colours  in  reverse  order :  it  is 
of  108°  diameter,  and  therefore  external  to  the  other. 

Now  the  explanation  of  this  miracle  of  beauty  is  simply  as  follows. 
While -the  sun  shines  upon  the  spherical  drops  of  falling  rain,  its  light  falls 
upon  the  whole  central  part  of  any  drop,  passes  completely  through,  but  that 
portion  which  enters  near  the  e%ge  of  the  drop,  as  at  a,  is  refracted  or  bent, 
and  reaches  the  back  surface  of  the  drop  a't  y  so 
slantingly,  or  an  angle  so  great,  that  it  suffers  there  Fig.  166. 

an  entire  reflection  instead  of  being  transmitted  ;  the 
ray,  therefore  is  turned  to  b,  where  it  escapes  from 
the  drop,  and  as  here  shown,  descends  to  the  earth 
or  eye  in  the  direction  b  e.  Thus  every  drop  of  rain 
on  which  the  sun  shines  is  a  little  mirror  suspended 
in  the  sky,  and  is  returning  at  a  certain  angle  all 
round  it,  viz.,  at  an  angle  of  41°,  a  portion  of  the 
light  which  falls  on  it ;  and  the  eye  placed  in  the  re- 
quired direction  receives  that  reflected  light.  If  in  this  case,  however,  there 
were  reflection  only,  and  not  also  refraction  with  separation  of  colours,  the 
rainbow  would  be  only  a  very  narrow  resplendent  arc  of  white  light  formed 
of  millions  of  little  images  of  the  sun;  but 
in  truth,  because  the  light,  which  enters  Fig.  167. 

near  the  edge  of  the  drop,  traverses  the 
surface  very  obliquely,  it  is  much  bent  or 
refracted  before  its  reflection,  as  seen  at  a, 
and  is  divided  into  rays  of  its  seven  col- 
ours, as  it  would  be  on  passing  through  a 
prism  (as  explained  at  page  337 ;)  and 
this  division  or  separation  continuing  after 
the  light  again  escapes  from  the  drop  at  b, 
instead  of  one  white  ray  descending  from 
each  drop  to  a  certain  point  of  the  earth, 
seven  rays  descend  (here  marked  by  dotted  lines  from  the  figure  1  on  the 
left  hand  to  7,  6,  5,  &c.,  on  the  right,  and  with  separation  greater  than  oc- 
curs in  reality  to  make  it  very  evident,)  and  of  these  rays,  an  eye  can  only 
receive  one  at  a  time  from  the  same  drop,  which  drop  will  then  appear  of  the 
colour  of  the  ray  :  but  for  the  same  reason  that  seven  eyes  placed  in  a  line 
from  above  downwards,  as  at  7,  6,  5,  &c.,  on  the  right  would  be  required  to 
see  the  seven  colours  from  one  drop  in  the  centre  of  the  bow,  so  one  eye  look- 
ing in  the  direction  of  seven  drops  situated  in  a  corresponding  row,  as  from 
1  -to  7  on  the  left,  will  catch  the  lower  or  red  ray  of  the  upper,  the  orange 
or  second  ray  of  the  next,  the  yellow  or  third  ray  of  that  which  follows,  and 
so  on,  while  it  will  lose  all  the  others,  and  thus  will  see  the  several  drops 
as  if  they  were  each  of  one  colour  only.  Of  such  elements,  then  found  in 
the  same  relative  directions  all  around  the  eye;  the  glorious  arch  is  formed. 


390  LIGHT. 

No  two  eyes  can  see  the  same  rainbow,  that  is,  can  receive  light  from  the 
same  drops  at  the  same  time ;  and  the  same  eye  does  not  for  two  instants 
receive  light  from  the  same  drops.  This  rainbow  can  never  appear  to  a  per- 
son on  a  plain,  unless  when  the  sun  is  within  41°  of  the  horizon,  for  other- 
wise the  centre  of  the  rainbow  would  be  more  than  41°  under  the  horizon, 
and  therefore  the  whole  circumference  would  be  below  it  too. 

We  have  described  above  what  is  called  the 

Fig.  168.  principal   bow,  formed   in    the    drops   by  two 

refractions,  and  one  reflection  of  light.  To 
produce  the  fainter  second  or  external  bow, 
mentioned  above,  and  of  which  the  colours  are 
in  reverse  order,  the  light  which  enters  on  the 
under  side  of  the  drop  as  at  a,  is  reflected  first 
at  y,  then  again  at  b,  and  escapes  at  c  towards 
the  eye :  after* two  reflections  as  well  as  two 
refractions.  As  the  semi-diameter  of  the  bow 
is  54°,  it  may  be  visible  when  the  internal  bow  is  not. 

An  artificial  rainbow  may  be  produced  in  sunshine  at  any  time  by  scatter- 
ing water-drops  from  a  bush  or  otherwise  ;  and  a  rainbow  is  often  seen  among 
the  spray  of  a  lofty  waterfall,  or  of  a  stormy  sea.  The  cut-glass  ornaments 
of  chandeliers,  &c.,  produce  colours  on  the  same  principle  as  rain-drops;  as 
do  also  mist  and  particles  of  frozen  water  between  a  luminous  body,  and  the  eye 
exhibiting  the  circular  coloured  halos  often  observed  around  the  sun  and  moon. 
A  white  halo  is  light  reflected  from  the  external  surfaces  of  drops  or  particles. 

"Mirrors  may  l>e plane,  convex,  or  concave;  and  certain  curvatures  v;ill 
produce  images  by  reflection,  just  as  lenses  produce  images  by  refraction  ; 
in  consequence,  there  are  reflecting  telescopes,  microscopes,  &c.,  as  there 
are  refracting  instruments  of  the  same  names."  (See  the  Analysis, 
page  325.) 

While  a  plane  surface  reflects  light,  so  that  what  is  called  the  image  in  it 
of  a  known  object  may  readily  be  mistaken  for  the  reality,  convex  or  con- 
cave mirror's  reflect  as  if  every  distinct  point  of  them  were  a  separate  small 
plane  mirror,  and  their  effects  on  light  correspond  with  a  relative  inclination 
of  the  different  parts.  The  only  forms  of  much  importance  are  the  regularly 
spherical  or  parobolic  concave  and  convex  mirrors.  We  shall  now  find  that 
these  produce  on  light  similar  effects  with  lenses,  only  the  concave  mirror 
answers  to  the  convex  lens,  and  the  convex  mirror  to  the  concave  lens.  It 
is  the  concave  mirror  which  gathers  the  light  to  form  images  in  the  most 
perfect  telescopes  that  exist,  as  those  of  Herschel  and  others.  Admirable  as 
is  the  refracting  telescope,  it  still  falls  short  in  certain  respects  of  the  teles- 
cope acting  by  reflection. 

In  a  hollow  sphere,  or  part  of  a  sphere  with 
polished  internal  surface,  if  rays  radiate  from  the 
centre  in  all  directions,  they  reach  every  part 
perpendicularly,  and  therefore  are  thrown  back 
to  the  centre.  Thus  if  A  B  were  a  concave 
spherical  mirror,  of  which  C.  were  the  centre, 
rays  issuing  from  C.  would,  in  obedience  to  the 
law  that  the  angles  of  incidence  and  reflection 
are  equal,  again  meet  at  C. 

It  can  be  proved  also,  that  any  ray  parallel  to 


CURVED    MIRRORS. 


391 


the  axis,  falling  upon  such  a  mirror,  will  be  Fig.  170. 

reflected  inwards  so  as  to  cut  the  axis  half- 
way between  the  mirror  and  its  centre,  viz., 
at  D,  the  centre  being  C.  Then  as  all  pa- 
rallel rays  must  meet  in  the  same  point, 
that  point  becomes  a  focus,  as  already  ex- 
plained for  lenses,  and  there  an  image  of  the 
sun  will  be  formed  when  the  mirror  is  held 
directly  towards  the  sun.  This  point  is  called  the  principal  focus  of  the 
mirror. 

For  the  same  reason  that  parallel  rays  meet  in  the  focus,  so  will  rays, 
issuing  from  the  focus  towards  the  mirror,  become  parallel,  after  reflection, 
as  seen  above  or  in  the  figure  at  page  271 ;  and  if  they  be  then  caught  in  a 
second  and  opposite  mirror,  as  also  represented  at  page  271,  corresponding 
effects  will  follow. 

Now,  for  a  concave  mirror,  as  already  explained  for  a  lens,  when  rays  fall 
on  it  obliquely  from  one  side  of  the  axis,  their  focus  will  be  on  the  opposite 
side,  and  therefore  the  mirror  will  form  an  inverted  image  of  any  body 
placed  before  it,  just  as  the  lens  does  ;  and  the  image  will  be  near  or  distant 
and  large  or  small,  according  to  the  divergence  of  the  approaching  rays, 
exactly  as  happens  with  lenses  j  and  thus  the  camera  obscura,  magic  lantern, 
telescopes  and  microscopes,  may  all  be  formed  by  mirrors,  as  they  may  be 
by  lenses.  Moreover,  concave  mirrors  magnify,  as  concave  lenses  of  the 
opposite  names  do.  The  two  subjects  of  images  by  refraction  and  by  reflec- 
tion run  so  nearly  parallel,  that  it  would  be  useless  repetition  here  to  enter 
upon  the  detailed  consideration  of  the  latter  subject,  and  we  shall  therefore 
content  ourselves  with  showing  why  a  concave  mirror  magnifies  and  why  a 
convex  mirror  minifies. 

A  concave  mirror  magnifies  "be-  Fig.  171. 

cause  the  light  from  the  top  of  the 
cross  at  A,  reaching  the  mirror 
where  it  can  be  reflected  to  an  eye 
placed  at  F,  viz.,  at  E,  seems  to 
the  eye  to  come  from  C,  and  the 
light  of  B  similarly  appears  to 
come  from  D,  so  that  the  cross  A 
B,  by  the  reflection,  seems  to  the 
eye  to  be  of  the  greater  dimensions  C  D. 

In  the  convex  mirror,  again,  for  corresponding  reasons,  the  cross  A  B 
appears  only  as  C  D,  and  therefore  much  smaller  than  the  reality. 

Concave  or  magnifying  mirrors  are  often 
used  by  persons  in  shaving. 

A  convex  mirror  is  a  common  ornament  of 
our  apartments,  exhibiting  a  pleasing  minia- 
ture of  the  room  and  its  contents. 

Any  polished  convex  body  is  a  mirror,  and 
therefore  the  ball  of  the  human  eye  is  one,  in 
which  we  may  contemplate  most  perfect 
miniatures  of  surrounding  things.  It  is  the 
image  of  the  window  or  of  the  sun  in  the 

convex  mirror  of  the  eye,  which  painters  usually  represent  by  a  spot  of 
white  paint  there ;  and  a  similar  luminous  spot  or  line  must  be  made  when 


Fig.  172. 


392  .    'LIGHT. 

they  have  to  represent  almost  any  of  the  pieces  of  furniture  which  have 
rounded  polished  surfaces  as  bottles,  glasses,  smooth  pillars,  &c., 

Convex  lenses  thus  are  also  mirrors  to  all  the  objects  around  them,  and 
very  strikingly  so,  owing  to  the  perfection  of  the  form  of  a  lens.  The 
polished  back  of  a  watch,  often,  in  the  same  way,  attracts  the  attention  of  a 
child,  who  wonders  to  see  there  so  clearly  "  the  little  baby." 

It  has  been  mathematical  amusement  to  calculate  what  kind  of  distor- 
tion mirrors  of  unusual  forms  will  produce,  and  then  to  make  distorted  draw- 
ings, which,  when  reflected  from  such  mirrors,  might  produce  in  the  eye  the 
natural  image  of  the  objects. 

When  a  concave  mirror  is  used  for  a  telescope,  the  image  formed  in  front 
of  it,  and  to  be  examined  through  the  magnifying  eye-glass,  may  be  viewed, 
— first,  as  in  Herschel's  telescope,  by  the  spectator  turning  his  back  to  the 
real  object,  and  looking  in  at  the  mouth  of  the  telescopic  tube,  near  to  the 
edge  of  which  the  image  is. thrown  by  a  slight  inclination  of  the  mirror  at  its 
bottom  : — or,  secondly,  as  in  the  Neictonian  telescope,  through  an  opening 
in  the  side  of  a  tube,  after  being  reflected  by  a  small  plane  mirror,  placed 
diagonally  in  the  centre  of  the  tube  : — or,  thirdly,  as  in  the  Gregorian 
telescope,  through  an  opening  cut  in  the  principal  mirror  or  speculum,  after 
being  reflected  towards  that  opening  by  a  small  mirror  placed  in  the  centre 
of  the  tube  ;  this  last  arrangement  .is  that  preferred  for  smaller  telescopes, 
because  the  spectator,  while  seeing  the  image,  is  also  looking  in  the  direc- 
tion of  the  object. 

Reflecting  telescopes  have  the  advantage  of  being  perfectly  achromatic, 
that  is,  of  producing  no  coloured  or  rainbow  edges  to  the  images  ;  for  com- 
pound light  is  reflected,  although  not  refracted  entire,  all  the  colours  follow- 
ing the  same  law  of  equal  angles  of  incidence  and  reflection. 

Herschel's  largest  telescope  had  a  niirrow^  of  48  inches  in  diameter,  and 
therefore  received  about  150,000  times  more  light  than  an  unassisted  eye, 
forming  with  that  light,  at  a  focal  distance  of  40  feet,  a  large  image  admira- 
bly distinct.  It  was  with  such  a  telescope  that,  in  the  obscurity  of  remote 
space,  Herschel  discovered  the  immense  planet  rolling  along,  which  in  honour 
of  his  royal  patron,  he  called  th%  Georgium  Sidus,  but  which  now,  by 
the  decision  of  the  scientific  world,  bears  his  own  name ; — arid  with  such 
he  discovered  moons  before  unseen  of  other  planets,  and  he  unravelled  the 
celestial  nebulas  and  clustered  -stars  of  the  milky  way  and,  in  a  word,  un- 
veiled vastly  more  than  had  before  been  done,  the  system  of  the  boundless 
universe.  If  this  world  were  to  last  for  millions  of  years,  the  discoveries 
made  by  Herschel's  telescope  would  mark  a  memorable  epoch  of  its  early 
history. 

"Light  returned  from,  or  passing  through  bodies  of  rougher  or  irregular 

surface,  or  which  have  other  peculiarities,  is  so  modified  as  to  produce  all 

i  those  phenomena  of  colour  and  varied  brightness  seen    among  natural 

bodies,  and  giving  them  their  distinctive  characters  and  beauty."     (See 

the  Analysis,  page  325.) 

General  remarks  on  this  part  of  our  subject  were  made  in  the  beginning 
of  the  section,  in  the  explanations  of  how  objects  not  self-luminious  become 
visible  by  reflecting  the  light  of  other  bodies,  and  of  how  the  prism  sepa- 
rates a  ray  of  white  light  into  rays  of  the  several  colours  which  are  seen 
also  in  the  rainbow — which  rays,  on  being  again  mixed,  become  white  light 


COMPARISON    OF    LIGHT    AND    SOUND.  893 

as  before : — and  much  beyond  these  remarks  we  have  not  the  intention  of 
now  proceeding  To  give  a  full  account  of  the  matters  that  might  come 
within  the  scope  of  this  department,  would  occupy  the  pages  of  a  large 
volume,  for  there  would  be  to  pass  in  review — the  various  opinions  which 
have  existed  on  the  intimate  nature  Q$  light, — the  facts  connected  with  what 
has  been  called  the  polarization  of  light, — the  relation  of  light  in  its  double 
refraction,  to  the  ultimate  structure  of  material  masses,  &c.,  all  which  sub- 
jects are  in  certain  respects  highly  interesting,  but — as  some  of  them  are  not 
yet  completely  investigated — as  respecting  others  various  opinions  prevail, — 
as  they  involve  few  matters  yet  applied  to  common  use, — as  the  reasonings 
about  them  are  far  removed  from  ordinary  trains  of  thinking,  and  refer  to 
facts  altogether  unknown  to  common  observation, — we  hold  them  not  to  be 
fit  parts  of  a  popular  treatise  on  light.  We  may  state,  however,  that  persons 
who  have  the  leisure  and  mathematical  preparation  necessary  for  pursuing 
the  study,  will  find  their  labour  in  it  richly  rewarded. 

What  we  deem  necessary  here  to  add,  is,  that  white  light,  in  falling  upon 
any  transparent  substance,  as  air,  water,  glass,  &c.,  reduced  to  thin  plates  of 
films,  is  so  affected,  that  for  certain  degrees  of  thinness,  different  for  each 
substance,  it  is  decomposed,  and  is  reflected  or  is  transmitted,  not  as  white 
light,  but  as  some  of  the  colours  of  the  rainbow,  and  the  colour  reflected  in 
any  case,  is  always  the  opposite  or  complement  of  that  which  is  transmitted, 
that  is  to  say,  is  such  that  the  two  brought  together  make  white  light  as 
before.  The  facts  may  be  studied  as  Newton  originally  studied  them,  in  the 
thin  plate  of  air  which  occupies  the  space  between  a  convex  lens  and  a  plane 
surface  of  glass  upon  which  the  lens  is  laid, — in  which  plate,  as  the  distance 
from  the  point  of  the  apparent  contact  of  the  glasses  increases,  there  are 
all  degrees  of  thinness,  and  with  these  appear  successive  rings  of  vivid  co- 
lours. The  same  trpth  is  exemplified  in  the  colours  of  a  soap-bubble,  which 
brighten  as  the  bubble  swells  and  becomes  of  thinner  substance,  and  are 
different  as  the  thickness  is  different  and  greater  from  above  downwards  ; — 
and  it  is  exemplified  also  in  the  colours  seen  in  the  fissures  of  cracked 
ice  or  crystalline  spars,  and  in  numerous  other  common  facts.  Now,  what- 
ever be  the  reasons  of  such  decomposition  of  light — and  the  explanation  is 
not  yet  complete — we  cannot  doubt  that  in  natural  bodies  generally,  the 
colours,  opacity,  transparency,  &c.,  depend  entirely  upon  the  volume  and 
arrangement  of  the  minute  fibres  or  plates,  with  included  interstices,  which 
constitute  the  volume  or  structure  of  each  mass  Accordingly,  whatever 
changes  that  arrangement  may  change  also  the  colour  of  the  mass  Thus, 
by  drawing  a  certain  number  of  minute  lines  on  a  certain  extent  of  any  me- 
tallic surface,  we  may  make  it  of  what  colour  we  please ;  and  mother-of-pearl 
owes  its  vivid  colours  and  beauty  entirely  to  its  furrowed  or  striated  surface, 
as  is  proved  by  our  making  an  impression  of  that  surface  on  sealing  wax  and 
perceiving  that  the  wax  exhibits  similar  colours. 

The  investigations  in  progress  respecting  the  phenomena  of  light,  are  fur- 
nishing new  proofs  of  the  extreme  simplicity  of  nature,  amidst  the  boundless 
extent  and  most  curious  variety.  When  men  thought  of  the  sense  of  touch 
only  as  it  exists  at  the  tips  of  the  fingers,  or  on  the  general  surface  of  the 
body  they  were  far  from  suspecting  that  the  sense  of  hearing  had  the  near 
relation  to  it  which  subsequent  discoveries  have  proved,  and  still  less,  that 
the  sense  of  sight  was  only  yet  a  finer  touch  than  hearing.  But  step  by 
step  they  have  ascertained,  1st.  in  relation  to  sound,  that  the  air  through 
which  it  usually  reaches  the  organ  of  hearing,  is  a  material  fluid  as  much  as 


394  LIGHT. 

water,  consisting  of  the  same  or  smaller  particles,  only  more  distant  among 
themselves, — and  that  a  motion  or  trembling  in  the  air,,  by  affecting  nerves 
exposed  in  the  ear,  produces  the  sensation  of  sound  by  slight  repeated  pressures 
on  these  nerves,  as  the  trembling  in  a  log  of  wood  caused  by  the  action  of  a 
saw  produces  a  peculiar  sensation  of  touch  in  the  nerves  of  a  hand  laid  on  the 
log  ; — and,  moreover,  that  sound  in  all  its  varieties,  is  merely  such  trembling 
affecting  a  structure  of  nerve  in  the  ear,  which  nerve  is  made  as  much  more 
readily  excitable  than  the  nerves  in  the  fingers  or  general  cutaneous  surface, 
as  the  action  or  impulse  of  trembling  air  is  more  delicate  than  the  stronger 
pressures  of  common  occurrence. — And,  2dly,  in  the  investigation  respecting 
light,  this  kind  of  comparison  is  carried  a  step  farther,  for  it  is  become 
matter  almost  of  certainty  that  the  sensation  of  light  is  produced  in  the 
suitable  nervous  tissue  of  the  eye,  called  the  retina  by  a  trembling  motion  in 
another  fluid  than  air,  which  fluid  pervades  all  space,  and  in  rarity  or  sub- 
tilety  of  nature  surpasses  air  yet  more  than  air  does  water  or  solids ; — and 
that,  while  in  sound,  different  tones  or  notes  depend  on  the  number  of 
vibrations  in  a  given  time,  so  in  light  do  different  colours  depend  on  the 
number  and  extent  of  the  vibrations.  Can  human  imagination  picture  to 
itself  a  simplicity  more  magnificent  and  fruitful  of  marvellous  beauty  and 
utility  than  all  this  ? — But  yet  farther,  as  air  answers  in  the  universe  so  many 
important  purposes  besides  that  of  conveying  sounds — although  this,  alone, 
comprehends  language,  which  almost  means  reason  and  civilization — so  also 
does  the  material  of  light  minister  in  numerous  ways,  in  the  phenomena  of 
heat,  electricity  and  magnetism. 

The  truths  now  positively  ascertained  with  respect  to  the  nature  of  light 
and  vision,  are  among  those  in  the  wide  field  of  human  iaquiry,  which,  . 
acting  on  ordinary  apprehension,  most  forciby  place  the  student,  as  it  were, 
in  the  very  presence  of  Creative  Intelligence,  awakening  in  him  the  most 
elevated  thoughts  of  which  the  human  mind  is  capable.  Had  there  been  no 
light  in  thje  universe,  all  its  other  perfections  in  regard  to  man  had  existed  in 
vain.  This  earth  would  have  been  to  its  human  inhabitants  what  any 
unknown  shore  would  be  to  exiles  abandoned  upon  it  after  their  eyes  were 
put  out:  every  movement  might  be  to  their  destruction,  for  their  perceptions, 
being  limited  by  the  length  of  their  arms,  and  of  their  fearful  groping  steps, 
the  wretched  beings  separating  when  impelled  by  hunger  to  search  for  food, 
would  probably  scatter  to  meet  no  more.  But  the  material  of  light  exists, 
pervading  all  space,  and  certain  impressions  made  upon  it  in  one  place,  ex- 
tend rapidly  over  the  universe,  the  progressive  impression  being  called  a  ray, 
or  beam  of  light.  The  beams  of  light,  then  from  all  parts  coming  to  every 
individual,  may  be  regarded  as  millions  of  supplementary  arms  or  feelers 
belonging  to  the  individual,  and  which  reach  to  the  end  of  the  universe,  so 
that  each  person,  instead  of  being  as  a  blind  point  in  space,  becomes  nearly 
omnipresent  \  then  these  limbs  or  feelers  have  no  weight,  they  are  never  in 
the  way,  they  impede  nothing,  and  they  are  only  known  to  exist  when  their 
use  is  required  ! 

But  this  miracle  of  LIGHT  would  have  been  totally  useless,  and  the  para- 
dise of  earth  would  have  been  to  man  still  a  dark  and  dreary  desert,  had  there 
not  been  farther  the  twin  miracle  of  the  EYE,  an  organ  of  commensurate  deli- 
cacy to  perceive  the  light.  In  the  Eye  we  have  to  admire  the  round  cornea, 
of  such  perfect  transparency,  placed  exactly  in  the  anterior  of  the  ball,  (and 
elsewhere  it  had  been  useless,)  then  exactly  behind  this,  the  beautiful  curtain, 
the  iris,  with  its  opening,  called  the  pupil,  dilating  and  contracting  to  suit  the 


PERFECTION    OF    THE    EYE.  395 

intensity  of  light — and  exactly  behind  the  iris,  again,  the  crystalline  lens  pos- 
sessing important  and  remarkable  properties,  and  which,  by  acting  on  the 
entering  light,  forms  of  it  on  the  retina  beautiful  pictures  or  images  of  the 
objects  in  front ; — the  most  sensible  part  of  the  retina  being  where  the  images 
fall.  Of  these  parts  and  conditions,  had  any  one  been  otherwise  than  as  it  is, 
the  whole  eye  had  been  useless,  and  light  useless,  and  the  great  universe  use- 
less to  man,  for  he  could  not  have  existed  in  it. — Then,  farther,  we  find  that 
this  precious  organ,  the  eye,  is  placed  in  the  person,  not  as  if  by  accident, 
any  where,  but  aloft  on  a  befitting  eminence,  where  it  becomes  the  glorious 
watch-tower  of  the  soul ;  and,  again,  not  so  that  to  alter  its  direction  the 
whole  person  must  turn,  but  in  £he  head,  which,  on  a  pivot  of  admirable 
structure,  moves  while  the  body  is  at  rest ;  besides  that,  the  ball  of  the  eye 
itself  can  roll  in  its  place,  and  is  furnished  with  muscles  which,  as  the  will 
directs,  turn  it  with  the  rapidity  of  lightning  to  sweep  along  the  horizon,  or 
take  in  the  whole  heavenly  concave ; — then  is  the  delicate  orb  secured  in  a 
strong  socket  of  bone,  and  there  is  over  this  the  arched  and  padded  eyebrow 
as  a  cushion,  to  mitigate  the  shock  of  blows,  and  with  its  inclined  hairs  to 
turn  aside  any  descending  perspiration  or  other  moisture  which  might  incom- 
mode ; — then  is  there  the  soft  and  pliant  eyelid  with  its  beauteous  fringes, 
incessantly  wiping  the  polished  surface,  and  spreading  over  it  the  pure  mois- 
ture poured  out  from  the  lachrymal  glands  above,  of  which  moisture  the 
superfluity,  by  a  fine  mechanism,  is  sent  into  the  nose,  there  to  be  evapor- 
ated by  the  current  of  the  breath ; — still  farther,  it  is  to  be  noted,  that  instead 
of  there  being  only  one  such  precious  organ,  there  are  two,  lest  one,  by  acci- 
dent, should  be  destroyed,  but  which  two  have  so  entire  sympathy,  that  they 
act  together  as  only  one  more  perfect;  then  the  sense  of  sight  continues 
perfect  during  the  period  of  growth  from  birth  to  maturity,  although  because 
the  eye  then  increases  in  size,  the  distance  between  the  lens  and  the  retina  is 
constantly  increasing ; — and  the  pure  liquid  which  fills  the  eye,  ifrrendered  tur- 
bid by  accident  or  disease,  is  by  the  actions  of  life,  afthough  its  source  be  the 
thick  red  blood,  gradually  restored  to  transparency. — The  mind  which  can 
suppose  or  admit  that,  within  any  limits  of  time,  one  single  such  apparatus 
of  vision  could  have  been  produced  by  accident,  or  without  design,  must 
surely  be  of  extraordinary  character,  or  must  have  received  unhappy  bias  in 
its  education ;  but  the  mind  which  can  still  farther  admi.t  that  the  millions 
of  human  eyes  which  now  exist  on  earth,  all  equally  perfect,  can  have  sprung 
from  accident — and  that  the  millions  of  millions  of  other  eyes  throughout 
the  almost  innumerable  species  of  the  living  creation,  where  each  is  adapted 
to  the  peculiar  nature  and  circumstances  of  the  animal  which  bears  it,  can 
be  accident ;  and,  lastly,  that  the  countless  millions  of  all  these  well  adapted 
kinds,  which  have  existed  in  past  ages,  were  all  but  accidents — the  mind 
which  can  admit  this,  must  have  some  of  its  highest  faculties  either 
benumbed  or  destroyed. 

As  a  concluding  reflection  with  respect  to  vision,  we  may  remark,  that  all 
the  provisions  above  considered  have  mere  utility  in  view,  for  any  one  of 
them  wanting  would  leave  a  necessary  link  in  the  chain  of  creation  wanting : 
but  we  have  shown,  in  a  preceding  part  of  the  work,  that  if  there  had  been 
white  light  only,  susceptible  as  now  of  different  degrees  of  intensity  and 
shade,  the  merely  useful  purposes  of  vision  should  have  been  answered  about 
as  perfectly  as  with  all  the  colours  of  the  rainbow — a  truth  instanced  in  the 
facts,  that  many  persons  do  not  distinguish  colours,  and  that  it  imports  not 
whether  a  person  view  objects  in  the  morning,  or  at  mid-day,  or  at  even-tide, 


396  LIGHT. 

or  through  plain  glass  or  coloured  glass,  provided  there  be  light  and  shade 
enough  to  show  them  clearly.  While,  therefore,  the  existence  of  light  gene- 
rally, and  of  the  eye,  speaks-  of  Creative  Power  and  Intelligence,  the  exist- 
ence of  colours,  or  of  that  lovely  variety  of  hues  exhibited  in  flowers,  in  the 
plumage  of  birds,  in  the  endless  aspects  of  the  earth  and  heavens — because 
appearing  expressly  planned  to  give  delight  to  animated  beings,  speaks  of 
Creative  Benevolence,  and  may  well  excite  in  us  towards  the  Being  in  whom 
these  attributes  reside,  the  feelings  associated  in  our  minds  during  this 
earthly  scene,  with  the  endearing  appellation  of  "  Father/' 


Fig.  173. 


MECHANISM    OF     THE    HUMAN    SKELETON.        399 

I 


PART   V. 

ANIMAL  AND  MEDICAL  PHYSICS. 

l 

SECTION  I. 
Mechanism  of  the  Human  Skeleton. 

HAVING  now  completed  our  study  of  general  mechanics,  we  shall  proceed, 
with  the  light  thence  derived,  to  examine  that  most  interesting  illustration 
of  many  of  the  truths — the  solid  frame-work  of  the  human  body — a  perfect 
work  of  an  unerring  Engineer  ! 

There  is  scarcely  a  part  of  the  animal  body,  or  an  action  which  it  per- 
forms, or  an  accident  that  can  befall  it,  or  a  piece  of  professional  assistance 
which  can  be  given  to  it,  that  does  not  furnish  illustration  of  some  truth  of 
natural  philosophy;  but  w?re  we  here  to  enter  into  much  detail,  we  should 
be  giving  minute  lessons  in  medical  science,  instead  of  explaining  general 
laws.  We  shall  therefore  only  touch  upon  as  many  particulars  as  will  make 
the  understanding  of  all  the  others  easy;  trying  to  conclude,  among  our 
illustrations,  such  matters  of  importance  as  would  be  likely  to  escape  the 
notice  of  a  hasty  student. 

The  cranium  or  skull  has  been  already  mentioned  as  an  instance  of  the 
arched  form  answering  the  purpose  of  giving  strength.  The  brain,  in  its 
nature,  is  so  tender  or  susceptible  of  injury,  that  slight  local  pressure  dis- 
turbs its  action.  Hence  a  solid  covering  like  the  skull  was  required  with 
those  parts  made  stronger  and  thicker  which  are  most  exposed  to  injury. 
An  architectural  dome  is  constructed  to  resist  one  kind  of  force  only,  always 
acting  in  one  direction,  viz.,  gravity;  and  therefore  its  strength  increases 
regularly  towards  the  bottom,  where  the  weight  and  horizontal  thrust  of  the 
whole  are  to  be  resisted ;  but  in  a  skull,  as  in  a  barrel  or  egg-shell,  the  mere 
tenacity  of  the  substance  is  many  times  greater  than  sufficient  to  resist 
gravity,  and  therefore  the  form  and  securities  are  calculated  to  resist  forces 
of  other  kinds  operating  in  all  directions.  When  we  reflect  on  the  strength 
displayed  by  the  arched  film  of  an*  egg-shell,  we  need  not  wonder  at  the 
severity  of  blows  w^ich  the  cranium  can  withstand. 

In  the  early  fetal  state,  that  which  afterwards  becomes  the  strong  bony 
case  of  the  brain  exists  only  as  a  tough  flexible  membrane.  Ossification 
commences  in  this  membrane  long  before  birth,  at  a  certain  number  of  points 
from  which  it  spreads,  and  the  portions  of  the  skull  formed  around  these 
points  soon  acquire  the  appearance  of  so  many  scales  or  shells  applied  on 
the  surface  of  the  brain,  and  held  together  by  the  remaining  membrane  not 
yet  ossified.  They  afterwards  become  firmly  fixed  together,  by  projections 
of  bone  from  each,  shutting  in  among  similar  projections  of  the  adjoining 
ones,  until  all  mutually  cohere  by  perfect  dove-tailed  joints,  like  the  work  of 
a  carpenter.  These  joints  are  called  sutures  of  the  cranium,  and  are  visible 


400  ANIMAL  AND 

• 

to  extreme  old  age.  Through  early  childhood,  the  cranium  remains  to  a 
certain  degree  yielding  and  elastic,  causing  the  falls  and  blows,  so  frequent 
.  during  the  lessons  of  walking,  £c.,  to  be  borne  with  comparative  impunity. 
The  mature  skull  consists  of  two  layers  or  tables,  with  a  soft  diploe  between 
them ;  the  outer  table  being  very  tough,  with  its  parts  dove-tailed  into  each 
other  as  tough  wood  is  joined  by  human  artificers;  while  the  inner  table  is 
harder  and  more  brittle,  (hence  called  vitreous)  with  its  edges  merely  lying 
in  contact. 

A  very  severe  partial  blow  on  the  skull  generally  fractures  and  depresses 
the  part,  as  a  pistol-bullet  would :  while  one  less  severe,  but  with  more  ex- 
tended contact,  being  slowly  resisted  by  the  arched  form,  often  injures  the 
skull  by  what  is  correspondent  to  the  horizontal  thrust  in  a  bridge,  and 
causes  a  crack  at  a  distance  from  the  place  struck — generally  half  way 
round  to  the  opposite  side.  The  French  in  speaking  of  this  effect,  use  the 
term  contre-coup.  Sometimes  in  a  fall  with  the  head  foremost,  the  skull 
would  escape  injury,  but  for  the  trunk  which  falls  upon  it,  and  drives  the 
end  of  the  spine  against  or  even  through  its  base. 

In  the  lower  jaw  we  have  to  remark  the  greater  mechanical  advantage,  or 
lever-power,  with  which  the  muscles  act,  than  in  other  parts  of  animals. 
The  temporal  and  masseter  muscles  pull  almost  directly,  or  at  right  angles 
to  the  line  of  the  jaw,  while  in  most  other  cases,  as  in  that  of  the  deltoid 
muscle  lifting  the  arm,  the  muscles  act  very  obliquely,  and  with  power 
diminished  in  proportion  to  the  obliquity.  An  object  placed  between  the 
back  teeth  is  compressed  with  the  whole  direct  power  of  the  strong  muscles 
of  the  jaw.  Hence  the  human  jaw  can  crush  a  body  which  offers  great 
resistance,  and  the  jaws  of  the  lion,  tiger,  shark,  and  crocodile,  &c.,  are 
stronger  still. 

The  teeth  rank  high  among  those  parts  of  the  animal  body  which  appear 
almost  as  if  they  were  severally  the  results  of  distinct  miraculous  agencies 
— so  difficult  is  it  to  suppose  a  few  simple  laws  of  life  capable  of  producing 
the  variety  of  form  and  fitness  which  they  exhibit.  They  constitute  a  beau- 
tiful set  of  chisels  and  wedges  so  arranged  as  to  be  most  efficient  for  cutting 
and  tearing,  and  grinding  the  food,  with  their  exterior  enamel  so  hard,  that 
few  substances  in  nature  can  make  an  impression  upon  it.  In  early  states, 
of  society,  teeth  were  used  for  many  purposes  for  which  steel  is  used  now. 
It  seems,  however,  as  if  the  laws  of  life,  astonishing  to  human  intellect  as 
they  are,  had  still  been  inadequate  to  cause  teeth  cased  in  their  hard  and 
polished  enamel,  to  grow  as  the  softer  bones  grow  ;  and  hence  has  arisen  a 
provision  more  extraordinary  still.  A  set  of  small  teeth  appear  soon  after 
birth,  and  serve  the  child  until  six  or  seven  years  of  age  :  these  then  fall 
out,  and  are  replaced  by  larger  ones,  which  endure  for  life ;  the  number  of 
the  latter,  however,  being  completed  cfaly  when  the  man  or  woman  is  full 
grown,  by  the  four  teeth,  called  wisdom  teeth,  from  coming  with  the  person's 
maturity,  to  fill  up  the  then  spacious  jaw. 

The  spine  or  back  bone,  in  its  structure,  has  as  much  of  beautiful  and 
varied  mechanism  as  any  part  of  our  wonderful  frame.  It  is  the  central 
pillar  of  support  and  great  connecting  chain  of  all  the  other  parts;  and  has 
at  the  same  time,  the  office  of  containing  within  itself,  and  of  protecting 
from  external  injury,  a  prolongation  of  the  brain,  called  the  spiral  marrow, 
more  important  to  animal  life  than  the  greater  part  of  the  brain  itself.  It 
has  united  in  it  the  apparent  incompatibilities  of  great  elasticity,  great 
flexibility  in  all  directions,  and  great  strength,  both  to  support  a  load  and  to 
defend  its  important  contents, — as  we  shall  now  perceive. 


MEDICAL    MECHANICS.  ,      401 

Elasticity. — The  head  rests  on  the  elastic  column  of  the  spine,  as  softly 
as  the  body  of  a  carriage  rests  upon  its  springs.  Between  each  two  of  the 
twenty-four  vertebrae  or  distinct  bones  of  which  the  spine  consists,  there  is 
a  soft  elastic  inter  vertebral  substance,  about  half  as  bulky  as  a  vertebrae,  and 
which  yields  readily  to  any  sudden  jar  :  then  the  spine  is  waved  or  bent  like 
an  italic/^  as  if  perceived  on  viewing  it  sideways,  or  in  profile,  and  by  this 
reason,  also,  it  yields  to  any  sudden  pressure  operating  against  either  end. 
The  bending  might  seem  a  defect  in  a  column  intended  to  support  weight, 
but  the  disposition  of  the  muscles  around  is  such  as  to  leave  all  the  elasticity 
of  that  form,  and  a  roomy  thorax,  without  any  diminution  of  strength. 

Flexibility. — The  spine  has  been  compared  to  a  chain,  because  it  consists 
of  many  distinct  pieces  (twenty-four.)  •  They  are  in  contact  by  smooth  rub- 
bing surfaces,  which  allow  of  a  degree  of  motion  in  all  directions ;  and  a  little 
motion  comparatively  between  each  two  adjoining  pieces,  becomes  a  great 
extent  of  motion  in  the  whole  line. 

The  strength — of  the  spine  as  a  whole,  is  shown  in  the  fact  of  a  man's 
easily  carrying  upon  his  head  or  back  a  weight  heavier  than  himself;  and  the 
strength  of  each  separate  vertebrae  surrounding  the  spinal  marrow,  is  evident 
in  its  being  a  double  arch,  or  strong  irregular  ring.  The  spine  increases  in 
size  towards  the  bottom,  in  the  justest  proportion,  as  it  has  more  weight  to 
bear.  The  articulating  surfaces  of  the  spine  are  so  many,  arid  so  exactly  fitted 
to  each  other,  and  are  connected  by  such  number  and  strength  of  ligaments, 
that  the  combination  of  pieces,  becomes,  in  reference  to  motion,  a  much 
stronger  column  than  a  single  bone  of  the  same  size  would  be. 

Considering  the  great  number  of  parts  forming  the  spine,  and  their  nice 
mutual  adaption,  it  might  be  expected  that  injuries  and  diseases  of  the  struc- 
ture would  be  very  frequent.  The  reverse,  however,  under  natural  circum- 
stances, is  true ;  so  that  while  hundreds  and  thousands  of  works  have  been 
published  on  the  diseases  of  almost  every  other  part  of  the  body,  hardly  any 
have  been  written  on  spine-affections,  and  what  have  appeared  are  of  very 
recent  date.  One  reason  of  this  is,  that  whatever  regards  health  and  disease 
is  now  much  more  completely  analyzed  than  formerly;  but  another  and  the 
chief  reason  is,  that  from  a  change  in  modern  times  introduced  into  the  system 
of  education  for  young  l§dies,  a  considerable  proportion  of  them  having  grown 
to  womanhood  with  weakened  and  crooked  spines. — The  subject  merits 
further  consideration  here. 

To  the  well-being  of  the  higher  classes  of  animals,  a  certain  degree  of  exer- 
cise of  their  various  parts  is  not  less  necessary  than  their  nourishment,  and 
if,  during  the  period  of  growth,  such  exercise  be  withheld  by  any  cause,  the 
body  never  acquires  its  due  proportions  and  strength.  To  prompt  young 
creatures  to  the  required  exertion,  nature  has  given  them  an  overflow  of  life 
and  energy,  as  evinced  in  the  ever-changing  occupation  of  a  child  in  the  quick 
succession  of  its  ideas,  in  its  jumping  and  skipping,  and  using  all  the  modes 
of  roundabout  action  to  expand  muscular  energy,  instead  of  seeking,  as  in  after 
life,  to  accomplish  its  ends  in  the  shortest  ways : — and  as  seen  among  the 
inferior  animals,  in  the  play  of  kittens,  puppies,  lambs,  &c.  But,  strongly  as 
nature  has  thus  expressed  herself,  tyrant  fashion,  with^a  usual  perversion  of 
common  sense,  had  of  late  times,  in  England,  for  young  women  of  the  higher 
classes  formed  a  school  discipline,  directly  at  war  with  nature's  dictate ;  so 
that  a  stranger  arriving  from  China,  might  almost  suppose  it  our  design  to 
make  crooked  and  weak  spines  by  that  discipline,  as  it  is  the  design  in  China 
to  make  little  feet  by  the  iron  shoe.  The  result  is  the  more  striking,  when  the 
brothers  of  the  female  victims,  and  who  of  course  have  similar  constitutions. 

26 


402  ANIMAL    AND 

are  seen  to  be  robust,  healthy,  and  well-formed.  A  peasant-girl,  when  her 
spirits  are  buoyant,  is  allowed  to  obey  her  natural  feeling,  and  at  proper  times 
to,  dance,  and  skip,  and  run,  until  healthy  exhaustion  asks  that  repose  which 
is  equally  allowed;  and  thus  she  grows  up  strong  and  straight,  but  the  young 
lady  is  receiving  constant  admonition  to  curb  all  propensity  to  such  vulgar 
activity,  and  often,  just  in  proportion  as  she  subdues  nature,  she  receives  the 
praise  of  being  well-bred.  The  multifarious  studies,  also,  of  the  latter  come 
powerfully  in  aid  to  the  admonition,  by  fixing  her  for  many  hours  everyday 
to  sedentary  employment;  and  the  consequences  soon  follow,  of  weakness 
in  the  body  generally  from  the  want  of  the  natural  quantity  and  variety  of 
muscular  exertion,  but  weakness  of  the  back  particularly,  from  the  manner 
in  which  the  sitting  is  usually  performed.  It  would  be  accounted  great  cruelty 
to  make  a  delicate  girl  stand  all  day,  because  her  legs  would  tire,  but  this 
very  cruelty  is  in  almost  constant  operation  against  her  back,  as  if  backs  could 
not  tire  as  well  as  legs.  When  she  is  allowed  to  sit  down  because  she  has 
been  long  standing,  great  care  is  taken  that  the  muscles  of  the  back,  which 
still  remain  in  action  as  she  sits,  shall  not  be  at  all  relieved;  for,  from  the 
idea  that  it  is  ungrace%il  to  loll,  she  is  either  upon  a  stool  which  has  no  back 
at  all,  or  upon  a  very  narrow  chair  with  a  perpendicular  back.  Now  neither 
of  these  seats  relieve  her  spine,  the  stool,  however,  being  less  hurtful  than 
the  chair,  because  it  allows  the  spine  to  bend  in  different  ways  so  as  to  rest 
the  different  sets  of  muscles  alternately,  while  the  chair  keeps  the  spine 
constantly  upright  and  nearly  unmoved.  The  excessive  fatigue  soon  causes 
the  spine,  somewhere,  to  give  way  and  to  bend,  and  the  curvature  often 
becomes  permanent.  Ajid,  as  when  a  bend  takes  places  in  one  situation, 
there  immediately  follows  an  opposite  bend  above  or  below,  to  keep  the  centre 
of  gravity  ef  the  body  always  directly  over  the  base,  the  curve  thus  becomes 
double,  like  an  italic/,  and  the  distortion  is  rendered  complete. — In  bending 
the  spine  is  sometimes  also  partially  rotated  or  twisted,  so  as  to  show  from 
behind  that  waving  profile  which  should  be  seen  only  from  the  side. 

When  owing  to  such  discipline  the  inclination  of  the  back  has  once  been 
begun,  it  is  often  rapidly  increased  by  the  means  used  to  correct  it.  Strong 
stiff  stays  are  put  on  to  support  the  back,  as  is  said,  but  which  in  reality,  by 
superseding  the  action  of  the  muscles  placed  there  l|y  nature  as  the  supports, 
cause  these  to  lose  their  strength,  and  to  be  unable,  when  the  stays  are  with- 
drawn, to  support  the  body.  Longer  sittings  in  the  narrow  upright  chair  are 
then  recommended,  and  sometimes  the  back  is  forcibly  stretched  by  pullies, 
so  the  patient  is  kept  all  day  and  night  lying  on  an  inclined  board,  losing  her 
health,  &c. ; — the  only  things  guarded  against  being,  the  patient  should  take 
due  exercise  and  air,  and  should  rest  properly  when  she  is  not  taking  exer- 
cise. With  many  persons  the  prejudice  had  at  last  grown  up,  that  strong 
stays  should  be  put  on  at  a  very  early  age,  to  prevent  the  first  approach  of 
the  mischief,  and  that  children^should  always  be  made  to  sit  on  straight- 
backed  chairs,  or  to  lie  on  hard  planes;  and  it  is  probable,  that  if  these  cures 
and  preventives  had  been  adopted  as  universally  and  strictly  as  many  deemed 
them  necessary,  we  should  now  scarcely  have  in  England  a  young  lady  of 
healthful  form.  What  would  be  said  of  the  person  who  should  try  to 
improve  the  strength  and  shape  of  a  young  race-horse  or  greyhound,  by 
binding  tight  splints  or  stays  round  its  beautiful  young  body,  and  then 
tying  it  up  in  a  stall!  But  this  is  the  kind  of  absurdity  and  cruelty  which 
has  been  so  commonly  practised  in  this  country  towards  beings  than  whom, 
as  nature  offers  them,  the  universe  surely  contains  none  more  faultless. 

A  pernicious  prejudice,  with  respect  to  such  curvature  or  distortion  of  the 


MEDICAL    MECHANICS.  403 

spine,  long  existed,  namely  that  it  was  a  scrofulous  affection ;  and  many 
mothers  concealed  it  as  much  as  possible,  and  sought  remedy  from  quacks 
far  from  home.  In  consequence,  until  within  a  few  years,  the  management, 
of  spine  diseases  was  chiefly  in  the  hands  of  some  irregular  members  of  the 
profession, — and  a  rich  source  of  wealth  it  became  to  them,  from  many  of 
their  remedies  being  calculated  rather  to  prolong  than  to  cure  the  evil.  The 
practice  in  such  cases,  however,  has  now  fallen  into  the  hands  of  the  pro- 
fession generally ;  the  science  having  detected  the  true  cause  of* the  evil,  its 
frequency  is  already  diminished.  It  has  been  shown  that  to  prevent  the 
disease  is  easy,  and  that  the  best  cures  are  those  conducted  on  the  general 
principles  of  improving  the  health  of  the  patient  by  fit  regimen,  of  prescrib- 
ing such  exercises  as  may  directly  strengthen  the  affected  part,  and  of  caus- 
ing the  patient, "when  reposing  to  assume  positions  which  directly  counteract 
the  morbid  tendency. 

Some  might  expect  here  a  long  description  of  machines  employed  in  the 
treatment  of  spine  affection  :  but  the  list  of  those  which  are  useful  or  safe 
is  very  short : — a  sofa  to  rest  upon  during  the  day  and  a  fit  bed  for  the 
night;  (the  "  hydrostatic  bed/')  proposed  by  the  author  of  this  work,  and 
described  ia  the  next  chapter,  has  certain  advantages ;)  choice  of  pleasant 
means  of  taking  exercise,  such  as  the  skipping-rope,  shuttle-cock,  dum-bells, 
a  rope-ladder  to  climb,  a  winch  to  turn,  &c.  : — and  where  it  is  much  desired 
that  the  young  lady  should  employ  herself  in  the  sitting  attitude,  as  in 
practising  music,  a  chair  may  be  used  with  crutches  rising  from  its  side,  or 
with  straps  descending  from  pullies  in  an  overhanging  canopy  or  crane,  and 
kept  tight  by  proper  weights  at  their  distant  ends,  to  support  the  head  and 
shoulders.  The  author  has  had  a  small  crane  of  wood  made,  which  well 
answers  the  last  mentioned  purpose,  and  may  be  attached  to  a  common 
chair.  It  would  be  out  of  place  here  to  detail  those  particulars  of  constitu- 
tional treatment  which,  in  peculiar  habits,  maybe  required  to  aid  the  effects 
of  the  means  above  described 

The  ribs — Attached  to  twelve  vertebrae  in  the  middle  of  the  back,  there 
are  the  ribs  or  bony  stretchers  of  the  cavity  of  the  chest,  constituting  a  struc- 
ture which  solves,  in  the  most  perfect  manner,  the  difficult  mechanical  pro- 
blem of  making  a  cavity  with  solid  exterior,  which  shall  yet  be  capable  of 
dilating  and  contracting  itself.  Each  pair  of  corresponding  ribs  may  be 
considered  as  constituting  a  hoop :  which  hangs  obliquely  down  from  the 
place  of  attachment  behind,  so  that  when  the  forepart  of  all  the  hoops  is 
lifted  by  the  muscles  the  cavity  of  the  chest  is  enlarged. 

We  have  to  remark  the  double  connection  of  the  rib  behind,  first  .to  the 
bodies  of  two  adjoining  vertebrae,  and  then  to  process  or  projection  from 
the  lower,  thus  affecting  a  very  steady  joint,  and  yet  leaving  the  necessary 
freedom  of  motion  :  and  we  observe  the  forepart  of  the  rib  to  be  joined  in  the 
breast-bone  by  flexible  cartilage,  which  allows  the  degree  of  motion  required 
there  without  the  complexity  of  a  joint,  and  admirably  guards,  by  its  elasti- 
city against  the  effects  of  sudden  blows  or  shocks. 

The  muscles,  which  have  their  origin  on  the  ribs  and  their  insertion  into 
the  bones  of  the  arm,  afford  us  an  example  worth  remembering  of  action  and 
reaction  being  equal  and  contrary.  When  the  ribs  are  fixed, 'these  muscles 
move  the  arm  ;  and  when  the  arm  is  fixed,  as  by  resting  on  a  chair  or  other 
object,  they  with  equal  force  move  the  ribs  The  latter  occurrence  is  seen 
in  fits  of  asthma  and  dyspnoea. 

The  human  skeleton,  with  its  naked  ribs,  is  so  associated  in  the  common 
mind,  with  ideas  of  death  and  loss  of  friends,  and  all  the  terrors  of  doubtful 


404  ANIMAL    AND 

futurity,  that  to  most  persons  it  is  an  object  of  abhorrence;  but  to  the  philo- 
sophic mind,  which  rises  superior  to  place  and  time,  the  so  admirable  adap- 
tation of  all  the  parts  to  their  purposes,  and  of  parts  which,  being  purely 
mechanical,  are  perfectly  understood,  makes  it  independently  of  all  profes- 
sional considerations,  an  object  of  the  most  intense  interest.  Such  mechanism 
reveals,  by  intelligible  signs,  the  hand  of  the  Creator;  and  a  man  may  be 
said  sublimely  to  commune  with  his  Maker,  who  contemplates  and  under- 
stands the  structure  aright. 

The  shoulder-joint  is  remarkable  for  combining  great  extent  of  motion 
with  great  strength.  The  round  head  of  the  shoulder-bone,  that  it  may  turn 
freely  in  all  ways,  rests  upon  a  shallow  cavity  or  socket  in  the  shoulder- 
blade;  and  the  danger  of  dislocation  from  this  shallowness  is  guarded  against 
by  two  strong  bony  projections  above  and  behind.  To  increase  the  range  of 
motion  to  the  greatest  possible  degree,  the  bone  called  the  shoulder-blade, 
which  contains  the  socket  of  the  arm,  slides  above  itself  upon  the  convex 
exterior  of  the  chest,  having  its  motions  limited  in  certain  directions  by  its 
connection,  through  the  collar-bone  or  clavicle,  with  the  sternum. 

The  scapula  or  blade-bone  is  extraordinary  as  an  illustration  of  the  me- 
chanical rules  for  combining  lightness  with  strength.  It  has  the  strength  of 
the  arch  from  being  a  little  concave,  like  the  dished  wheel  already  described, 
and  its  substance  is  chiefly  collected  in  its  borders  and  spines,  with  thin 
plates  between,  as  the  strength  of  a  wheel  is  collected  in  its  rim,  and  spokes, 
and  nave. 

The  bones  of  the  arms,  considered  as  levers,  have  the  muscles  which 
move  them  attached  very  near  to  the  fulcra,  and  very  obliquely,  so  that  the 
muscles,  from  working  through  a  short  distance,  comparatively  with  the 
displacement  of  the  resistances  at  the  extremities  require  to  be  of  great 
strength.  It  has  been  calculated  that  the  muscles  of  the  shoulder-joint,  in 
the  exertion  of  lifting  a  man  upon  the  hand,  pull  with  the  force  of  two 
thousand  pounds. 

Notwithstanding  all  the  securities  of  the  shoulder-joint  now  described  in 
the  infinite  variety  of  twists,  and  falls  and  accidents  to  which  men,  in  the 
busy  scene  of  society,  are  liable,  the  joint  is  frequently  dislocated,  that  is, 
the  rounded  head  of  the  humerus  or  arm-bone  slips  from  his  socket,  with 
instant  lameness  as  a  consequence. 

In  the  treatment  of  dislocations  and  fractures  of  the  frame-work  of  the 
human  body,  the  surgeon  cannot  avoid  displaying  strikingly  either  his  pro- 
fessional skill  or  ignorance.  With  what  ease  does  the  displaced  arm  or  thigh- 
bone return  to  its  socket,  under  the  guidance  of  the  skilful  hand ;  and  to 
what  horrible,  and  often  unavailing  torture,  is  the  patient  subjected,  when, 
in  such  a  case,  ignorance  dares  to  act !  It  is  very  painful  to  allow  the  imagi- 
nation to  dwell  upen  the  records  of  ancient  surgery,  and  to  be  made  present, 
as  it  were,  to  the  stretching  of  patients  on  the  rack  with  pullics  and  power- 
ful engines,  to  do,  what  better  information  could  have  accomplished  with 
such  gentleness.  And  would  that  the  records  of  modern  times  contained 
no  instances  of  individuals  crippled  for  life  by  bad  practice.  To  a 
practioner  in  this  branch,  impunity  and  a  quiet  conscience  can  now  be  se- 
cured only -by  his  having  a  perfect  knowledge  of  anatomy,  and  familiarity 
with  the  laws  of  mechanical  philosophy. 

With  our  present  information  on  these  subjects,  we  are  suprised  at  the 
detail  of  the  practices  and  errors  promulgated  in  former  times,  owing  to  im- 
perfect knowledge  of  mechanics,  even  by  authors  of  the  highest  credit.  It 
would  hardly  be  believed  that  so  distinguished  an  ornament  of  English  sur- 


MEDICAL    MECHANICS.  405 

gery  as  Mr.  Pott,  should  assign  as  one  reason  for  not  pulling  by  the  hand 
or  foot,  in  reducing  a  dislocation  of  the  shoulder  or  hip,  that  the  intervening 
joints  prevented  the  strain  from  reaching  the  part  desired. 

Some  surgeons,  possessing  a  certain  degree  of  knowledge  in  mechanics, 
but  only  that  degree  which  is  dangerous,  having  heard  that  the  lever  was 
a  powerful  engine,  have  tried  to  replace  bones  'solely  by  leverage,  as  it  was 
called.  Thus,  a  man's  dislocated  arm  has  been  placed  over  the  back  of  a 
chair  as  a  fulcrum,  or  over  the  top  of  a  door,  and  while  the  weight  of  the 
suffering  body  was  hanging  to  it  on  one  side  as  the  resistance,  force  has  been 
applied  to  the  other  side,  enough  sometimes  to  break  the  bone,  or  to  tear 
away  the  ligaments  and  soft  parts  about  the  joint. 

Other  surgeons,  after  learning  in  the  same  way  the  effects  of  the  pulley, 
have  wished  to  do  all  by  irresistible  extension,  and  instead  of  borrowing  the 
moderate  assistance  which  might  be  useful,  have  torn  muscles  and  ligaments 
from  their  attachments. 

It  is  not  the  object  of  this  work  to  enter  into  an  extended  examination  of 
the  accidents  which  befall  the  body  requiring  mechanical  skill  for  their  proper 
management,  for  this  would  be  to  deliver  a  course  of  instruction  on  practical 
surgery;  but  it  is  wished  to  awaken  the  attention  of  the  medical  student  to 
those  valuable  general  principles  which  may  furnish  direction  in  most  diffi- 
culties. Knowing  these  principles,  and  possessing  good  sense,  he  will  often, 
be  a  more  effective  minister  of  his  art  than  a  man  full  of  learned  precedents, 
who  knows  them  not.  To  make  this  lesson  more  impressive  to  his  young 
readers,  the  author  may  take  the  liberty  of  adducing  his  own  experience. 
When  he  was  himself  very  young,  and  had  not  yet  had  extensive  practical 
experience,  he  was  thrown  into  a  situation  where  a  heavy  medical  charge 
developed  upon  him,  and  where,  through  accidents  among  a  numerous  crew, 
during  a  very  eventful  voyage,  which  led  to  intercourse  with, the  savage 
inhabitants  of  unfrequented  coasts,  he  had,  within  twenty-six  months,  more 
practice  in  singular  wounds,  dislocations,  and  fractures,  than  falls  to  the  lot 
of  many  practitioners  during  a  life  : — in  that  time  he  became  strongly  im- 
pressed with  the  importance  to  the  medical  man  of  such  knowledge  as  he  now 
recommends  :  and  he  had  reason  to  rejoice  that  although  Natural  Philosphy 
was  not  then  much  insisted  upon  in  the  course  of  professional  education,  cir- 
cumstances had  led  him  to  look  carefully  at  the  body  through  that  medium. 

The  os  humerij  or  bone  of  the  upper  arm,  is  not  perfectly  cylindrical,  but 
like  most  of  the  other  bones  called  cylindrical,  it  has  ridges  to  give  strength, 
on  the  principle  explained  in  the  chapter  "  on  strength  of  materials." 

The  elbow  joint  is  a  correct  hinge,  and  so  strongly  secured  that  it  is  rarely 
dislocated  without  fracture. 

The  fore-arm  consists  of  two  bones  with  a  strong  membrane  between 
them.  Its  great  breadth,  from  this  structure,  affords  abundant  space  for  the 
origin  of  the  many  muscles  which  go  to  move  the  hand  and  fingers  :  and  the 
very  peculiar  mode  of  connection  of  the  two  bones,  gives  man  that  most 
useful  faculty  of  turning  the  hand  round,  into  what  are  called  the  positions 
of  pronation  and  supination, — exemplified  in  the  action  of  twisting  or  of 
turning  a  gimblet. 

The  old  surgeons,  who  acted  frequently  by  rules  of  routine  rather  than 
by  reasons,  in  the  accident  of  fracture  to  one  or  both  bones  of  the  fore-arm, 
often  applied  a  tight  bandage,  which  pulled  the  bones  at  the  fractured  part 
close  to  each  other,  and  thus  injured  the  future  shape  and  strength  of  the  arm. 

The  wrist.     The  many  small  bones  forming  the  wrist  have  a  signal  effect 


406  ANIMAL    AND 

of  deadening,  in  regard  to  the  parts  above,  the  shocks  or  blows  which  the 
hand  receives. 

The  annular  ligament  is  a  strong  band  passing  round  the  joint,  and 
keeping  all  the  tendons  which  pass  from  the  muscles  above  to  the  fingers, 
close  to  the  joint.  It  answers  the  purpose  of  so  many  fixed  pullies  for 
directing  the  tendons  :  without  it,  they  would  all,  on  action,  start  out  like 
bow-strings,  producing  deformity  and  weakness. 

The  human  hand  is  so  admirable,  from  its  numerous  mechanical  and 
sensitive  capabilities,  that  one  opinion  at  one  time  prevailed,  that  man's 
superior  reason  depended  on  his  possessing  such  an  instructor  and  such  a 
servant.  Now,  although  reason,  with  hoofs  instead  of  fingers,  could  never 
have  raised  man  much  above  the  brutes,  and  probably  could  not  have  secured 
the  continued  existence  of  the  species, — still,  the  hand  is  no  more  than  a 
fit  instrument  of  the  Godlike  mind  which  directs  it. 

The  pelvis,  or  strong,  irregular  ring  of  bone  on  the  upper  edge  of  which 
the  spine  rests,  and  from  the  sides  of  which  the  legs  spring,  forms  the  centre 
of  the  skeleton.  A  broad  bone  was  wanted  here  to  connect  the  central 
column  of  the  spine  with  the  lateral  columns  of  the  legs,  and  a  circle  was 
the  lightest  and  strongest.  If  we  attempt  still  farther  to  conceive  how  the 
circle  could  be  modified  so  as  to  fit  it — for  the  spine  to  rest  on,  for  the  thighs 
to  roll  in,  for  muscles  to  spring  from,  both  above  and  below,  for  the  person 
to  be  able  to  sit,  &c.,  we  shall  find,  on  inspection,  that  all  our  anticipations 
are  realized  in  the  most  perfect  manner.  In  the  pelvis,  too,  there  are  the 
thyroid  hole  and  ischiatic  notches,  furnishing  subordinate  instances  of  con- 
trivance to  save  material  and  weight : — they  are  merely  deficiencies  of  bone 
where  solidity  could  have  given  no  additional  strength.  The  broad  ring  of 
the  pelvis  protects  most  securely  the  important  organs  placed  within  it. 

The  hip  joint  exhibits  the  perfection  of  the  ball  and  socket  articulation. 
It  allows  the  feet  to  move  round  in  a  circle,  as  well  as  to  have  the  great 
range  of  backward  and  forward  motion,  exhibited  in  the  action  of  walking. 
When  we  see  the  elastic,  tough,  smooth  cartilage  which  lines  the  deep  socket 
of  this  joint,  and  the  similar  glistening  covering  of  the  ball  or  head  of  the 
thigh-bone,  and  the  lubricating  synovia  poured  into  the  cavity  by  appro- 
priate secretaries,  and  the  strong  ligaments  giving  strength  all  around,  we 
feel  how  far  the  most  perfect  of  man's  work  falls  short  of  the  mechanism 
exhibited  in  nature. 

The  thiyh-bone  is  remarkable  for  its  two  projections  near  the  top,  called 
trochanters,  to  which  the  moving  muscles  are  fixed ;  and  which  lengthen 
considerably  the  lever  by  which  the  muscles  work.  The  shaft  of  the  bone 
is  not  straight,  but  has  a  considerable  forward  curvature.  Short-sightedness 
might  suppose  this  a  weakness,  the  bone  being  a  pillar  to  support  a  weight ; 
but  the  bend  gives  it  in  reality  the  strength  of  the  arch,  to  bear  the  action 
of  the  mass  of  muscles  called  vastus,  which  lies  and  swells  upon  its  fore  part, 

The  knee  is  a  hinge  joint  of  complicated  structure,  claiming  the  most 
attentive  study  of  the  surgeon.  The  rubbing  parts  are  flat  and  shallow,  and, 
therefore,  the  joint  has  little  strength  from  form  ;  but  it  derives  security  from 
the  numerous  and  singularly  strong  ligaments  which  surround  it.  The  liga- 
ments on  the  inside  of  the  knees  resemble,  in  two  circumstances,  the  annular 
ligaments  of  joints,  viz.,  in  having  a  constant  and  great  strain  to  bear,  and 
yet  in  becoming  stronger  always  as  the  strain  increases.  The  line  of  the  leg, 
even  in  the  most  perfect  shapes,  bends  inward  a  little  at  the  knee,  requiring 
the  support  of  the  ligaments ;  and  in  many  persons  it  bends  very  much ;  but 


MEDICAL    MECHANICS.  407 

the  inclination  does  not  increase  with  age.  The  legs  of  many  weakly  in- 
kneed  children  become  straight  by  exercise  alone.  This  inclination  at  the 
middle  joint  of  the  leg,  by  throwing  a  certain  strain  on  the  ligaments,  gives, 
in  such  actions  as  jumping,  running,  &c.,  an  increase  of  elasticity  to  the  limb. 

In  the  knee  there  is  a  singular  provision  of  loose  cartilages  between  the 
ends  of  the  bones.  They  have  been  called  friction-cartilages,  from  a 
supposed  relation  in  use  to  friction-wheels,  but  their  real  effect  seems  to  be, 
to  accommodate,  in  the  different  positions  of  the  joint,  the  surfaces  of  the 
rubbing  bones  to  each  other. 

Under  the  head  Pneumatics,  we  shall  find  that  the  bones  forming  the 
joints,  are  held  together,  independently  of  their  ligaments,  by  a  constant 
pressure  of  the  atmosphere,  amounting  in  the  knee,  for  instance,  to  upwards 
of  sixty  pounds. 

The  great  muscles  on  the  fore-part  of  the  thigh  are  contracted  into  a  ten- 
don a  little  above  the  knee,  over  and  in  front  of  which  the  tendon  has  to 
pass  to  reach  the  top  of  the  leg,  where  its  attachment  is.  The  part  of  the 
tendon  over  the  joint  becomes  bony,  and  forms  the  patella  or  knee-pan, 
often  called  the  pulley  of  the  knee.  This  peculiarity  enables  the  muscles 
to  act  more  advantageously,  by  increasing  the  distance  of  the  rope  from  the 
centre  of  the  motion.  The  petella  is,  moreover,  a  sort  of  shield  or  protection 
to  the  fore-part  of  this  important  joint. 

The  leg  below  the  knee,  like  the  fore-arm  already  described,  has  two 
bones.  They  offer  spacious  surface  of  origin  for  the  numerous  muscles 
required  for  the  feet,  and  they  form  a  compound  pillar  of  greater  strength 
than  the  same  quantity  of  bone  as  one  shaft  would  have  had.  The  indi- 
vidual bones  also  are  angular  instead  of  round,  hence  deriving  greater  power 
to  resist  blows,  &c. 

The  ankle-joint  is  a  perfect  hinge  of  great  strength.  There  is  in  front  of 
it  an  angular  ligament,  by  which  the  greater  part  of  the  tendons  passing 
downwards  to  the  foot  and  toes  are  kept  in  their  places.  One  of  these  ten- 
dons passes  behind  and  under  the  bony  projection  of  the  inner  ankle,  in  a 
smooth,  appropriate  groove,  exactly  as  if  a  little  fixed  pulley  were  there. 

The  heel,  by  projecting  so  far  backwards,  is  a  lever  for  those  strong  mus- 
cles to  act  by,  which  form  the  calf  of  the  leg  and  terminate  in  the  tendo- 
achillis.  The  muscles  by  drawing  at  it,  lift  the  body,  in  the  actions  of 
standing  on  the  toes,  walking,  dancing,  &c.  In  the  foot  of  the  negro,  the 
heel  is  so  long  as,  in  European  estimation,  to  appear  ugly  ]  and  its  great 
length  rendering  the  effort  of  smaller  muscles  sufficient  for  the  various  pur- 
poses, the  calf  of  the  negro's  leg  is  smaller  than  of  other  races  of  men. 

In  a  graceful  human  step  the  heel  is  always  raised  before  the  foot  is  lifted 
from  the  ground,  as  if  the  foot  were  part  of  a  wheel  rolling  forward  ;  and 
the  weight  of  the  body,  supported  by  the  muscles  of  the  calf  of  the  leg,  as 
just  described,  rests  for  the  time  on  the  fore-part  of  the  foot  and  toes.  There 
is  at  that  time  a  bending  of  the  foot  in  a  certain  degree.  But  where  strong 
wooden  shoes  are  used,  or  any  shoe  so  stiff  that  it  will  not  yield  and  allow 
this  bending  of  the  foot,  the  heel  is  not  raised  at  all  until  the  whole  foot  rises 
with  it,  so  that  the  muscles  of  the  calf  are  scarcely  used,  and  in  consequence 
soon  dwindle  in  size,  and  almost  disappear.  Many  of  the  English  farm- 
servants  wear  heavy  stiff  shoes,  and  in  London  may  constantly  be  seen  i»s 
the  drivers  of  country  wagons,  with  fine  robust  body  and  arms,  but  with  legs 
which  are  fleshless  spindles,  producing  a  gait  most  awkward  and  unmanly. 
The  brothers  of  these  men,  otherwise  employed,  are  not  so  mis-shapen ; 
and  even  they  themselves,  when  they  choose  to  become  soldier,  and  are 


408  ANIMAL    AND 

trained  in  military  exercises,  lose  their  peculiarity.  What  a  pity  that,  for 
the  sake  of  a  trifling  saving,  graceful  nature  should  be  thus  deformed.  An 
example  of  an  opposite  kind  is  seen  in  Paris,  where,  as  the  streets  have  no 
side  pavements,  and  the  ladies  are  obliged  consequently  to  walk  almost  con- 
stantly on  tiptoe,  the  great  action  of  the  muscles  of  the  calf  has  given  a 
conformation  of  the  leg  and  foot,  to  match  which  the  Parisian  belles  proudly 
challenge  all  the  world, — not  aware,  probably,  that  it  is  a  defect  of  their 
city  to  which  the  boasted  peculiarity  is  mainly  due. 

A  person  confined  to  his  bed  for  a  weqk  or  two  by  sickness,  has  generally 
to  remark  a  much  greater  wasting  of  the  legs  than  of  the  arms :  the  reason 
of  which  is,  that  the  muscles  of  the  leg,  in  ordinary  cases,  being  more  in 
use  than  those  of  the  arms,  their  ordinary  bulk  is  more  dependent  on  use, 
and  they  suffer  a  corresponding  change  from  inaction. 

Such  facts  as  now  mentioned,  bear  directly  on  the  subject  so  near  the 
hearts  of  many  English  mothers,  viz.,  the  weak  and  crooked  backs  of  their 
daughters.  From  such  they  may  understand  that  strong  stays,  which  in 
part  supercede  the  action  of  the  muscles  placed  by  nature  around  the  spine 
to  support  it,  cause  these  muscles  to  dwindle,  and  afterwards,  when  the  sup- 
port of  the  stays  fails  or  becomes  unequal,  leave  the  back  to  bend  or  twist. 
Stays,  therefore,  can  neither  help  to  make  strong  and  well-formed  backs, 
originally,  nor  can  they  be  a  remedy  after  the  weakness  has  commenced. 
A  healthy  young  woman  from  the  country,  with  spine  lying  deep  between 
the  firm  cushions  of  muscles  which  support  it,  if,  according  to  town  fashion, 
braced  up  in  tight  stays,  will  frequently,  at  the  end  of  a  short  time,  exhibit 
such  a  wasting  of  the  flesh,  that  the'  points  of  bone  in  the  spine  may  be 
counted  by  the  eye,  all  the  way  down. 

The  arch  of  the  foot  is  to  be  noticed  as  another  of  the  many  provisions 
for  saving  the  body  from  shocks  by  the  elasticity  of  the  supports.  The  heel 
and  the  ball  of  the  toes  are  the  two  extremes  of  the  elastic  arch,  and  the 
leg  rests  between  them. 

Connected  with  elasticity,  it  is  interesting  to  remark  how  imperfectly  a 
wooden  leg  answers  the  purpose  of  a  natural  one.  The  centre  of  the  body, 
when  supported  by  the.  wooden  leg,  which  always  remains  of  the  game 
length,  must  describe,  at  each  step  a  portion  of  a  circle  of  which  the  bottom 
nob  of  the  leg  is  the  centre;  and  the  body  is,  therefore,  constantly  rising  and 
falling; — but  with  the  natural  legs,  which,  by  gentle  flexture  at  the  knee, 
are  made  shorter  or  longer  in  different  parts  of  the  step  as  required,  the  body 
is  carried  along  in  a  manner  perfectly  or  nearly  level.  In  like  manner,  a 
man  riding  on  horseback,  if  he  kept  his  back  upright  and  stiff,  has  his  head 
jolted  by  every  step  of  the  trotting  animal;  but  the  experienced  horseman, 
even  without  rising  in  the  stirrups,  by  letting  the  back  yield  a  little  at  each 
movement,  as  a  bent  spring  yields  during  the  motion  of  a  carriage,  can 
carry  his  head  quite  smoothly  along. 

In  a  general  review  of  the  skeleton,  we  have  to  remark,  1st,  the  nice 
adaptation  of  all  the  parts  to  one  another,  and  to  the  strains  which  they  have 
respectively  to  bear;  as — in  the  size  of  the  spinal  vertebrae  increasing  from 
above  downwards — the  bones  of  the  leg  being  larger  than  thos£  of  the  arm, 
and  so  on.  2dly,  the  objects  of  strength  and  lightness  combined  ;  as  by  the 
hollowness  of  the  long  bones — their  angular  form — their  thickening  and 
flextures  in  particular  places  where  great  strain  has  to  be  borne — the  enlarge- 
ment of  the  extremities  to  which  the  muscles  are  attached,  lengthening  the 
lever  by  which  these  acts,  &c.  3dly,  we  have  to  remark  the  nature  and 


MEDICAL     MECHANICS.  409 

strength  of  material  in  different  parts,  so  admirably  adapted  to  the  purposes 
which  the  parts  serve :  there  is  bone  for  instance,  in  one  place  nearly  as 
hard  as  iron,  where,  covered  with  enamel,  it  has  the  form  of  teeth,  with  the 
office  of  chewing  and  tearing  all  kinds  of  matter  used  as  food;  in  the  cranium 
again,  bone  is  softer  but  tough  and  resisting ;  in  the  middle  of  the  long  bones 
it  is  compact  and  little  bulky,  to  leave  room  for  the  swelling  of  the  muscles 
lying  there;  while  at  either  end,  with  the  same  quantity  of  matter,  it  is 
large  and  spongy,  to  give  a  broad  surface  for  articulation ;  and  in  the  spine, 
the  bodies  of  the  vertebrae,  which  rest  on  an  elastic  bed  of  intervertebral  sub- 
stance are  light  and  spongy,  while  their  articulating  surfaces  and  processes 
are  very  hard.  Tn  the  joints  we  see  the  tough  elastic  smooth  substance  called 
cartilage,  covering  the  ends  of  the  bones,  defending  and  padding  them,  and 
destroying  friction.  In  infants  we  find  all  the  bones  soft  or  gristly,  and  there- 
fore calculated  to  bear  with  impunity  the  falls  and  blows  incidental  to  their 
age ;  and  we  see  certain  parts,  where  elasticity  is  necessary  or  useful,  re- 
maining cartilage  or  gristle  for  life,  as  at  the  anterior  extremities  of  the  ribs. 
About  the  joints  we  have  to  remark  the  ligaments  which  bind  the  bones  to- 
gether, possessing  a  tenacity  scarcely  equalled  in  any  other  known  substance; 
and  we  see  that  the  muscular  fibres  whose  contractions  move  the  bones  and 
thereby  the  body, — because  they  would  have  rendered  the  limbs  clumsy 
even  to  deformity  had  they  all  passed  over  the  joints  to  the  parts  which 
they  have  to  pull, — attach  themselves,  at  convenient  distances,  to  a  strong 
cord  called  a  tendon,  by  means  of  which,  like  a  hundred  sailors  at  a  rope 
they  make  their  effort  effective  at  any  distance.  The  tendons  are  remarka- 
ble for  the  great  strength  which  resides  in  their  slender  forms,  and  for  the 
lubricated  smoothness  of  their  surfaces.  Many  other  striking  particulars 
might  be  enumerated,  but  these  may  suffice.  Such,  then,  is  the  skeleton 
or  general  frame-work  of  the  human  body ;  less  curious  and  complicated, 
perhaps,  than  some  other  parts  of  the  system  which  we  have  yet  to  examine, 
but  so  perfect  and  so  wonderful,  that  the  mind  which  can  attentively  con- 
sider it  without  emotion  is  in  a  state  not  to  be  envied.* 

This  living  force  of  man  has  been  used  as  a  working  power  in  various 
ways,  as  in  turning  a  winch — pulling  a  rope — walking  in  the  inside  of  a 
large  wheel  to  move  it,  as  a  squirrel  or  turnspit  dog  moves  his  little  wheel, 
&c.  Each  of  these  has  some  particular  advantage :  but  the  mode  in  which 
for  many  purposes,  the  greatest  effect  may  be  produced,  is  for  a  man  to  carry 
up  to  a  height  his  body  only,  and  then  to  let-it  work  by  its  weight  in  de- 
scending. A  bricklayer's  labourer  would  be  less  fatigued  to  lift  twice  as 
many  bricks  to  the  top  of  a  house  in  the  course  of  a  day,  by  ascending  the 
ladder  without  a  load,  and  raising  bricks  of  nearly  his  own  weight  over  a 
pully  each  time  in  descending,  than  by  carrying  fewer  bricks  and  himself 
up  together,  and  descending  again  without  a  load,  as  is  still  usually  done. 

Reflection,  independently  of  experiment,  would  naturally  anticipate  the 
above  stated  result,  for  the  load  which  a  man  should  be  best  able  to  carry, 
is  surely  that  from  which  he  can  never  free  Jiimself, — the  load  of  his  own 
body.  Accordingly  the  strength  of  muscles  and  disposition  of  parts  are  all 
such  as  to  make  his  body  appear  very  light  to  him. 

The  question  which  was  agitated  with  such  warmth  some  time  ago  as  to 

*  In  the  second  and  third  editions  of  this  work  a  criticism  was  introduced  at  this 
place  of  a  treatise  on  "Animal  Mechanics,"  published  as  a  part  of  the  Library  of 
Useful.  Knowledge;  in  which  treatise  the  author  from  imperfect  acquaintance  with 
Natural  Philosophy  had  fallen  into  many  grave  errors.  That  note,  having  answered  its 
purpose  is  not  repeated  here. 


410  ANIMAL    AND 

the  propriety  of  condemning  men  and  women  to  work  on  the  tread-mill  re- 
ceives an  easy  decision  here.  They  work  by  climbing  on  the  outside  of  a 
large  wheel  or  cylinder,  which  is  turned  by  their  weight,  and  on  which,  to 
avoid  falling  from  their  proper  situation,  they  must  advance  just  as  fast  as  it 
turns.  There  are,  on  the  outside  of  the  cylinder,  projections  or  steps  for  the 
feet,  and  the  action  to  the  workers  is  exactly  that  of  ascending  an  acclivity. 
Now,  as  nature  has  fitted  the  human  body  for  climbing  hills  as  well  as  for 
walking  on  plains,  the  work  of  the  tread-mill,  under  proper  restrictions  as 
to  duration,  must  be  nearly  as  natural  and  healthful  as  any  other.  Its 
effects  have  ultimately  proved  it  to  be  so. 

Animal  power  being  exhausted  in  proportion,  as  well  to  the  time  during 
which  it  is  acting,  as  to  the  intensity  of  force  exerted,  there  may  often  be  a 
great  saving  of  power  by  doing  work  quickly,  although  with  a  little  more 
exertion  during  the  time.  Suppose  two  men  of  equal  weight  to  ascend  the 
same  stair,  one  of  whom  takes  only  a  minute  to  reach  the  top,  and  the  other 
takes  four  minutes,  it  will  cost  the  first  but  a  little  more  than  a  fourth  part 
of  the  fatigue  which  it  costs  the  second,  because  the  exhaustion  has  relation 
to  the  time  during  which  the  muscles  are  acting.  The  quick  mover  may 
have  exerted,  perhaps,  one-twentieth  more  force  in  the  first  instant,  to  give 
his  body  the  greater  velocity  which  was  afterwards  continued,  but  the  sloth 
supported  his  load  four  times  as  long. 

A  healthy  man  will  run  rapidly  up  a  long  stair,  and  his  breathing  will 
scarcely  be  quickened  at  the  top ;  but  if  he  walk  up  slowly,  his  legs  will 
feel  fatigued,  and  he  will  have  to  wait  some  time  before  he  can  speak  calmly. 

For  the  same  reason  coach-horses  are  much  spared  by  being  made  to, 
gallop  up  a  short  hill,  and  then  being  allowed  to  go  more  slowly  for  a  little 
time,  so  as  to  rest  at  the  top. 

The  rapid  waste  of  muscular  strength  which  arises  from  continued  action, 
is  proved  by  keeping  the  arm  extended  horizontally  for  some  time.  Few 
persons  can  continue  the  exertion  beyond  a  minute  or  two.  In  animals 
which  have  long  horizontal  necks,  there  is  a  wonderful  provision  of  nature 
in  a  strong  elastic  substance  on  the  back  or  upper  part  of  the  neck  which 
nearly  supports  the  head  independently  of  muscular  exertion. 

In  farther  illustration  of  the  truth  that  strength  is  saved  in  many  cases  by 
doing  work  quickly,  we  may  recall  the  fact  explained  at  page  60,  that  a  body 
thrown  or  shot  upwards  with  double  velocity,  rises  four  times  as  far  when 
that  with  a  single  velocity,  or  half  the  other. 

"Instruments.1' 

The  following  remarks  regard  some  instruments  used  by  medical  men,  and 
which  range  under  the  present  division  of  "  Mechanics." 

The  obstetric  forceps — As  the  blades  beyond  the  joint  or  fulcrum  are 
longer  than  the  handles,  the  pressure  on  the  head  included  in  them  is  less 
than  that  exerted  by  the  hand  that  uses  them,  but  its  degree  should  always 
be  kept  present  to  the  mind  of  the  operator. 

The  vectiSj  or  lever  used  instead  of  the  forceps  just  mentioned,  is  a  dan- 
gerous instrument  in  unskilful  hands.  In  fact  whenever  it  is  used  as  a 
lever,  in  the  common  acceptation  of  the  term,  with  some  part  of  the  pelvis 
as  the  fulcrum,  the  use  of  it  is  a  piece  of  unskilful  cruelty;  for  the  soft  parts 
between  the  bone  and  the  instrument  are  bruised  not  only  with  the  whole 
force  of  the  hand,  but  with  twice  or  thrice  as  much,  according  as  the  resist- 


MEDICAL    MECHANICS.  411 

ance  is  nearer  to  the  fulcrum  than  to  the  hand.  The  instrument  is  safely 
used,  only  when  the  operator  makes  one  of  his  hands  the  fulcrum,  and  uses 
the  other  as  the  power,  or  makes  different  parts  of  the  same  hand  answer 
both  purposes ;  and  then  there  is  a  resemblance  between  the  action  of  the 
vectis  and  of  a  hook. 

The  levator,  or  lever  for  rising  the  broken  and  depressed  portion  of  the 
skull  in  trepanning,  has  a  fulcrum  attached  to  it  by  a  joint.  Care  should 
be  taken  to  place  this  fulcrum  where  its  pressure  cannot  be  injurious. 

The  circular-saw  or  crown-saw  of  the  trephine,  should  be  worked  with  a 
quick  motion  and  gentle  pressure,  for  the  reason  given  at  page  107,  when 
treating  of  cutting  instruments.  The  purpose  is  thereby  attained,  both 
sooner  and  better,  and  the  head  of  the  patient  is  less  shaken. 

For  the  same  reason,  in  amputation,  a  light  and  quick  motion  of  the 
straight  saw  causes  less  jarring,  and  effects  an  easier  division  of  the  bone. 

In  using  the  amputation  knife,  the  speed,  neatness  and  success  of  the 
operation  are  all  favoured  by  blending  the  drawing  or  saw  motion  of  the  knife, 
with  the  pressure  towards  the  bone 

These  last  observations  are  of  a  hundred  similar,  which  might  be  made  to 
prove  the  extreme  importance  to  a  surgeon  of  having  familiarity  with  the 
use  of  tools  and  instruments.  Perhaps  a  person  cannot  better  acquire  this 
than  by  practising,  while  young,  some  amusing  work  of  carpentry.  Manual 
dexterity,  and  a  little  readiness  at  mechanical  contrivance,  so  frequently 
prove  of  importance  to  persons.in  all  stations,  that  it  is  a  great  defect  in  the 
prevailing  system  of  general  education,  not  to  cultivate  them  with  greater 
attention.  ^ 

The  tooth-key  is  an  instrument  found  in  many  hands,  and  persons  who  do 
not  pretend  to  more  than  the  lowest  degree  of  skill  in  the  healing  art,  are 
not  afraid  to  use  it.  The  consequence  is,  that  perhaps  scarcely  a  day  passes 
in  which  teeth  are  not  broken  and  jaws  splintered,  and  gums  bruised  even 
to  sloughing,  by  the  unskilful  or  awkward  use  of  it.  The  common  tooth-key 
may  be  compared  to  wheel  and  axle,  the  hand  of  the  operator  acting  on  two 
spokes  of  the  wheel  to  work  it,  while  the  tooth  is  fixed  to  the  axle  by  the 
claw,  and  is  drawn  out  as  the  axle  turns.  The  gum  and  alveolar  process  of 
the  jaw  form  the  support  on  which  the  axle  rolls.  The  common  errors  in 
tooth-drawing  by  the  key,  are  these  : 

1st.  Turning  the  key  towards  that  side  where  the  adjoining  teeth  are  so 
close  that  the  tooth  to  be  drawn  cannot  pass,  without  either  breaking  one  of 
them,  or  being  itself  broken.  Sometimes  two  or  even  three  teeth  are  thus 
unfixed  instead  of  one. 

2d.  Neglecting  the  natural  inclination  of  the  tooth.  By  winding  it  round 
in  the  direction  in  which  it  already  inclines,  and  in  accordance  with  a  bend 
which  is  generally  found  in  it,  the  operation  is  easy  and  safe ;  but  by  draw- 
ing it  in  the  opposite  way,  it  not  unfrequently  is  broken,  or  it  splinters  the 
part  of  the  jaw-bone  in  which  it  was  set. 

3d.  If  the  tooth-claw  be  blunt,  its  point  may  slip  upon  the  tooth,  so  as 
to  produce  a  jar  which  is  very  apt  to  break  the  tooth 

4th.  Unless  the  axle  or  fulcrum  of  the  key  be  made  to  rest  as  evenly  as 
possible  on  the  gum,  it  will  tear  or  otherwise  injure  the  gum.  It  should 
rest,  if  possible,  over  the  part  of  the  bone  in  which  the  tooth  is  set,  for  if 
not — as  when  a  back  tooth  is  drawn  by  an  instrument  resting  on  a  part  con- 
siderably anterior  to  it — the  twist  produced  is  painful,  and  there  is  danger 
of  splintering. 


412  ANIMAL    AND 

A  man  who,  after  making  these  reflections,  operates  leisurely  a  few  times 
on  the  dead  subject,  will  be  able  to  give  instant  and  safe  relief  to  very  com- 
mon and  most  intense  suffering.  And  it  is  hardly  excusable  in  any  medi- 
cal man  who  may  be  placed  where  a  professed  dentist  cannot  be  procured, 
to  neglect  acquiring  a  talent  so  easy. 

Some  dentists,  by  a  strong  forceps  made  for  the  purpose,  pull  teeth 
directly  out;  others  by  a  simple  sharp-pointed  lever  push  them  out;  and 
others  use  a  forceps  in  the  manner  of  the  tooth-key,  by  resting  one  side  of 
it  on  the  gum  as  a  fulcrum,  and  then  giving  it  a  twisting  motion  :  in  the 
latter  case,  the  resting  side  of  the  forceps  is  formed  like  the  bolster  of  a 
tooth-key.  But  much  more  in  all  cases  depends  on  the  dexterity  of  the 
operator  than  on  the  form  of  the  instrument. 

Steel  trusses  for  ruptures  are  among  the  blessings  to  suffering  humanity 
which  modern  ingenuity  has  supplied.  From  the  unhealthy  employments 
of  some  men,  and  the  early  dissipations  or  unnatural  modes  of  life  of  others, 
debilitated  constitutions  are  frequent,  and  are  often  transmitted  to  offspring; 
and  one  of  the  lamentable  effects  is  that  weakness  of  the  flesh  forming  the 
sides  of  cavities  which,  and  on  occasions  of  strong  effort,  allows,  at  particular 
points  the  living  parts  to  protrude  from  within,  so  as  to  form  tumours  under 
the  skin.  A  person  perfectly  healthy  may  suffer  the  same  injury  from  a 
very  violent  strain  or  pressure  on  the  abdoman.  The  occurrence  is  called 
hernia  or  rupture  ;  the  most  common  hernia  is  that  of  the  small  intestine 
through  the  groins. 

Formerly  this  occurrence  disabled  for  life.  A  man  who  had  hernia  was 
discharged  from  the  army  or  navy ;  he  could  not  ride  on  horseback,  or  take 
usual  exercise ;  he  could  not  lift  a  weight ;  and  in  a  word  he  often  became 
a  miserable  burden  to  himself  and  others.  Now,  by  fitting  the  pad  of  a 
good  steel  truss  to  the  part,  the  rupture  is  as  perfectly  restrained,  as  if  the 
hand  of  a  skilful  surgeon  were  constantly  there.  The  truss  may  be  put  on 
and  off,  with  as  little  reflection  or  trouble,  as  a  part  of  the  ordinary  dress, 
and  the  man  becomes  again  almost  as  fit  for  all  the  duties  of  life  as  if  he 
were  without  his  ailment. 

The  old  and  still  common  form  of  the  steel  truss  is  that  of  a  half  or  three- 
quarter  hoop,  so  bent  and  tempered,  that  when  put  upon  the  patient,  one 
end  which  has  a  pad  upon  it,  presses  with  a  given  force,  over  the  opening  by 
which  the  rupture  protrudes,  and  the  remainder  tightly  embraces  the  body. 
The  objections  to  this  kind  of  truss  are,  the  difficulty  of  making  it  to  fit 
exactly;  its  being  rather  troublesome  to  put  on  and  off;  and  its  pressing 
disagreeably  round  the  body. 

Another  kind  of  truss,  free  from  these  objections,  consists  of  a  little  more 
than  half  a  hoop,  with  a  pad  at  each  end  :  one  of  the  pads  supports  the 
weakness,  and  the  other  rests  upon  the  centre  of  the  back,  to  bear  all  the 
strain  there,  while  the  hoop  itself  passing  round  the  hip  farthest  from  the 
hernia,  reposes  loosely  on  the  hip.  This  truss  may  be  called  self-adjusting, 
for  it  almost  of  itself  falls  into  its  place,  and  needs  no  fastenings ;  the  same 
truss  fits  all  persons  of  one  size,  whatever  their  shape;  and  the  strength  may 
be  adjusted  by  changing  the  number  of  plates  in  the  spring-hoop. 

Tourniquets,  creches,  splints,  &c.,  &c.,  are  so  simple  in  all  respects  as  not 
to  merit  special  notice  here. 

This  section  contains  some  of  the  reflections  which,  in  contemplating  the 
human  skeleton,  occur  to  a  person  familiar  with  mechanical  philosophy :  and 
the  more  complete  such  a  person's  knowledge  is  of  anatomy,  physiology, 


MEDICAL    MECHANICS.  413 

surgery  and  medicine,  the  more  numerous  will  be  the  professional  objects  on 
which  this  philosophy  will  shed  a  light,  dissipating  doubt  and  error.  The 
author  has  not  entered  into  more  minute  detail,  because  it  would  have  been 
encroaching  upon  the  office  of  the  teachers  of  particular  departments,  and 
because  he  thinks  that  any  one  who  is  not  enabled,  by  the  examples  here 
given,  to  make  the  applications  of  the  general  laws  to  all  possible  cases, 
may  account  the  study  ©f  the  healing  art  unsuited  to  the  faculties  with 
which  he  is  endowed. 


414  FLUIDITY    IN    RELATION    TO    ANIMALS. 

• 

PART  V. 

(CONTINUED.) 


SECTION  II.— DOCTRINES   OF   FLUIDITY  IN    RELATION   TO 

ANIMALS. 


In  the  preceding  sections  on  the  laws  of  fluidity,  occasional  illustrations 
have  been  taken  from  the  animal  economy-  but  there  are  many  otherpar- 
ticulars  of  the  same  class  and  of  great  interest,  which  it  is  convenient  to 
consider  apart  under  the  four  heads  of,  1st.  The  circulation  of  the  Hood  ; 
2d.  The  respiration  and  voice;  3d.  The  \digestion^  and  4th.  The  pelvic 
phenomena.  It  is  important  to  remark  here,  that  this  section  cannot  be 
understood  by  a  person  ignorant  of  those  which  precede. 

THE    CIRCULATION   OF   THE   BLOOD. 

PERHAPS  there  are  few  points  more  remarkable  in  the  history  of  the  pro- 
gress by  which  man  has  arrived  at  his  present  knowledge  of  the  universe, 
than  that  he  was  so  long  ignorant  of  the  fact  that  the  blood  in  his  own  and 
in  other  animal  bodies  is  constantly  circulating.  England  claims  as  one  of 
her  sons,  the  man  whose  powerful  intellect  at  last  established  this  truth,  in 
opposition  to  strong  appearances,  and  to  the  most  fixed  prejudices.  Dr. 
Harvey  published  his  proofs  in  the  year  1619.  A  person  who  tries  to  ima- 
gine what  the  science  of  medicine  could  have  been  while  it  took  no  account 
of  this  fact,  on  which,  as  a  basis,  much  of  the  certain  reasoning  about  the 
phenomenon  of  life  must  rest,  is  prepared  for  what  old  medical  books  exhibit 
of  the  writhings  of  human  reason,  in  attempts  to  explain  and  to  form  theories, 
while  a  fatal  error  was  mixed  with  every  supposition. — The  chief  circum- 
stances which  prevented  the  earlier  discovery  of  the  circulation  was,  that  on 
examining  dead  bodies,  the  arteries  were  always  found  empty  of  blood ; — 
which  was  the  reason,  also,  of  those  vessels  being  called  arteries  or  air- 
tubes. 

We  now  know,  that  as  the  Thames  water  spreads  over  London  in  pipes. 
to  supply  the  inhabitants  generally,  and  to  answer  the  particular  purposes  of 
brewers,  bakers,  tanners,  and  others,  and  is  then  in  great  part  returned  to 
where  the  current  sweeps  away  the  impurities,  so,  nearly  in  the  human 
body,  does  the  blood  spread  from  the  centre,  through  the  arteries,  to  nourish 
all  the  parts,  and  to  supply  material  of  secretion  to  the  liver,  the  kidneys, 
the  stomach,  and  other  viscera;  and  returns  from  these  by  the  veins  towards 
the  heart  and  lungs,  to  be  purified  and  to  have  its  waste  replenished,  that  it 
may  again  renew  its  course. 

The  circulation  may  be  more  particularly  described  thus  :  From  the  left 


THE    CIRCULATION.  415 

chamber  or  ventricle  of  the  strong  muscular  mass,  the  heart,  a  large  tube 
arises,  called  the  aorta  ;  and  b*y  a  continued  division  or  ramification,  opens 
a  way  for  the  bright  scarlet  blood  to  the  very  minutest  part  of  the  living 
frame, — the  extreme  divisions  of  twigs  being  so  small  that  they  are  called 
capillary  or  hair-like  tubes.  At  the  termination  of  these  vessels,  the  blood, 
after  answering  the  purposes  of  general  nutrition,  &c.,  by  which  it  loses  its 
bright  colour,  enters  the  commencement  of  the  venous  tree,  or  returning 
channel ;  and  gliding  successively  from  smaller  to  larger  branches,  returns 
towards  the  right  chamber  or  ventricle  of  the  heart  requiring  purification  and 
partial  renewal."  Considering  the  great  arterial  and  venous  systems  of  the 
body  as  twin  trees — the  scarlet  and  the  purple,  with  corresponding  and 
meeting  branches,  and  with  trunks  which  touch  each  other  at  the  heart — it 
will  appear  that  they  again  similarly  meet  or  inosculate  by  their  extreme 
roots,  and  thus  form  a  continued  or  circular  channel.  The  root  of  the  venous 
tree,  by  which  the  blood  spreads  from  the  right  chamber  of  the  heart  through 
the  lungs  is  called  the  pulmonary  artery,  and  that  of  the  arterial  tree,  by 
which  the  blood  returns  to  the  left  chamber,  is  called  the  pulmonary  vein. 
Both  of  these  ramify  in  the  spongy  masses  of  the  lungs  forming  a  great  part 
of  the  pulmonary  substance.  Fresh  material  for  the  blood  is  brought  from 
the  digestive  organs  by  the  I  icteal  absorbents  and  thoracic  duct,  and  is  con- 
stantly pouring  into  a  large  vein  near  the  heart,  to  be  completely  mixed  with 
the  dark  or  returning  blood  by  a  violent  agitation  or  churning  during  its 
passage  through  the  heart.  The  mixture,  on  leaving  the  right  ventricle,  is 
strained  through  the  minute  ramifications,  the  vessels  in  the  lungs,  and  at 
the  same  time  is  exposed  to  the  action  of  the  air  entering  the  cells  of  the 
lungs  in  respiration,  by  which  exposure  the  dark  purple  blood  becomes  again 
pure  scarlet,  and  when  it  reaches  the  left  chamber  or  ventricle,  is  ready  to  set 
out  on  its  journey  as  before,  charged  with  new  life  and  nutriment.  The 
two  chambers  or  ventricles  of  the  heart  have  each  an  anti-chamber  or  auricle, 
(so  called  from  an  external  resemblance  to  a  dog's  ear,)  into  which  the  blood 
is  first  received  from  the  veins ',  and  there  are  valvular  doors  between  the 
auricle  and  ventricle,  which  allow  the  blood  to  pass  readily  into  the  ventricle, 
but  oppose  its  recoil  during  the  ventricular  contraction.  Similarly  acting 
valves  are  placed  between  the  ventricles  and  great  arteries.  There  are  valves, 
also,  in  many  of  the  veins,  over  the  body,  to  secure  the  natural  course  of 
the  circulation.  Besides  the  important  change  of  purification  which  the 
blood  undergoes  in  passing  through  the  lungs,  its  composition  is  much 
influenced  by  the  action  of  the  kidneys,  of  the  exhalent  of  the  skin,  and  of 
the  liver, — the  two  former  relieving  it  of  superfluous  moisture  and  salts,  the 
last  from  a  large  quantity  of  matter  in  the  form  of  bile. 

The  description  given  above  of  the  circulation  of  the  bloo'd  is  only  an  out- 
line; and  yet  by  showing  the  manner  in  which  fresh  material  enters,  it  con- 
tains more  than  Harveyknew  of  the  subject.  In  this  department  of  knowledge, 
as  in  most  others,  we  have  advanced  from  the  very  general  and  vague,  to  the 
more  particular  and  precise , — and  just  as  the  general  nature  of  steam  was 
known  long  before  it  served  in  steam-engines,  and  as  the  period  of  the  moon's 
revolutions  had  been  accurately  observed  for  thousand  of  years  before,  the 
fluctuations  in  her  velocity  could  be  calculated  so  as  to  make  her  the  mari- 
ner's best  guide  in  his  courses  across  the  ocean,  so,  when  Harvey  had  proved 
the  general  fact  of  the  circulation  of  the  blood,  he  had  left  much  yet  to  be 
done,  by  observing  and  raising  from  subordinate  facts,  to  render  the  know- 
ledge available  for  the  many  useful  purposes  which  it  is  calculated  to  serve. 

Within  a  few  years  only,  has  the  importance  of  the  subordinate  circum- 


416  FLUIDITY    IN    RELATION     TO    ANIMALS. 

stances  been  fully  appreciated, — as  is  evinced  by  the  numerous  works 
composed  to  elucidate  them  ;  but  many  of  which  works  have  served  only  to 
prove,  that  if  the  difficulties  were  to  be  solved  by  natural  philosophy,  medical 
men  in  general  had  not  yet  studied  it  sufficiently  to  be  able  to  use  it  suc- 
cessfully. In  this  section  it  will  be  attempted  to  place  certain  important 
points  of  the  subject  in  a  clear  light :  and  by  referring  directly  to  the  general 
laws  of  nature,  as  explained  in  the  body  of  the  work,  to  settle  existing 
disputes  on  some  of  these  points,  to  remove  remaining  doubts  on  others, 
and  to  suggest  some  important  new  applications. 

The  fact  of  the  circulation  of  the  blood  being  once  admitted,  an  inquirer 
who  contemplates  the  apparatus  by  which  it  is  effected,  is  led  by  the  general 
analogies  of  nature  to  conclude — 1st.  That  the  ventricle  of  the  heart,  at  each 
contraction,  empties  itself  into  the  great  artery,  as  a  forcing-pump  at  each 
stroke  empties  itself  into  its  pipe  ; — 2d.  That  the  consequent  jet  causes  a 
wave  to  pass  along  the  extremities  of  the  arterial  tree,  (accounted  simply 
elastic,)  so  as  to  produce  every  where,  what  is  called  the  pulse  ;  3d.  That 
the  force  of  the  heart  acting  along  the  arteries,  forces  the  blood  through 
their  open  capillary  extremities  into  the  commencing  veins,  and  along  the 
veins  back  to  the  heart  again.  Now  these  assumptions,  which  Harvey 
believed  completely  to  describe  the  circulation,  are  all  nearly  true  :  but  are 
still  so  far  from  being  either  the  exact  or  the  whole  truth,  that  they  leave 
important  facts  unexplained.  Thus  : — 1st.  The  pulse,  instead  of  being  the 
wave,  as  slowly  progressive  as  the  view  above  given  anticipates,  is  almost  as 
instantaneous  over  the  whole  body  as  a  shock  of  electricity;  2d.  The  arteries 
are  all  found  empty  after  death,  although  they  have  no  power  of  emptying 
themselves  ;  and  if  an  artery  be  tied  in  the  living  body,  the  part  beyond  the 
ligature,  and  cut  off,  therefore,  from  the  influence  of  the  heart,  is  equally 
emptied  ; — 3d.  The  rapidity  of  the  blood's  passage  through  the  capillaries 
varies  very  much,  but  it  does  not  vary  in  exact  accordance  with  the  changes 
in  the  rapidity  or  force  of  the  heart's  action. — These  and  other  facts  ascer- 
tained since  Harvey's  day,  not  exactly  squaring  with  his  views,  have  rendered 
further  investigation  necessary;  and  it  is  now  additionally  known — 1st. 
That  the  coats  of  the  arteries  are  not  simpl}7  elastic  but  actively  contractile ; 
and  2dly.  That  the  capillary  vessels  can  move  the  blood  independent  of  the 
heart.  In  analyzing  this  subject,  it  is  convenient  to  follow  the  blood  round 
from  the  heart  to  the  heart  again,  through  the  three  stages  of,  1st.  The 
arteries  ;  2d.  The  capillaries  ;  3d.  The  veins. 

Motion  of  the  Wood  in  the  arteries. 

The  contractions  of  the  heart  inject  the  blood  into  the  arteries  with  a  force 
maintaining  such  a  tension  in  them,  that  according  to  the  interesting  experi- 
ments of  Dr.  Hales,  recorded  in  his  Statistical  Essays,  if  any  artery  of  a 
large  animal  like  a  horse  be  made  to  communicate  with  an  upright  tube,  the 
blood  will  ascend  in  the  tube  to  a  height  of  about  ten  feet  above  the  level  of 
the  heart  and  will  there  continue,  rising  and  falling  a  few  inches  with  each 
pulsation  of  the  hear^  Now  a  column  of  ten  feet,  as  explained  at  page  120, 
indicates  a  pressure  of  about  four  and  a  half  pounds  on  the  square  inch  of 
surface  :  this,  therefore,  is  the  force  of  the  heart  urging  the  blood  along  the 
arteries  into  the  veins. — The  opposing  tension  of  the  veins  is  much  less,  be- 
cause, as  will  be  explained  under  the  proper  head,  the  blood  readily  escapes 
from  them  into  the  heart ;  Hales  found  that  in  a  tube  communicating  with  a 
vein,  the  blood  stood  only  a  few  inches  higher  than  the  level  of  the  heart. 


THE    CIRCULATION.  417 

In  small  animals  he  ascertained  the  tension  of  artery  and  vein  to  be  less  than 
in  large  ones }  and  the  ratios  deduced  for  the  human  body,  under  ordi- 
nary circumstances,  were  an  eight  feet  column,  or  nearly  four  pounds  per 
inch  for  the  arteries ;  and  half  a  foot  column,  or  a  quarter  of  a  pound  per 
inch  for  the  veins. 

Arteries  examined  after  death  are  found  to  consist  of — 1st,  an  outer  coat 
of  strong  elastic  substance;  2d,  a  middle  coat  of  circular  fibres;  and  3d,  an 
invex  inner  coat  of  smooth  Kniny  membrane.  Their  elasticity  or  power  of 
resisting  change  of  dimension,  and  of  returning  to  a  middle  state  from  either 
dilatation  or  compression,  because  remaining  in  the  dead  artery,  was  the  most 
obvious  property,  and  was  that  first  attended  to.  Minute  observation  of  the 
phenomena  of  life  has  since  determined  the  following  facts,  proving  and  illus- 
trating a  contractility  resident  in  the  fibrous  coat. 

1.  A  small  living  artery,  cut  across,  soon  contracts  so  as  to  close  its  canal 
and  arrest  haemorrhage. 

2.  While  an  animal  is  bleeding  to  death,  the  arteries,  in  accommodating 
themselves  to  the  decreasing  quantity  of  blood,  contract  far  beyond  the  de- 
gree to  which  their  simple  elasticity  would  carry  them  j  and  they  relax  again 
after  death.     Dr.  Hales  took  seventeen  quarts  of  blood  from  a  horse  before 
it  died,  in  whose  body  only  three  quarts  more  were  found  altogether,  and  yet 
the  moment  before  death  the  tension  of  the  arteries  sustained  a  column  of 
two  feet  of  blood  in  his  experimental  tube. 

3.  The  artery  of  a  living  animal,  if  exposed  by  dissection  to  the  air,  some- 
times will  contract  in  a  few  minutes  to  a  great  degree :  and  in  such  a  case, 
only  a  single  fibre  of  the  artery  may  be  affected,  narrowing  the  channel  like 
a  thread  tied  round  it.     (See  Parry  on  the  Pulse.) 

4.  When  a  living  artery  is  tied,  the  part  between  the  ligature  and  the 
nearest  branch  on  the  side  of  the  heart  gradually  contracts,  and  becomes  at 
last  a  solid  or  impervious  cord. 

5.  Fluctuation  in  the  vital  action  of  parts,  is  often  attended  with  sudden 
increase  or  diminution  of  caliber  in  the  arteries  concerned. 

Although  these  facts  prove  indubitably  a  contractility  in  the  coats  of  arte- 
ries distinct  from  their  elasticity,  still,  because  the  circular  fibres  do  not 
resemble  common  muscles  in  colour  or  in  chemical  composition,  or  in  being 
immediately  obedient  to  the  stimuli  of  electricity,  pricking,  great  heat,  &c., 
their  contractility  was  by  many  persons  for  a  long  time  denied.  The  dis- 
pute, however,  was  often  more  about  the  words  contractility  and  muscularity, 
than  about  facts. 

The  pulse  in  the  arteries,  chiefly  as  regards  its  almost  instantaneous  occur- 
rence over  the  whole  system,  in  all  states  of  arterial  dilatation,  and  its  great 
strength  and  sharpness  in  very  small  and  remote  branches,  points  also  to  the 
active  contractility  of  the  arterial  coats  :  for, 

1.  Were  the  arterial  tree  in  the  living  body  a  system  of  simply  elastic  tubes 
as  readily  admitting  of  further  dilatation  as  in  the  dead  body,  the  first  part 
or  trunk  would  affect  the  motion  of  the  blood  beyond  it,  nearly  as  the  air- 
vessel  (see  page  126)  placed  at  the  commencement  of  artificial  arrangements 
of  water-pipes  affects  the  motion  of  the  water  in  them ; — that  is  to  say,  as  the 
air-vessel  converts  the  sudden  and  interrupted  jets  of  water  from  pumps  of 
fire-engines,  town  supplying  pipes,  &c.,  into  a  uniform  stream  with  scarcely 
a  remnant  of  shock,  so,  in  the  arterial  branches,  simple  elasticity  would 
cause  a  more  tranquil  flow  than  indicated  by  the  remarkable  gushes  from  a 
wounded  artery,  and  a  quieter  beat  than  that  bounding  pulse  of  life  felt  in  the 
remote  artery  of  the  wrist,  as  sensibly,  in  proportion,  as  near  the  heart  itself. 

27 


418  FLUIDITY    IN     RELATION    TO     ANIMALS. 

2.  Were  the  pulse  a  wave  advancing  in  tubes  that  yielded  as  readily  as  the 
dead  arteries  in  their  middle  states  of  dilatation,  it  would  be  more  slowly 
progressive  from  the  heart  to  the  extremities  ;  but  it  is  felt  so  instantly  over 
the  whole  body,  as  to  be  commonly  compared  to  a  shock  of  electricity. 

3.  A  pulse  may  be  produced  artificially  in  the  arteries  of  a  body  recently 
dead  by  filling  them  with  water  to  the  tension  of  life,  and  then  injecting  at 
intervals,  by  a  syringe,  as  much  water  as  the  heart  throws  off  blood  at  a  beat; 
but  although  the  artery  is  then  distended  nearly  to  the  limit  of  its  dilatability, 
and  is,  therefore,  rendered  rigid,  the  beats  are  weaker  than  those  of  the  liv- 
ing pulse.     A  similar  experiment,  tried  by  connecting  the  artery  of  a  dead 
animal  with  the  corresponding  artery  of  a  living  one,  has  a  similar  result. 

4.  A  tube  extensively  elastic,  that  it  might  convey  a  wave  of  liquid  with 
a  velocity  approaching  to  that  of  the  pulse,  would  require  to  be  so  tense, 
from  fulness,  as  to  be  discernible  always  by  the  touch,  through  any  imbed- 
ing  medium,  such  as  flesh,  like  a  hard  cylinder  or  cord ;  and  it  would  be  act- 
ing constantly  as  a  spring  tending  to  straighten  itself,  and,  therefore,  would 
be  stiffening  the  parts   through  which  it  passed.     Now  the  living  arteries, 
between  their  pulsations,  are  almost  as  soft  and  compressible  as  the  surround- 
ing flesh,  and  they  offer  no  perceiveable  opposition  to  bending,  in  any  move- 
ment of  the  parts.     This  may  be  verified  by  examination  of  the  lips,  for  in- 
stance, or  of  the  fingers;  but  when  a  person  sits  cross-legged,  the  well-known 
shaking  of  the  suspended  foot,  in  unison  with  the  pulse,  shows  the  recurring 
efforts  of  the  artery  to  straighten  itself,  during  the  moments  of  greater  tension. 

5.  A  bulky  wave  in  elastic  vessels  would  have  to  recoil  from  the  extremi- 
ties, or  to  pass  through  them  as  a  gush ;  and  the  recoil  would  be  particularly 
observeaSle  near  the  ligature  of  the  tied  artery ;  but  examination  has  not 
detected  such  effects  in  the  living  body.     The  operation  for  aneurism, — in 
which  the  artery  is  tied  beyond  the  tumour,  instead  of,  as  usual  on  the  side 
next  the  heart, — if  it  checked  a  strong  wave,  would  almost  certainly  produce 
bursting ;  yet  Mr.  Wardrope  and  others  have  lately  performed  this  operation 
with  successful  issue. 

6.  The  wave  would  be  more  interrupted  by  the  bandage  in  the  operation 
of  bleeding,  than  the  living  pulse  is. 

7.  The  pulse  of  a  paralytic  limb  often  seems  more  affected,  than  mere 
change   of  size  in  the  artery  will  account  for.     The  same  is  true,  in  an 
opposite  way,  of  the  pulse  in  an  artery  leading  to  an  inflamed  part. 

8.  If  the  abdomen  of  a  living  animal  be  opened,  the  mesenteric  artery,  in 
all  its  ramifications,  is  seen  stiffened  and   raised  up  suddenly  with  every 
pulsation,  in  a  manner  which  the   spreading  of  newly  received  blood  in  a 
very  yielding  vessel  does  not  account  for. 

9.  In  the  interesting  experiments  of  Bichat,  Parry  and  others,  to  ascer- 
tain the  exact  extent  of  the  supposed  dilatation  and  contraction  of  arteries, 
during  a  pulse,  not  the  slightest  degree  of  either  was  discernible,  even  when 
sought  for  with  microscopes. 

To  explain  these  and  other  phenomena,  then,  it  seems  necessary  to  admit, 
as  occurring  throughout  the  whole  body,  and  almost  simultaneously  with  the 
contraction  of  the  heart  itself,  such  an  action  of  the  contractile  fibres  of  the 
arteries,  so  as  to  modify  the  elasticity  of  the  arteries,  and  to  render  them  rigid 
enough,  in  all  degrees  of  dilation,  for  the  heart  to  produce  its  effects  through 
them  almost  as  it  would  through  tubes  of  metal. — Dr.  Young,  in  a  paper 
published  in  the  Philosophical  Transactions  for  1809,  and  characterized  by 
the  usual  elegance  and  precision  of  his  writings,  has  adduced  experiments 


THE    CIRCULATION.  419 

and  calculations,  to  show  that  waves  in  elastic  vessels  advance  more  quickly 
than  was  before  imagined  :  but  the  spreading  of  the  pulse  seems  to  be  yet 
more  rapid  than  his  calculation  anticipates. — It  is  evident,  that  when  arteries, 
In  consequence  of  depletion,  are  contracted  beyond  the  middle  station  of 
their  elasticity,  their  tension  and  power  of  quickly  conveying  the  pulse  must 
be  dependent  altogether  on  the  condition  of  their  contractile  fibres. 

The  careful  experiments  which  could  detect  no  change  of  size  in  the 
arteries  during  the  pulse,  while  they  disprove  the  ancient  belief  of  a  con- 
siderable tumefaction  or  wave  passing  along,  or  of  a  considerable  filling  and 
emptying  of  arteries,  like  what  occurs  in  the  heart,  might  also  be  supposed 
positively  to  disprove  the  occurrence  of  any  general  constriction  of  the  vessels 
on  their  contents — but  erroneously  : — for  if  a  man's  arterial  system,  con- 
sidered as  one  cavity,  be  supposed  to  contain  five  pounds  of  blood  (which  is 
near  the  truth,)  and  if  the  vessels  be  thought  to  embrace  their  contents,  even 
between  the  pulses,  with  force  enough  to  have  all  a  rounded  or  cylindrical 
form,  although  remaining  soft  and  yielding  to  the  pressure  of  the  finger ; 
and  if  we  suppose  their  coats,  during  the  pulse  to  be  thrown  into  a  sudden 
contraction,  as  if  in  obedience  to  an  electrical  shock,  still,  because  blood  is  in- 
compressible, and  because  just  as  much  enters  the  arteries  with  every  pulse 
as  escapes  from  them  before  the  next,  their  bulk  would  not  sensibly  diminish 
by  the  strongest  conceivable  action  of  their  coats  j  of  which  action  the  only 
sensible  effects  would  be,  that  the  soft,  yielding,  aad,  in  some  places,  com- 
pressed tubes  would  be  suddenly  converted  into  hard  or  resisting  cylinders ; 
and  that  wherever,  by  any  accidental  pressure,  an  artery  had  been  flattened, 
it  would,  in  regaining  its  cylindrical  form,  strike  or  pulsate  against  the  com- 
pressing body. — Whether  such  an  action  as  this  contributes  to  produce  the 
arterial  pulse  will  be  considered  under  the  head  of  "  the  pulse,"  after  we 
have  seen  how  the  blood  moves  in  the  capillaries  and  veins. 

In  any  admissible  view,  however,  of  arterial  agency,  we  find  that  the 
arteries  contribute  to  the  circulation  of  the  blood,  but  as  tubes  which  convey 
it,  'their  own  permanent  tension,  and,  therefore,  the  force  with  which  the 
blood  is  pressed  into  the  capillaries,  being  derived  from  the  heart  alone. 
Even  if  there  be  a  momentary  arterial  contraction,  such  as  alluded  to  above, 
at  the  instant  of  the  pulse,  it  is  of  too  short  duration  to  have  an  appreciable 
effect,  and  probably  any  effect  would  be  counterbalanced  by  the  same  action 
pervading  the  capillaries.  Many  physiologists  have  had  a  confused  belief 
that  the  arteries  aided  very  actively  in  propelling  the  blood  ;  but  a  little 
reflection  would  have  shown,  that  as  they  have  no  vermicular  or  progressive 
contraction,  like  the  intestines,  they  no  more  propel  the  fluid  within  them, 
than  any  other  tubular  conduits  do. — Although  they  be  thus  in  no  degree 
instrumental  in  the  propulsion  of  the  blood,  still,  by  more  permanently 
enlarging  or  diminishing  their  caliber,  that  is,  by  merely  becoming  larger  or 
smaller  conduits,  they  may  much  influence  its  local  distribution,  and  the 
speed  of  its  transmission.* 

*  It  has  long  been  a  subject  of  dispute  whether  the  arteries  exercise  any  active  power  in 
the  circulation  of  the  blood,  and  many  ingenious  experiments  have  been  instituted  to  deter- 
mine the  question.  Our  author,  in  denying  that  the  arteries  are  in  any  degree  instrumental 
in  the  propulsion  of  the  blood,  is  not 'borne  out  by  recent  investigations.  M.  Poiseuille, 
indeed,  seems  satisfactorily  to  have  proved  that  the  contractility  of  the  arteries  does  assist 
in  the  propulsion  of  the  blood.  By  a  series  of  well  devised,  and  apparently  accurate  experi- 
ments, M.  Poiseuille  arrived  at  the  unexpected  result,  that  the  force  of  the  blood  in  the 
arteries  will  support  a  column  of  mercury  of  the  same  height  with  whatever  part  of  the 
course  of  the  arterial  circulation  the  column  is  placed  in  connection — whether  for  example 
it  is  connected  with  the  origin  of  the  carotid,  or  with  a  branch  derived  by  repeated  subdivi- 
sion from  the  crural  artery  :  and  he,  therefore,  concludes  that  the  force  with  which  a  mole- 


420  FLUIDITY    IN    RELATION    TO    ANIMALS. 

The  nature  of  this  work  does  not  allow  us  to  record  historically  the  vari- 
ous errors  into  which  even  able  men  have  fallen,  in  attempting  to  explain 
the  office  of  the  arteries,  but  we  shall  glance  at  the  following  — Dr.  Monro 
and  John  Hunter,  two  of  the  most  able  physiologists  that  the  world  ha$ 
seen,  believed  that  the  arteries  did  almost  as  much  in  propelling  the  blood 
as  the  heart  itself.  We  need  not  repeat  the  refutation  of  this  opinion.  The 
ingenious  Hichat,  again,  unable  to  detect  either  momentary  contraction  or 
dilatation  in  the  arteries,  thought  that  the  blood  was  pushed  along  them  by 

cule  of  blood  moves,  is  the  same  throughout  the  whole  arterial  circulation.  Following  out 
these  researches,  he  has  investigated  the  manner  in  which  the  original  impulse  communi- 
cated hy  the  heart,  is  transmitted  unimpared  to  distant  parts  of  the  circulation,  notwith- 
standing the  retarding  tendency  of  friction,  and  the  yielding  of  the  parieties  of  the  vessels. 
His  first  object  here  was  to  ascertain  whether  the  arteries  are  dilated  by  the  stroke  of  the 
heart,  and  impulse  communicated  to  the  blood,  and  what  the  amount  of  the  dilatation  may 
be.  By  many  physiologists  a  very  extravagant  idea  used  to  be  entertained  of  the^mount 
of  their  dilatation;  on  the  other  hand,  the  latter  researches  of  Parry,  and  other  experi- 
mentalists have  assigned  exceedingly  narrow  limits  to  it;  nay,  by  one  eminent  physiologist, 
Bichat,  it  has  been  denied  altogether.  M.  Poiseuille  has  determined  the  point  by  means  of 
a  very  satisfactory  set  of  experiments  with  an  apparatus  of  his  own  invention,  and  has 
ascertained  that  dilatation  ia  affected  ;  but  that  it  is  so  small  as  certainly  to  be  indistin- 
guishable in  an  artery  by  the  unaided  senses.  This  apparatus  cannot  be  thoroughly  de- 
scribed without  a  diagram ;  it  will  be  sufficient,  therefore,  for  us  to  mention,  that  it  is  so 
contrived  as  to  contain  about  eight  inches  of  the  carotid  artery  of  the  horse  in  ^a  vessel 
filled  with  water  and  made  water  tight,  except  at  one  point,  from  which  a  small  horizontal 
glass-tube  issues,  about  an  eighth  of  an  inch  in  diameter.  At  each  contraction  ofjthe 
animal's  heart  it  was  found  that  the  water  in  the  small  tube  advanced  two  inches  and 
eight-tenths,  and  that  it  retired  to  its  former  place  during  the  diastole  of  the  heart.  The 
diameter  of  the  artery  was  seven-twentieths  of  an  inch.  Hence  it  may  be  calculated  that 
at  each  pulsation  its  capacity  was  increased  about  a  thirtieth  part. 

Having  ascertained  this  fact,  M.  Poiseuille  goes  on  to  inquire,  whether  the  power  which 
is  expended  by  the  blood  in  causing  this  dilatation  is  restored  by  the  subsequent  contrac- 
tion of  the  artery.  For  this  purpose  a  portion  of  the  common  carotid  artery  of  the  horse, 
ten  inches  long,  and  seven-twentieths  of  an  inch  in  diameter,  taken  immediately  after  death, 
was  connected  with  the  end  a,  of  the  tube,  (see  Figure  ;)  a  stop-cock,  however,  being  pre- 
viously fitted  between  aand  b.  The  other  end  of  the  artery  was  fixed  on  a  tube  of  analogous 
construction,  different  in  fact  only  in  so  far  as  the  limb  c  d  was  inclined  at  about  half  a  right 
angle  instead  of  being  vertical,  and  the  stop- cock  was  placed  near  the  end  d.  The  whole  of 
the  first  tube,  the  artery  and  part  of  the  descending  limb  b  c  of  the  second  tube  was  filled 
with  water,  a  little  mercury  then  filled  the  curvature  of  the  second  tube,  and  the  ascending 
inclined  limb  of  that  tube  above  the  mercury  was  filled  with  water.  The  stop-cock  of  the 
last  tube  being  closed,  and  that  on  the  first  tube  being  opened,  mercury 
Fig.  174.  was  poured  into  the  former  by  its  end  d,  till  the  pressure  on  the  artery 

amounted  to  ninety  five  millimetres  or  about  3.8  inches.  The  stop- 
"t  cock  of  the  first  tube  was  then  closed  and  th;»t  on  the  second  tube  was 
opened ;  upon  which  the  water  rose  insta.neously  in  the  latter,  a 
portion  flowed  out  at  the  top,  and  theremainder  then  sank  a  little,  and 
assumed  a  fixed  level.  On  making  the  necessary  computations,  M. 
Poiseuille  found  that  the  point  to  which  the  mercury  was  raised  in  the 
second  tube  at  the  moment  of  the  contraction  of  the  artery  indicated 
an  elevation  of  one  hundred  and  ten  millimetres  or  4.4  inches.  Hence 
the  power  with  which  the  arterial  coatu  contract  upon  themselves  after 
being  dilated,  exceeds  that  which  is  expended  in  dilating  them.  In  the 
present  experiment  the  excess  was  equivalent  to  six-tenths  of  an  inch 
of  mercury,  or  three  nineteenths  more  than  the  dilating  force.  In  three 
other  experiments,  the  excess  of  the  column  of  mercury  was  9.20,  14. 
20,  19.20  of  an  inch.  When  repeated  with  the  artery  of  an  animal 
which  had  been  killed  four  days  before,  the  excess  was  less  than  4.20 
It  is  evident,  therefore,  that  whatever  force  the  blood  after  issuing  from 
the  heart  loses  in  consequence  of  its  acting  on  yielding  vessels,  is  com- 
pletly  restored  by  the  elastic  contraction  of  their  parietes.  The  me- 
moirs of  M.  Poiseuille  will  be  found  in  the  Eepertoir  General  d'Ana- 
tomie  et  de  Physiologie,  Tom,  VII. 

Dr.  Badham  in  an  interesting  paper  in  the  London  Medical  Gazette  (vol.  viii.  p.  549) 
has  adduced  some  strong  evidences  of  the  existence  of  independent  arterial  action,  and 
gives  a  sketch  of  various  pathological  phenomena,  which  appears  explicable  on  such  an 
admission,  and  inexplicable  without  it. — Am.  Ed. 


THE    CIRCULATION.  421 

the  heart  instantly  through  their  whole  extent,  as  a  solid  rod  of  metal  or 
wood  is  advanced  by  an  -impulse  at  one  end.  Dr.  Parry  took  nearly  the 
same  view  of  the  subject,  and  illustrated  his  idea  by  referring  to  the  experi- 
ment of  moving  a  whole  line  of  billiard-balls  by  striking  the  extreme  one. 
Both  these  authors  erred  by  neglecting  the  hydrostatical  truth,  that  pressure 
in  a  fluid  operates  equally  in  all  directions,  and,  therefore,  that  fluid  pressed 
into  a  tube  tends  to  dilate  the  tube,  just  as  powerfully  as  to  drive  the  fluid 
forward ;  and  they  did  not,  advert  to  the  fact  that  the  progress  of  the  blood  in 
the  small  arteries  is  not  by  waves  or  successive  jets,  but  is  nearly  a  uniform 
stream.  The  blood  could  only  advance,  as  they  supposed,  by  the  arteries 
becoming,  for  an  instant,  absolutely  rigid,  in  consequence  of  a  strong  action 
of  their  contractile  fibres. 

It  merits  notice  here,  although  not  strictly  a  mechanical  fact,  that  arteries 
permanently  increase  or  diminish  in  size  when  a  permanent  change  takes 
place  in  the  demand  for  their  service.  The  arteries  of  the  gravid  uterus,  or 
of  an  increasing  tumour,  grow  with  the  part  supplied,  while  on  the  contrary, 
those  of  the  stump  left  after  amputation  soon  remarkably  diminish.  If  the 
chief  artery  of  a  limb  be  obliterated  by  any  cause,  as  after  the  operation  for 
aneurism,  the  small  collateral  anastomozing  branches  increase  in  size  to  do 
its  duty. 

It  is  farther  remarkable,  that  when  arteries  are  called  upon  to  carry  an 
increased  quantity  of  blood,  they  often  become  tortuous  or  serpentine,  as 
well  as  larger;  and  that  arteries  leading  to  parts  whose  actions  are  naturally 
intermitting,  or  fluctuating,  have  generally  the  tortuous  form.  Of  these 
truths,  the  arteries  leading  to  rapidly  growing  tumours,  or  to  varicose 
aneurisms,  and  the  arteries  of  the  uterus  and  testes,  may  serve  as  instances. 
This  bending  of  arteries,  and  the  very  curious  divisions  into  many  branches 
which  again  re-unite,  found  in  those  leading  to  the  brains  of  some  animals, 
do  not  seem  intended  to  slacken  the  rapidity  of  the  sanguineous  current, 
but  to  give  the  artery  a  greater  control  over  the  supply. 

/ 

Passage  of  the  blood  through  the  capillaries. 

We  have  seen  that  the  heart  keeps  up  a  tension  or  pressure  in  the  arteries 
of  about  four  pounds  on  the  square  inch  of  their  surface  j  and  with  this  force, 
therefore,  is  propelling  the  blood  into  the  capillaries.  If  these  last  were  pas- 
sive tubes,  constantly  open,  such  force  would  be  sufficient  to  press  the  blood 
through  them  with  a  certain  uniform  velocity;  but  they  are  vessels  of  great 
and  varying  activity ;  it  is  among  them  that  the  nutrition  of  the  different  tex- 
tures of  the  body  takes  place,  as  of  muscle,  bone,  membrane,  &c. ;  and  that 
all  the  secretions  from  the  blood  are  performed,  as  of  bile,  gastric  juice,  or 
saliva,  &c.  ;  and  to  perform  such  varied  and  often  fluctuating  offices,  they 
require  to  be  able  to  control,  in  all  ways,  the  motion  of  the  blood  passing 
through  them.  The  capillaries  of  the  cheek,  under  the  influence  of  shame, 
dilate  instantly,  and  admit  more  blood  producing  what  is  called  a  blush  ;  — 
under  the  influence  of  anger  or  fear,  they  suddenly  empty  themselves,  and 
the  countenance  becomes  pallid — tears  or  saliva,  under  certain  circumstances, 
gush  in  a  moment,  and  in  a  moment  again  are  arrested — if  a  person  having 
inflammation  in  one  hand  be  blooded  from  corresponding  veins  in  both  arms 
at  the  same  time,  twice  or  thrice  as  much  blood  will  flow  from  the  diseased 
side  as  from  the  other.  Similar  changes  occur  in  many  other  instances. 
Now  the  only  action  of  cylindrical  vessels,  capable  of  causing  these  pheno- 
mena, is  contraction  and  dilatation  of  their  coats ;  and  with  reference  to  such 


422  FLUIDITY    IN    RELATION    TO    ANIMALS. 

caction  it  merits  notice,  that  arterial  branches  have  more  of  the  fibrous  or 
contractile  coat  in  proportion  as  they  are  smaller.- 

A  muscular  capillary  tube  strong  enough  to  shut  itself  against  the  arterial 
current  from  the  heart,  is  also  strong  enough  to  propel  the  blood  to  the  heart 
again  through  the  veins,  even  if  the  resistance  on  the  side  of  the  veins  were 
as  great  as  the  force  on  the  side  of  the  arteries.  For  if  we  suppose  the  first 
circular  fibre  of  the  tube  to  close  itself  completely,  it  would,  of  course,  be 
exerting  the  same  repellent  force  on  both  sides,  or  as  regarded  both  the 
artery  and  vein.  If,  then,  the  series  of  such  fibres  forming  the  tube  were  to 
contract  in  succession  towards  the  vein,  as  the  fibres  of  the  intestinal  canal 
contract  in  propelling  the  contents  of  that  canal,  it  is  evident  that  all  the 
blood  in  the  capillary  would  thereby  be  pressed  into  the  vein  towards  the 
heart.  If  after  this  the  capillary  again  relaxed  on  the  side  of  the  artery,  so 
as  to  admit  more  blood,  and  again  contracted  towards  the  vein  as  before,  it 
would  produce  a  forward  motion  of  the  blood,  first  towards  the  vein,  and  then 
in  it  independently  of  the  heart.  As  capillary  action,  however,  is  not  visi- 
ble, its  nature  has  not  yet  been  positively  ascertained  : — some  persons  have 
deemed  it  electrical. 

It  is  capillary  action  which  absorbs  and  moves  the  fluids  of  the  classes  of 
animals  which  have  no  heart.  It  must  also  be  the  power  which  moves  the 
blood  in  warm-blooded  monsters  formed  without  hearts.  There  are  cases  of 
apparent  death  among  human  beings,  where  the  heart  remains  inactive  for 
days  and  yet  a  degree  of  circulation  sufficient  to  preserve  life  is  carried  on 
by  the  capillaries.  In  illustration  of  capillary  action,  we  have  also  the  absorp- 
tion, by  the  lacteals,  of  nutriment  from  the  alimentary  canal ;  and,  perhaps, 
to  a  certain  extent,  the  circulation  of  blood  in  the  livers  of  animals.  In 
this  last  case,  the  blood  collected  by  veins  from  the  abdominal  viscera,  instead 
of  going  directly  to  the  heart,  is  again  distributed  through  the  liver  by  the 
branches  of  the  vena  portae,  and  is  then  again  collected  by  the  ordinary  veins 
of  the  liver,  and  carried  to  the  heart :  it  thus  moves  through  two  sets  of 
capillaries  in  passing  from  the  arteries  to  the  heart  again. 

The  action  of  the  capillaries  is  the  cause  of  that  singular  fact  which  pre- 
vented the  ancients  from  discovering  the  circulation  of  the  blood,  viz.,  the 
empty  state  of  the  arteries  after  death.  All  the  muscular  parts  of  an  animal, 
including,  therefore,  the  contractile  coats  of  vessels,  retain  their  life,  or  power 
of  contracting,  for  a  considerable  time  after  respiration  has  ceased — as  is 
seen  in  the  recovery  of  persons  apparently  drowned  or  suffocated  :  in  the 
leaping  of  a  heart  taken  from  an  animal  recently  killed;  in  the  actions  resem- 
bling life  which  can  be  produced,  by  the  agency  of  galvanism,  in  a  body 
recently  dead  :  but  the  fact  is  seen  still  more  amply  for  our  purpose,  in  the 
total  disappearance  of  a  local  inflammation  after  the  death  of  the  patient, — 
for  inflammation  involves  a  gorging  or  over-tension  of  the  capillaries,  into 
which,  when  the  heart  has  ceased  to  press  blood,  the  contractile  force  remain- 
ing in  them,  even  under  disease  and  in  a  dead  animal,  is  sufficient  to  squeeze 
the  blood  out  of  them,  and  often  to  remove  all  trace  of  the  malady  which  has 
been  fatal.  In  ordinary  cases,  then,  the  capillaries  throughout  the  body  re- 
main alive  and  active  for  a  considerable  time  after  breathing  has  ceased,  work- 
ing like  innumerable  little  pumps,  and  empyting  the  arteries  into  the  veins. 
As  the  red  blood  is  their  proper  sustenance  as  well  as  stimulus,  they  work  as 
long  as  there  is  any  of  it  coming  into  them  from  the  arteries  behind ;  except, 
however,  the  capillaries  of  the  lungs,  which  soon  cease  to  act,  because,  after 
breathing  has  ceased,  they  receive  only  black  blood,  and  are  moreover  com- 
pressed by  the  collapse  of  the  chest;  and  all  the  blood  accumulates  behind 


TflE    CIRCULATION.  423 

them.  The  capillaries  may  continue  to  be  filled  from  the  arteries,  either  in 
consequence  of  their  elasticity  opening  them  with  what  is  called  a  suction 
power,  or  of  an  absorbant  powor  depending  on  life,  like  that  of  the  lacteals 
and  of  the  absorbents  all  over  the  body,  and,  perhaps,  of  the  vessels  in  the 
roots  of  vegetables.  When  death  is  produced  by  lightning,  or  by  the  poisons 
which  destroy  all  muscular  irritability,  and  therefore  that  of  capillaries,  the  ar- 
teries after  death  are  found  to  contain  blood  like  the  veins.  In  a  living  body, 
if  an  artery  be  tied,  the  part  beyond  the  ligature  is  soon  emptied  into  the  veins, 
and  becomes  flat. — The  experiment  has  be  made  even  upon  the  aorta  itself. 
The  empty  state  of  the  arteries  after  death  has  been  ascribed,  by  some 
teachers,  to  the  momentum  with  which  they  supposed  the  blood  to  be  thrown 
out  from  the  heart  in  its  last  contraction — sufficient,  said  they,  to  squirt  it  fairly 
through  the  most  distant  capillaries;  a  doctrine  exemplifying  the  carelessness 
with  which  men  sometimes  receive  and  repeat  opinions,  to  which  their  atten- 
tion has  never  been  fully  awakened.  Such  an  effect  would  not  follow,  even 
if  the  action  of  a  dying  heart  were  the  strongest  possible  ;  while,  in  reality, 
it  is  in  most  cases  so  feeble,  that  the  pulse  for  sometime  ceases  to  be  per- 
ceptible at  the  extremities,  and  the  diminished  circulation  lets  them  become 
cold. — Other  physiologists  have  taught  that  an  artery  is  capable  of  contract- 
ing directly  upon  its  contents,  so  as  to  expel  even  the  last  drop  ; — but  large 
arteries,  when  emptying,  do  not  contract  roundly  like  an  intestine  ;  they 
become  flat  like  elastic  tubes  of  leather  sucked  empty,  and  no  contractile 
action  of  the  vessel  itself  could  bring  its  sides  together  in  such  a  manner. 
If  arteries  emptied  themselves  by  their  own  action,  the  pulmonary  artery 
should  be  more  certainly  empty  than  the  aorta,  because  it  is  shorter;  yet  it 
is  always  full ;  for  the  reason  already  stated,  that  the  pulmonary  capillaries 
cease  to  act  after  respiration  has  ceased,  on  account  of  the  blood  in  them 
being  venous  or  .dark  blood,  and  therefore  not  life-supporting  or  stimulant  to 
them. 

Passage  of  Hood  through  the  veins. 

The  veins  have  much  thinner  coats  than  the  arteries,  and  if  taken  altogether 
have  much  greater  capacity  :  for  besides  being  larger  than  the  corresponding 
arteries,  they,  exist  in  many  situations,  as  double  sets,  an  exterior  and  an 
interior  :  they  have  also  very  frequent  inosculations  or  communications  with 
each  other  throughout  their  whole  course. 

The  simple  weight  of  the  column  of  blood  in  any  descending  artery  is  just 
sufficient  to  raise  the  blood  through  open  capillaries  to  an  equal  height  in 
the  corresponding  vein,  according  to  the  hydrostatical  law,  that  fluids  attain  the 
same  level  in  all  communicating  vessels  ;  and  therefore,  as  the  arch  of  the 
aorta  rises  considerably  above  the  heart,  the  gravitating  pressure  of  the  de- 
scending arterial  column  of  blood  would  be  sufficient  to  lift  that  in  the  veins 
not  only  up  to  the  heart,  but  considerably  beyond  it.  In  addition  to  this  in- 
fluence of  gravity  on  the  venous  current,  the  blood  is  pressed  into  the  arteries, 
and  from  them,  therefore,  towards  the  veins,  with  a  force  from  the  heart  itself, 
as  stated  above,  of  about  four  pounds  to  the  square  inch,  or,  in  other  words, 
as  if  there  were  a  column  of  blood  eight  feet  higher  than  the  heart  urging  the 
current.  It  might  be  expected  from  the  law  of  equal  diffusion  of  pressure  in 
fluids,  that  these  causes  would  soon  produce  a  tension  in  the  veins  as  great 
as  in  the  arteries  :  and  this  does  not  happen,  only  because  the  blood  has  a 
ready  escape  from  the  veins  through  the  right  ventricle  of  the  heart.  Under 
ordinary  circumstances,  there  can  be  no  greater  tension  in  the  veins  than  what 
is  sufficient  to  lift  the  blood  to  the  heart  and  to  overcome  the  friction  ; — as  in 


424  FLUIDITY    IN    RELATION    TO    ANIMALS. 

an  upright  leathern  tube,  open  at  top,  and  receiving  water  at  its  bottom  from 
a  powerful  forcing  pump,  there  never  can  be  a  greater  tension  or  pressure  than 
what  corresponds  to  the  height  of  the  fluid  column  in  the  tube,  and  to  the 
friction  between  the  fluid  and  tube.  In  Dr.  Hale's  experiments,  already 
alluded  to,  a  tube  connected  with  a  vein  so  as  to  receive  its  blood,  became 
filled  with  blood  to  a  height  only  of  about  six  inches  above  the  level  of  the 
heart.  As  Dr.  H.  generally  cut  the  vein  completely  across,  and  inserted  the 
tube  into  the  portion  leading  from  the  capillaries,  he  would  have  discovered 
the  whole  power  with  which  the  blood  is  pushed  along  the  veins  from  the 
capillaries,  but  for  the  free  lateral  communication  of  veins  with  each  other, 
which  reduces  the  tension  even  in  an  obstruct  branch,  to  the  degree  exist- 
ing in  the  system  generally.  When,  from  agitation  of  the  animal,  or  any 
straining  exertion,  the  passage  of  the  blood  into  the  heart  was  impeded,  all 
the  veins  became  tense,  and  a  tube  inserted  into  the  returning  jugular  had 
blood  running  over,  at  a  height  of  three  feet  above  the  heart. 

If  the  blood  did  not  escape  from  the  veins,  as  above  described,  the  only 
cause  which  could  prevent  the  venous  tension  from  becoming  as  great  as  the 
arterial,  would  be  obstruction  in  the  connecting  capillaries  :  but  the  following 
facts  and  considerations  prove  that  these  vessels,  which,  in  the  dead  body, 
allow  the  passage  of  injections,  in  the  living  body  freely  allow  the  passage 
of  blood.  1st.  Magendie  laid  bare  the  chief  artery  and  vein  of  a  living  limb, 
and  at  the  part,  detached  them  from  the  flesh  underneath,  so  that  he  could 
apply  a  tight  bandage  round  the  limb  without  including  them,  and  could  thus 
render  them  the  only  channels  of  circulation  for  the  limb  beyond  the  band- 
age. He  then  found,  that  when  a  separate  ligature  was  put  upon  the  vein, 
to  prevent  the  return  of  its  blood  to  the  heart,  and  a  puncture  was  made 
beyond  the  ligature,  the  flux  of  blood  from  the  puncture  was  rapid  or  slow 
according  as  the  heart  was  allowed  to  produce  a  greater  or  less  degree  of 
tension  in  the  artery : — this  tension  was  regulated  by  his  compressing  the 
artery  between  the  fingers.  2d.  After  a  similar  preparation  of  the  parts,  the 
blood  will  ascend  in  a  tube  from  the  obstructed  vein  very  nearly  as  high  as 
from  the  artery  3d.  In  the  common  operation  of  bleeding  at  the  moment 
of  puncturing  the  vein,  the  blood  often  jets  from  it  as  from  an  artery,  stain- 
ing even  the  top  of  the  bedstead.  4th.  The  microscope  discovers  in  the 
capillaries,  a  uniform  forward  motion  of  the  blood,  as  if  it  were  obeying  the 
steady  pressure  of  the  arterial  tension,  and  not  any  intermitting  action.  5th. 
Disturbed  action  of  the  heart,  by  obstructing  the  passage  of  the  blood  through 
it,  is  very  soon  attended  with  a  tumefaction  of  all  the  veins  leading  to  the 
heart :  the  tumefaction  becomes  very  visible  about  the  neck  and  head,  and 
in  the  liver  produces  swelling  and  acute  pain.  6th.  Dr.  Young,  from  experi- 
ments made  by  him,  and  reported  in  the  philosophical  transactions  for  1809, 
concluded  that  perfectly  open  capillaries,  of  the  size  existing  in  the  living 
body,  should  just  retard  a  flow  of  blood  urged  by  the  usual  arterial  tension, 
in  the  degree  which  really  occurs  : — a  correspondence  proving  that  they  must 
be  open;  and  open  vessels,  however  small,  and  how  slowly  soever  they 
transmit  the  blood,  still,  if  the  escape  of  blood  from  the  veins  were  arrested, 
would  transmit  the  arterial  tension  without  diminution.  7th.  The  fact  that 
after  death  the  capillaries  empty  the  arteries  into  the  veins,  proves  that,  under 
certain  circumstances,  the  venous  tension  may  become  even  greater  than  the 
arterial. — These  facts  then  and  others  that  might  be  mentioned,  prove  incon- 
testably,  that  the  blood  is  pressed  into  the  veins  from  the  arteries  and  capilla- 
ries, with  force  sufficient  to  lift  it,  not  only  to  the  heart  again,  but  many  feet 
farther,  viz.,  about  as  far  as  it  would  ascend  in  a  tube  rising  from  the  tense 


THE    CIRCULATIO.N.  425 

arteries  themselves.  So  little,  however,  has  this  important  truth  been  under- 
stood, that  in  elementary  works  of  authority  lately  published,  the  venous 
current  is  treated  of  as  a  very  obscure  subject;  and  some  authors,  in  their 
anxiety  to  explain  it,,  have  assigned  causes  for  it,  which,  as  will  appear  here- 
after, are  positive  absurdities  in  physics.  The  difficulty  in  the  question 
seems  to  have  arisen  from  the  great  disparity  observed  between  the  tension 
in  the  arteries  and  in  the  veins,  while  the  reflection  did  not  occurr,  that  the 
disparity  was  owing  to  there  being  a  free  passage  or  outlet  from  the  veins 
through  the  heart. 

The  illustrious  Bichat,  with  an  inattention  to  facts,  extraordinary  in  him, 
persuaded  himself  that  the  influence  of  the  heart  ceased  entirely  at  the  capil- 
laries and  that  the  blood  was  returned  through  the  veins  by  the  action  of  the 
capillaries,  alone.  How  could  he  avoid  the  single  reflection,  that,  if  the  pur- 
pose of  the  arteries  had  been  merely  to  convey  the  blood  to  the  capillaries, 
and  not  also  to  bear  the  force  which  pressed  it  into  and  through  them,  the 
extraordinary  strength  of  the  arterial  coats,  and  the  great  power  of  the  heart 
to  fill  them  and  keep  up  the  tension  described,  would  have  been  quite  super- 
fluous ? — and  he  knew  that  nature  does  nothing  in  vain.*  The  reflection 
applies  strikingly  to  the  pulmonary  artery,  of  which  no  branch  exceeds  a 
few  inches  in  length. 

The  uniform  current  of  blood  along  the  veins,  so  apparent  in  the  operation 
of  bleeding,  and  produced,  as  now  explained,  by  the  combined  influence  of 
the  heart  and  capillaries,  suffers  a  considerable  disturbance  in  the  neigh- 
bourhood of  the  heart  from  three  causes.  1st.  As  there  is  no  valve  between 
the  veins  and  the  auricles  of  the  heart,  each  contraction  of  the  right  auricle 
tends  to  throw  the  blood  back  into  the  veins,  as  well  as  forward  into  the 
ventricle,  and  thus  produces  the  venous  pulse  often  felt  in  the  neighbour- 
hood of  the  chest.  2d.  When  the  chest  is  expanded  by  inspiration,  it  is 
more  roomy  than  during  the  collapse  of  expiration,  and  the  blood  then  enters 
it  more  readily.  3d.  While  the  chest  is  inhaling  or  drawing  in  air,  that  is 
to  say,  expanding  so  as  to  diminish  the  tension  or  pressure  of  the  air  within 
it,  (see  Pneumatics^)  it  is  by  the  same  action  favouring  the  entrance  of  blood 
through  the  veins  towards  the  heart  placed  in  it ; — on  the  contrary,  while  it 
is  exhaling  or  throwing  out  air,  it  is,  with  equal  force,  resisting  the  entrance 
of  blood,  and  slackening,  or  even  causing  recoil  of  the  inward  current.  This 
favouring  or  resisting  force,  however,  as  will  be  hereafter  shown,  is  only 
such  as  to  lift  or  support  a  column  of  blood  of  about  half  an  inch  in  height. 
It  appears,  then,  that  the  entrance  of  blood  into  the  chest  fluctuates  by  rea- 
son of  the  respiration,  &c.,  as  the  entrance  of  a  river  stream  into  the  sea 
fluctuates  by  reason  of  the  ebbing  and  flowing  of  the  tide.  An  eye  watch- 
ing the  jugular  vein,  under  favourable  circumstances,  may  see  it  tense  or 
slack  in  accordance  with  the  opening  and  shutting  of  the  chest. 

It  still  remains  to  be  ascertained  whether  or  not  veins  have  in  themselves 
any  contractile  power,  such  as  can  partially  empty  a  lower  portion  into  a 
higher  portion  beyond  an  adjoining  valve.  If  so,  the  valve  by  then  bearing 
the  pressure  would  let  more  blood  be  easily  raised  from  below  into  the  por- 
tion so  relieved  :  and  the  action,  without  being  equal  to  the  office  of  com- 
pletely emptying  any  portion  of  a  vein,  would  still  have  the  effect  of  dividing 

*  This  incorrect  and  inconclusive  mode  of  reasoning  is  so  common  that  we  may  be  per- 
mitted to  protest  against  it.  The  influence  of  the  heart  may  cease  with  the  capillaries, 
and  yet.  nature  has  done  nothing  in  vain.  Before  we  would  be  justified  in  making  such  a 
charge  against  nature  we  must  possess  an  infinitely  more  precise  knowledge  of  the  circula- 
tory forces  and  of  the  functions  of  the  arterial  system  than  we  do  at  present.  Am.  Ed. 


426     FLUIDITY  IN  RELATION  TO  ANIMALS. 

a  long  heavy  column  into  a  number  of  short  columns  of  comparatively  little 
resistance.  It  is  certain,  at  least,  that  the  valves  in  the  veins,  by  preventing 
the  falling  back  of  blood  which  has  once  passed  towards  the  heart,  must 
affect  its  flow  during  bodily  exercise;  for  every  time  that  pressure  is  made 
on  a  vein  by  a  swelling  muscle  or  otherwise,  the  blood  in  the  part  must  be 
forced  forward,  and  cannot  return. 

The  veins  which  are  surrounded  by  muscles  are  thinner  anc^weaker  than 
those  supported  only  by  the  skin.  The  external  veins  of  the  legs  are 
almost  as  strong  as  arteries.  Proving,  however,  that  the  fabric  of  veins  is 
much  weaker  than  that  of  arteries,  any  vein  in  the  living  body,  made  to 
communicate  directly  with  an  artery,  soon  exhibits  what  is  called  a  varicose 
aneurism,  and  swells  to  bursting.  Veins  possess  power,  to  a  great  extent, 
of  adapting  themselves  to  the  varying  quantity  of  blood. 

Some  recent  authors,  as  stated  above,  either  not  aware  of  the  facts  which 
prove  that -the  blood  is  everywhere  pressed  into  the  veins  with  force  much 
more  than  sufficient  to  raise  it  to  the  heart  again  ;  or,  being  unable,  from 
their  little  familiarity  with  mechanical  science,  to  draw  exact- conclusions 
from  the  facts,  or  to  avoid  errors  in  their  own  hypotheses,  have  promulgated 
the  opinion  that  the  progression  of  the  blood  in  the  veins  is  greatly  owing 
to  a  partial  vacuum  or  a  suction  power  in  the  heart  or  chest ;  that  is  to  say, 
to  the  atmospheric  pressure  remaining  constant  on  the  body  generally,  while 
it  is,  at  intervals,  lessened  about  the  heart.  Now  the  whole  influence  of 
this  effect  or  circumstance,  as  stated  above,  is  merely  a  slight  disturbance  of 
the  uniformity  of  the  venous  current  near  the  chest.  Such  a  doctrine  could 
not  be  proposed  or  entertained  for  a  moment  by  a  person  understanding  the 
principle  of  a  common  household  pump;  and  its  having  been  published,  and 
tolerated  by  certain  professional  men  in  the  present  time,  will  remain  a  proof 
to  posterity  of  the  deficiency,  as  regards  fundamental  science  or  natural 
philosophy,  now  existing  in  the  ordinary  medical  education.  Much  ingenuity 
has  been  wasted  upon  it,  particularly  by  Drs.  Carson  and  Barry,  the  latter 
of  whom,  after  making  laborious  experimental  investigations  on  living  ani- 
mals, has  even  attempted  to  build  upon  it  a  superstructure  of  medical  theory 
and  practice !  To  say  that  the  influence  of  the  heart  or  chest  is  the  power 
which  draws  the  blood  to  the  heart  from  the  general  system,  is  as  if  one 
asserted  that  the  rising  and  falling  of  the  tide  at  the  mouth  of  a  river  is  the 
power  which  collects  the  tributary  streams  in  the  interior  country. 

We  shall  enter  into  a  little  detail  on  this  subject,  because  the  discussion 
will  elucidate  some  minor  points  connected  with  the  circulation. 

Presuming,  then,  that  the  reader  perfectly  understands  the  theory  of 
pumps,  and  therefore  of  atmospheric  pressure,  as  explained  under  Pneu- 
matics, he  will  readily  understand  the  two  following  propositions,  either  of 
which  proves  it  to  be  a  physical  impossibility,  that  a  sucking  action  of  the 
heart  or  chest  can  be  a  cause  of  the  blood's  motion  along  the  veins.  1st. 
The  veins  are  pliant  tubes  free  to  collapse,  and  no  pump  can  lift  liquid 
through  such.  2d.  The  suction-power  of  the  chest  in  healthy  respiration  is 
too  weak  to  lift  liquid  even  one  inch  through  tubes  of  any  kind. 

A  practical  illustration  of  the  first  proposition  is  afforded  by  putting  the 
point  of  a  syringe  into  a  piece  of  gut,  or  eel-skin,  or  vein  filled  with  water, 
and  then  trying  to  pump  up  the  water.  The  result  will  be,  that  the  fluid 
close  to  the  mouth  of  the  syringe  will  enter  it,  and  then  the  sides  of  the 
pliant  tube  will  collapse  against  the  syringe,  making  an  end  of  the  experi- 
ment. In  exact  proportion  to  the  rigidity  of  the  tube  will  be  the  distance  to 
which  the  influence  of  the  syringe,  will  extend  in  it;  if,  for  instance,  half  an 


THE    CIRCULATION.  42T 

ounce  of  pressure  on  the  square  inch  of  its  surface  be  required  to  make  it 
collapse,  then  the  pump  will  draw  up  one  inch  of  water,  and  so  for  other 
proportions.  If,  during  the  action  of  the  syringe,  the  tube  were  allowed  to 
open  freely  at  the  bottom  into  a  vessel  of  water,  instead  of  the  syringe  then 
drawing  any  more  water  from  the  vessel  into  the  tube,  the  original  contents 
of  the  tube  would  straightway  be  discharged  downwards  into  the  vessel. 

The  explanation  of  all  these  facts  is  found  in  the  pressure  of  the  atmos- 
phere, (see  from  page  158  to  page  158)  seeking  entrance  everywhere  at  the 
surface  of  the  earth,  with  a  force  of  fifteen  pounds  per  square  inch,  and 
overcoming  any  opposing  force  less  than  this ; — a  pressure  which  is  suffi- 
cient, therefore,  to  push  a  column  of  water  thirty-four  feet  in  height,  up 
through  a  rigid  tube  into  the  vacuum  of  a  pump,  but  will  cause  the  sides  of 
the  tubes  to  collapse,  unless  able  to  sustain  its  force.  When  nature  intends 
a  tube  to  resist  any  degree  of  suction,  the  tube  is  made  rigid  in  proportion ; 
—witness  the  wind-pine  and  its  branches,  which  are  the  only  instances  in 
the  human  body.  jSid  if  tubes  prepared  for  sucking  light  air  only  have 
received  such  rigidity,  how  much  stronger  would  tubes  have  been  made  for 
sucking  blood. 

Some  bad  reasoners  on  this  subject  have  believed,  that  if  a  suction  power 
exist,  capable  of  lifting  one  inch  of  a  column  of  liquid,  any  column,  however 
long,  must  follow  the  first  inch  when  acted  upon  by  the  power  ]  for,  say 
they,  the  atmospheric  pressure,  by  preventing  a  vacuum,  will  prevent  sepa- 
ration of  the  liquid.  Now,  in  the  first  place,  this  reasoning  is  quite  inap- 
plicable to  pliant  tubes,  because  the  ready  collapse  of  their  sides  will  both 
allow  the  separation  and  prevent  the  vacuum ;  and,  in  the  second  place, 
with  respect  to  rigid  tubes,  it  is  equivalent  to  asserting  that  a  force  just 
capable  of  lifting  one  link  of  a  chain,  must,  therefore,  be  able  to  lift  any 
number  of  connected  links.  Water,  in  a  rigid  tube,  to  which  the  air  has 
no  admittance,  may  truly  be  considered  as  a  chain,  for  it  is  held  together  by 
a  force  of  fifteen  pounds  per  inch,  pressing  inwards  at  the  two  ends  ;  and  by 
force  inferior  to  this,  cannot  lift  one  portion  of  it  away  from  another,  and, 
therefore,  cannot  draw  out  a  drop  but  by  lifting  the  whole.  A  man  cannot 
suck  any  water  from  a  rigid  tube  which  is  closed  at  the  bottom ;  and  if  the 
bottom  be  open,  and  he  has  not  power  to  support  the  whole  contained  fluid, 
it  will  sink  from  his  tantalized  lips  to  stand  at  an  elevation  corresponding 
to  his  suction  power. 

To  illustrate  the  second  proposition  respecting  the  trifling  suction  power 
really  residing  in  the  chest,  we  may  state  that  a  person  of  ordinary  strength 
using  the  whole  power  of  the  chest,  (but  not  of  the  mouth  separated,  which 
is  a  smaller  and  much  more  powerful  pump  than  the  chest,)  cannot,  through 
a  rigid  tube,  suck  water  from  more  than  about  two  feet  below  his  lips,  and 
therefore  not  half  way  so  far  as  from  the  extremities  of  his  body ;  while, 
in  the  opposite  action  of  blowing  outwards,  as  in  the  attempt  to  blow 
through  a  tube  which  is  dipping  into  water,  he  finds  nearly  the  same  limit. 
But  in  ordinary  breathing,  instead  of  force  corresponding  to  a  liquid  column 
of  two  feet,  or  &  fifteenth  of  the  atmospheric  pressure,  the  increase  and  di- 
minution of  air-density  in  the  chest  are  measured  by  a  column  of  less  than 
one  inch  or  about  a  five-hundredth  of  the  atmospheric  pressure.  This  fact 
is  easily  shown  by  breathing  through  the  nose,  while  holding  in  the  mouth 
one  end  of  a  glass  tube,  the  other  end  of  which  is  immersed  in  water,  and 
then  noting  how  much  the  water  in  the  tube  rises  above  the  surrounding 
level  during  inspiration,  and  sinks  below  it  during  expiration.  The  mouth, 
during  this  experiment,  may  be  considered  as  a  part  of  the  general  cavity  of 


428  FLUIDITY    IN    RELATION    TO    ANIMALS. 

the  chest,  to  and  from  which  air  is  passing  by  the  narrow  openings  of  the 
nostrils.  In  tranquil  breathing,  with  both  nostrils  open,  the  fluctuation  in 
the  tube  is  less  than  half  an  inch  each  way ;  with  on.e  nostril  closed  and  the 
other  a  little  compressed,  it  may  amount  to  a  whole  inch ;  and  with  hurried 
or  convulsive  breathing,  like  that  of  an  animal  in  terror  and  in  pain,  it  may 
exceed  twelve  inches.  Although  the  measures  thus  obtained  from  the 
mouth  are  somewhat  too  small  for  the  changes  in  the  chest  itself,  because 
the  chest  is  more  remote  from  the  opening  by  which  the  external  air  enters, 
the  difference  is  very  trifling,  as  may  be  proved  during  such  experiments,  by 
stopping  the  nostrils  altogether,  while  the  same  respiratory  efforts  are  con- 
tinued ;  and  as  is  also  proved  by  the  agreement  of  the  results  witlf  strict 
calculation  founded  on  the  inertia  and  velocity  of  the  air  respired — a  calcu- 
lation similar  to  that  required  in  adjusting  the  index  to  the  machine  men- 
tioned at  page  215,  for  measuring  water-currents.  In  common  healthy 
breathing,  then,  while  the  mouth  is  open,  the  fluctuation  of  pressure  in  the 
chest  would  be  measured  by  less  than  half  an  inch  ucrotion  each  way  of  the 
liquid  column.  Dr.  Barry,  not  aware  that  this  point  could  be  so  easily 
determined  by  the  bloodless  experiment  described  above,  or  even  by  a  simple 
calculation,  sought  the  solution  by  numerous  trials  on  living  animals,  into 
some  part  of  whose  chest  he  forced  a  tube.  But  even  if  farther  experiments 
had  been  at  all  necessary,  those  of  Dr.  B.  could  not  have  decided  the  ques- 
tion, first,  because  the  pain  and  agitation  of  the  dying  animals  rendered  the 
breathing  violent  or  unnatural;  and,  secondly,  because  his  experimental 
tube  often  or  always  became  a  syphon,  and  he,  not  adverting  to  this  fact, 
has  not  recorded  the  difference  of  level  in  the  liquids  at  the  two  ends.  That, 
the  external  level  was,  for  the  most  part,  higher  than  the  internal,  is  proved 
by  his  having  noticed,  almost  solely,  the  inhaling  action  of  the  chest,  although 
the  exhaling  is  often  a  more  powerful  effort. 

Calling  an  inch  column  of  blood,  then,  the  measure  of  the  greatest  suges- 
cent  and  repellent  powers  of  the  chest  during  ordinary  respiration,  we  see 
that  the  force  which  really  sends  the  blood  from  below  to  the  heart,  may 
have  to  lift  a  column  one  inch  shorter  during  inspiration,  and  one  inch 
longer  during  expiration :  but  this  is  the  full  and  true  measure  and  nature 
of  the  influence  of  the  inspiration  on  the  blood's  return  to  the  heart.  To 
say,  then,  that  the  atmospheric  pressure,  modified  by  respiration,  is  the 
great  power  which  moves  the  venous  blood,  is  just  as  if  we  said,  that  a  boy, 
standing  near  the  fly-wheel  of  a  steam  engine  of  a  hundred  horses'  power, 
and  giving  it  his  Lilliputian  thrust,  alternately  backward  and  forward,  were 
the  prime  mover  of  the  machinery. 

The  truth  explained  above,  that  no  kind  of  pump  can  lift  fluid  through 
pliant  tubes,  free  to  collapse,  like  the  veins,  renders  it  unnecessary  farther  to 
speak  here  of  the  pumping  action  of  the  heart  itself,  insisted  on  by  Dr.  Car- 
son, or  of  that  other  action,  mentioned  in  a  subsequent  part  of  this  work,  to 
which,  also,  he  attributes  great  influence,  viz.,  the  tendency  towards  a  vacuum 
external  to  the  lungs  and  around  the  heart,  produced  by  the  disposition  of 
the  lungs  to  collapse.  It  maybe  remarked,  however,  that  this  last  influence 
is  more  considerable  than  the  simple  inspiratory  action  dwelt  on  by  Dr. 
Barry,  and  that  it  operates  during  expiration  nearly  as  much  as  during 
inspiration,  varying  in  force  with  the  degrees  of  expansion  of  the  chest.  It 
is  weaker  in  the  living  than  in  the  dead  body,  because  the  rigidity  of  the 
distended  pulmonary  arteries  helps  to  support  the  weight  of  the  living  lungs. 

Were  it  necessary  to  give  proofs  to  persons  unable  to  follow  the  above 
argument,  that  a  suction-power  in  the  heart  or  chest  is  not  the  force  which 


THE    CIRCULATION.  429 

draws  the  blood  from  the  extreme  veins,  reference  might  be  made  to  many 
notorious  facts  quite  incompatible  with  that  supposition ;  such  for  instance, 
as  those  recorded  at  page  424,  and  others.  A  vein  tied,  fills  tensely  below 
the  ligature — a  vein  cut  across  bleeds  from  the  orifice  which  is  distant  from 
the  heart,  and  will  fill  a  lofty  tube  connected  with  it — the  circulation  goes  on 
in  persons  holding  their  breath — the  veins  of  fishes,  which  do  not  breathe, 
return  the  blood  as  well  as  those  of  men,  &c.,  &c.* 

After  the  explanations  now  given,  it  is  almost  superfluous  to  remark  that 
absorption  in  animals  cannot  depend  on  atmospheric  pressure,  and  that  the 
effect  of  cupping-glasses  applied  to  extract  blood,  or  to  prevent  the  absorption 
of  poison  in  wounds,  in  no  way  depends  upon  the  fluctuating  density  of  the 
air  in  the  chest. •(•  Dr.  Barry's  reasonings  upon  these  subjects  involve  the 
same  fallacies  as  his  reasonings  on  the  venous  current.  With  respect  to 
absorption,  they  neglect  the  facts  of  fluids  having  weight;  and  with  respect 
to  cupping-glasses,  of  which  the  true  action  is  explained  at  page  175,  they 
are  equivalent  to  asserting  that  the  action  of  pumps  drawing  water  from  a 
river  among  the  hills  is  influenced  by  tides,  or  pumps  operating  at  its  mouth 
in  the  sea. 

If  the  fluids  in  animal  vessels  had  no  weight,  it  is  true,  that  in  absorption 
at  external  atmospheric  pressures  of  fifteen  pounds  per  inch  might  force  new 
matter  into  a  receiving  orifice,  at  the  instant  during  inspiration,  when  the 
opposing  pressure  in  the  chest,  at  the  other  ends  of  the  vessels,  were  half  an 
ounce  per  inch  less, — there  would  be  no  physical  absurdity  in  opposing  this, 
although  there  are  physiological  facts  that  disprove  it — but  when  we  reflect, 
that  in  all  vessels  under  the  level  of  the  heart,  the  weight  of  the  contained 
fluids  causes  an  additional  outward  pressure  of  about  half  an  ounce  troy  for 
every  perpendicular  inch  of  fluid  column,  making  an  excess  of  outward  pres- 
sure at  the  toes,  for  instance,  even  at  the  more  favourable  time  of  absorption, 

*  The  influence  of  inspiration  of  the  cavity  in  the  chest  exterior  of  the  lungs,  and  the  ex- 
pansive power  of  the  heart,  on  the  circulation  of  the  blood  in  the  veins,  have  no  doubt  been 
greatly  overestimated  by  Drs.  Barry,  Carson  and  others,  but  »ur  author  appears  to  us  to 
have  undervalued  their  eflfect.  Their  joint  power  is  more  considerable  than  the  reader 
might  be  led  to  suppose  from  the  perusal  of  the  preceding  pages. 

The  influence  of  inspiration  has  been  estimated  by  our  author,  perhaps  justly,  as  only 
sufficient  to  raise  a  column  one  inch  ;  if  this  force  acted  through  rigid  tubes  of  the  length 
of  the  veins,  it  would  produce  no  movement  of  the  contained  fluid;  but  acting  through  pliant 
tubes,  it  would  rise  one  inch  of  the  blood  out  of  the  vein  nearest  the  heart,  and  if  this  power 
acted  alone,  its  effect  would  here  cease.  But  the  vis  a  tergo,  produced  by  the  propulsive 
power  of  the  capillaries,  and,  perhaps,  also  of  fbe  heart,  prevents  the  collapse;  the  vein  is 
kept  full,  and  at  every  inspiration  this  power  is  renewed. 

The  influence  of  the  tendency  towards  a  vacuum  external  to  the  lungs,  and  around  the 
heart,  from  the  contractile  disposition  of  the  resilience  of  the  lungs,  is  admitted  by  our 
author  to  be  more  considerable  than  the  int-piratory  effort,  and  it  in  fact  is,  we  think,  greater 
than  is  suspected.  There  are  reasons  for  believing  that  the  lungs  do  not  entirely  fill  the 
cavity  in  which  they  are  contained;  the  influence  of  this  space  is  therefore  constant,  though 
greater  during  inspiration,  and  of  course  diminished  during  expiration. 

The  capillaries,  our  author  has  most  satisfactorily  shown,  have  a  vital  expansive  power; 
and  though  he  does  not  assert  that  the  heart  has  no  such  power,  he  denies  that  it  can  have 
any  influence  on  the  movement  of  the  venous  blood,  since  it  must  act  through  pliant  tubes. 
This  would  be  the  fact  if  the  expansion  of  the  heart  were  the  only  moving  power,  but  the 
vis  a  tergo  prevents  their  collapse,  and  the  effect  of  the  expansive  power  of  the  heart, 
whatever  that  may  be,  is  allowed  to  act. 

While,  therefore,  the  action  of  the  capillaries  and  perhaps  of  the  left  ventricle  of  the 
heart  must  be  considered  as  the  main  forces  by  which  the  blood  is  propelled  through  the 
veins,  the  expansive  power  of  the  heart — respiration  and  the  resilience  of  the  lungs,  or 
atmospheric  pressure — ought  to  be  viewed  as  necessary  forces,  though  their  precise  power 
cannot  readily  be  estimated. — Am.  Ed. 

f  The  effect  of  cupping-glasses  in  preventing  the  absorption  of  poisons  has  been  shown 
by  Dr.  Pennock  to  be  owing  to  mechanical  pressure.  See  the  interesting  experiments  in 
the  American  Journal  of  the  Medical  Sciences,  Vol.  II. — Am.  Ed. 


430  FLUIDITY    IN    RELATION    TO    ANIMALS. 

of  about  two  pounds  per  inch,  we  see  that  absorption  must  be  a  strong  action 
of  life,  able  to  overcome  a  great  excess  of  mechanical  resistance,  instead  of 
a  passive  phenomenon  obeying  an  excess  of  mechanical  force.  If  a  mere 
balance  of  pressures  acted  at  the  orifices,  as  Dr.  B.  supposes,  the  blood  and 
other  fluids  would  be  constantly  oozing  out  from  all  orifices  below  the  heart, 
as  blood  really  does  from  an  artificial  opening,  with  force  that  would  fill  a 
tube  reaching  as  high  as  the  heart.  It  would  be  good  news  for  proprietors 
of  mines,  and  other  persons  having  to  raise  water,  if  by  taking  off  an  ounce 
or  two  per  inch  of  the  atmospheric  pressure  at  the  top  of  a  full  pipe,  the  atmo- 
spheric pressure  continuing  elsewhere  would  then  force  water  in  at  openings 
below  and  cause  the  upward  current: — but  in  truth  to  make  the  atmosphere 
efficient  below,  powerful  steam-engines  or  other  means  must  be  used  to  take 
off  a  pressure  above,  of  at  least  half  an  ounce  per  square  inch,  for  every  inch 
in  herght  the  water  has  to  rise. 

Another  erroneous  conception  of  atmospheric  pressure,  akin  to  that  which 
we  have  been  considering,  is  expressed  in  the  following  reasoning  on  the 
progress  of  blood  in  the  veins.  "  The  atmosphere  presses  15  Ibs.  per  square 
inch  on  all  thing's ;  the  blood  therefore,  in  a  vein  which  has  20  inches  of  sur- 
face is  pressed  upon  through  the  flesh,  with  a  force  of  20  times  15,  or  300 
Ibs.,  while  a  cross  section  of  the  vein  near  the  heart  would  measure  less  than 
one  inch.  The  blood,  therefore,  is  always  running  towards  the  heart,  to 
escape  from  a  powerful  excess  of  atmospheric  pressure." — This  paradox  is 
solved  by  the  law  of  fluid  pressure,  explained  at  page  131.  The  same  rea- 
soning would  prove  that  an  eel-skin  suspended  by  its  lip,  and  filled  with  water, 
when  exposed  to  the  pressure  of  the  atmosphere,  should  quickly  be  emptied; 
and  nearly  the  same  would  prove  that  a  long  sharp  wedge  thrown  into  water, 
should  be  always  moving  in  a  direction  away  from  its  point;  and  that  a  ship 
formed  like  the  wedge,  should  make  quick  speed  across  the  sea  without  either 
oar  or  sail. 

A  knowledge  of  thefacte  detailed  under  the  three  heads  of  arteries,  capillaries 
and  veins,  prepares  us  for  the  discussion  of  the  following  subjects. 

The  force  of  the  heart. 

The  arterial  tension  of  four  pounds  to  the  square  inch,  marked  by  its  sup- 
porting in  a  tube  connected  with  the  arteries,  a  column  of  blood  eight  feet 
high,  (see  .page  417,)  is  produced  by  the  action  of  the  heart ;  but  as  the 
heart,  while  injecting  the  blood  against  this  resistance,  has  moreover  to  over- 
come the  inertia  both  of  the  quantity  injected  and  of  the  mass  in  the  great 
artery,  first  moved  by  the  injection,  as  also  the  resisting  elasticity  of  the 
vessel  which  yields  to  momentary  increase  of  pressure,  the  heart  must  act 
with  a  force  exceeding  four  pounds  on  the  inch.  And  as  the  left  ventricle 
of  the  human  heart,  when  distended,  has  about  ten  square  inches  of  internal 
surface,  the  whole  force  exerted  by  it  is  a  matter  of  simple  calculation.  It 
is  remarkable,  as  there  is  this  easy  means  of  solving  the  question,  that  the 
accurate  Magendie,  in  his  recent  elements  of  physiology,  should  speak  of  it 
as  undetermined ;  and  should  cite,  as  the  best  approximation,  an  estimate 
made  from  the  obscure  circumstance  of  a  loaded  foot  shaking  in  unison  with 
the  pulse,  when  suspended  in  the  cross-legged  sitting -attitude.- 

Some  physiologists  have  expressed  surprise  that  the  force  of  the  heart 
should  be  so  great  as  it  is,  remarking  that  much  less  would  have  sufficed  to 
propel  the  blood  to  the  most  distant  capillaries;  but  they  did  not  reflect  that 
the  heart,  besides  carrying  on  the  general  circulation,  has  to  force  blood  into 


THE    CIRCULATION.  431 

those  parts  of  the  flesh  which,  in  the  various  positions  of  sitting,  lying, 
standing,  &c.,  are  for  the  time  compressed  by  the  whole  weight  of  the  body  ; 
for  that,  if  it  were  not  strong  enough  for  this  purpose,  either  the  compressed 
parts,  deprived  of  their  nourishment,  would  quickly  die,  or  the  person  obliged 
to  be  every  moment  changing  position,  could  obtain  no  lengthened  repose. 
In  illustration  of  this  point,  we  may  advert  to  the  frequent  occurrence,  in 
diseases  where  the  power  of  the  heart  is  for  the  time  weakened,  for  slough- 
ings,  or  bed-sores  in  the  bearing  parts,  causing  many  cases  of  illness  to 
terminate  fatally  which  would  otherwise  soon  have  terminated  in  health. 
The  author  of  tnis  work  has  had  great  satisfaction  in  suggesting  a  means 
of  entirely  preventing  deplorable  termination,  namely,  that  which  he  is 
now  about  to  describe  under  the  title  of 

The  HYDROSTATIC  BED  for  Invalids. 

In  many  of  the  diseases  which  afflict  humanity,  more  than  half  of  the  suf- 
fering and  danger  is  not  really  a  part  of  the  disease,  but  the  effect  or  conse- 
quence of  the  confinement  to  which  the  patient  is  subjected.  Thus  a  fracture 
of  a  bone  of  the  arm  is  as  serious  a  local  injury  as  a  fracture  of  one  of  the 
bones  of  the  leg  ;  but  the  former  leaves  the  patient  free  to  go  about  and  amuse 
himself,  or  attend  to  business  as  he  wills,  and  to  eat  and  drink  as  usual — in 
fact,  hardly  renders  him  an  invalid ;  while  the  latter  imprisons  the  patient 
closely  upon  his  bed,  and  brings  upon  him,  first,  irksomeness  of  the  conti- 
nued position,  and  then  the  pains  of  the  unequal  pressures  borne  by  the  parts 
on  which  the  body  rests.  These,  in  many  cases  of  confinement,  disturb  the 
sleep  and  the  appetite,  and  excite  fever,  or  such  constitutional  irritation  as 
much  to  retard  the  cure  of  the  original  disease,  and  not  unfrequently  to  pro- 
duce new  and  more  serious  disease.  That  complete  inaction  should  prove 
hurtful  to  the  animal  system,  may  by  all  be  at  once  conceived;  the  operation 
of  the  continued  local  pressures  will  be  understood  from  the  following  state- 
ments. '  The  health,  and  even  life,  of  every  part  of  the  animal  body,  depend 
on  the  sufficient  circulation  through  it  of  fresh  blood,  driven  in  by  the  force 
of  the  heart.  Now  when  a  man  is  sitting  or  lying,  the  parts  of  his  flesh  com- 
pressed by  the  weight  of  the  body,  do  not  receive  the  blood  so  readily  as  at 
other  times ;  and  if  from  any  cause  the  action  of  his  heart  has  become  weak, 
the  interruption  will  both  follow  more  quickly  and  be  more  complete.  A 
peculiar  uneasiness  soon  arises  where  the  circulation  is  thus  obstructed,  im- 
pelling the  person  to  change  of  position ;  and  a  healthy  person  changes  as 
regularly,  and  with  as  little  reflection,  as  he  winks  to  wipe  and  moisten  his 
eyeballs.  A  person  weakened  by  disease,  however,  while  he  generally  feels 
the  uneasiness  sooner,  as  explained  above,  and  therefore  becomes  what  is 
called  restless,  makes  the  changes  with  much  fatigue;  and  should  the  sensa- 
tions after  a  time  become  indistinct,  as  in  the  delirium  of  fever,  in  palsy,  &c,, 
or  should  the  patient  have  become  too  weak  to  obey  the  sensation,  the  com- 
pressed parts  are  kept  so  long  without  their  natural  supply  of  blood  that  they 
lose  their  vitality,  and  become  what  are  called  sloughs  or  mortified  parts. 
These  have  afterwards  to  be  thrown  off,  if  the  patient  survive,  by  the  pro- 
cess of  ulceration,  and  they  leave  deep  holes,  requiring  to  be  filled  up  by 
new  flesh  during  a  tedious  convalescence.  Many  a  fever,  after  a  favourable 
crisis,  has  terminated  fatally  from  this  occurrence  of  sloughing  on  the  back 
or  sacrum  ;  and  the  same  termination  is  common  in  lingering  consumptions, 
palsies,  spine  diseases,  &c.,  and  generally  in  diseases  which  confine  the 
patients  long  to  bed. 


432     FLUIDITY  IN  RELATION  TO  ANIMALS. 

It  is  to  mitigate  all,  and  entirely  to  prevent  some  of  the  evils  attendant  on 
the  necessity  of  remaining  in  a  reclining  posture,  that  the  hydrostatic  bed  is 
intended.  It  was  first  used  under  the  following  circumstances. 

A  lady  after  her  confinement,  which  occurred  prematurely,  and  when  her 
child  had  been  for  some  time  dead,  passed  through  a  combination  and  suc- 
cession of  l^v  fever,  jaundice,  and  slight  phlegrnasia  dolens  of  one  leg.  In 
her  state  of  extreme  depression  of  strength  and  sensibility,  she  rested  too 
long  in  one  posture,  and  the  parts  of  the  body  on  which  she  had  rested  all 
suffered  :  a  slough  formed  on  the  sacrum,  another  on  the  heeLj  and  in  the 
left  hip,  on  which  she  had  lain  much,  inflammation  began,  which  terminated 
in  abscess.  These  evils  occurred  while  she  was  using  preparations  of  bark, 
and  other  means,  to  invigorate  the  circulation,  and  while  her  ease  and  com- 
fort were  watched  over  by  the  affectionate  assiduity  of  her  mother  with 
numerous  attendants.  After  the  occurrence,  she  was  placed  upon  the  bed 
contrived  for  invalids  by  Mr.  Earle,  furnished  for  this  case  with  pillows  of 
down  and  of  air  of  various  sizes,  and  out  of  its  mattrass  portions  were  cut  op- 
posite f to  the  sloughing  parts;  and  Mr.  Earle  himself  soon  afforded  his 
valuable  aid.  Such,  however,  was  the  reduction  of  the  power  of  life,  that 
in  spite  of  all  endeavors,  the  mischief  advanced,  and  about  a  week  later, 
during  one  night,  the  chief  slough  on  the  back  was  much  enlarged,  another 
had  formed  near  it,  and  a  new  abscess  was  proceeding  in  the  right  hip.  An 
air-pillow  had  pressed  where  these  sloughs  appeared.  The  patient  was  at 
that  lime  so  weak  that  she  generally  fainted  when  her  wounds  were  dressed ; 
she  was  passing  days  and  nights  of  uninterrupted  suffering,  and  as  all  known 
means  seemed  insufficient  to  relieve  her,  her  life  was  in  imminent  danger. 

Under  these  circumstances,  the  idea  of  the  hydrostatic  bed  occurred  to  met 
Even  the  pressure  of  an  air-pillow  had  killed  her  flesh ;  and  it  was  evident 
that  persons  in  such  condition  could  not  be  saved  unless  they  could  be  sup- 
ported without  sensible  inequality  of  pressure.  I  then  reflected,  that  the 
support  of  water  to  a  floating  body  is  so  uniformly  diffused,  that  every  thou- 
sandth of  an  inch  of  the  inferior  surface  has,  as  it  were,  its  own  separate 
liquid  pillar,  and  no  part  bears  the  load  of  its  neighbour — that  a  person 
resting  in  a  bath  is  nearly  thus  supported — that  this  patient  might  be  laid 
upon  the  surface  of  a  bath  over  which  a  large  sheet  of  the  water-proof  India- 
rubber  cloth  were  previously  thrown,  she  being  rendered  sufficiently  buoyant 
by  a  soft  mattress  placed  beneath  her — thus  would  she  repose  on  the  face  of 
the  water,  like  a  swan  on  its  plumage,  without  sensible  pressure  anywhere, 
and  almost  as  if  the  weight  of  her  body  were  annihilated.  The  pressure  of 
the  atmosphere  on  our  bodies  is  of  fifteen  pounds  per  square  inch  of  its  sur- 
face, but  because  uniformly  diffused,  is  not  felt.  The  pressure  of  a  water- 
bath  or  depth  to  cover  the  body,  is  less  than  half  a  pound  per  inch,  even  on 
the  under  side  where  it  is  greatest,  and  is  similarly  unperceived.  A  bed 
such  as^then  planned,  was  immediately  made.  A  trough  of  convenient  dimen- 
sions (6  feet  long,  2  feet  8  inches  wide,  and  11  inches  deep,  are  good  com- 
mon dimensions)  was  lined  with  metal  to  make  it  water-tight;  it  was  about 
half  filled  with  water,  and  over  it  was  thrown  a  sheet  of  the  India-rubber 
cloth  as  large  as  would  be  a  complete  lining  to  it  if  empty.  Of  this  sheet 
the  edges,  touched  with  spirit  varnish  to  prevent  the  water  creeping  round 
by  capillary  attraction,  were  afterwards  secured  in  a  water-tight  manner  all 
round  to  the  upper  border  or  top  of  the  trough,  shutting  in  the  water  aa 
closely  as  if  it  had  been  in  bottles,  the  only  entrance  left  being  through  an 
opening  at  one  corner,  which  could  be  perfectly  closed.  Upon  this  beautiful 
dry  sheet  a  suitable  mattress  was  laid,  and  constituted  a  bed  ready  to  receive 


THE    CIRCULATION.  433 

its  pilow  and  bed-clothes,  and  not  distinguishable  from  a  common  bed  but 
by  its  most  surpassing  softness  or  yielding.  The  bed  was  carried  to  the 
patient's  house,  and  she  was  laid  upon  it;  she  was  instantly  relieved  in  a 
remarkable  degree :  sweet  sleep  came  to  her;  she  awoke  refreshed;  she 
passed  the  next  night  much  better  than  usual ;  and  on  the  following  day, 
Mr.  Earle  found  that  all  the  sores  had  assumed  a  healthy  appearance ;  the 
healing  from  that  time  went  on  rapidly,  and  no  new  sloughs  were  formed. 
When  the  patient  was  first  laid  upon  the  bed,  her  mother  asked  her  where 
the  down  pillows,  which  she  before  had  used,  were  to  be  placed;  to  which 
she  answered,  that  she  knew  not,  for  that  she  felt  no  pain  to  direct;  in  fact, 
she  needed  them  no  more. 

It  may  be  here  recalled  to  mind,  that  the  human  body  is  nearly  of  the 
specific  gravity  of  water,  or  of  the  weight  of  its  bulk  of  water,  and  therefore, 
as  is  known  to  swimmers,  is  just  suspended  or  upheld  in  water  without  exer- 
tion when  the  swimmer  rests  tranquilly  on  his  back  with  his  face  upwards. 
He  then  displaces  water  equal  to  his  own  body  in  weight  as  well  as  in  bulk, 
and  is  supported  as  the  displaced  water  would  have  been.  If  his  body  be 
two  and  a  half  cubical  feet  in  bulk,  (a  common  size,)  he  will  just  displace  two 
and  a  half  cubic  feet  of  water,  equal  in  weight  to  his  body.  If,  however, 
instead  of  displacing  the  water  with  his  mere  body,  he  choose  to  have  some- 
thing around  or  under  him  which  bulky  with  little  weight,  as  the  mattrass 
of  the  bed  above  described,  then,  after  his  weight  is  forced  two  cubical  feet 
of  that  under  the  level  of  the  water  around,  he  will  float  with  four-fifths  of  his 
body  above  the  level,  and  will  sink  much  less  into  his  floating^  mattrass  than 
a  person  sinks  in  an  ordinary  feather-bed.  It  thus  appears  that  by  choosing 
a  certain  thickness  of  mattrass,  and  if  unusual  positions  are  required  by 
having  different  thicknesses  in  different  parts,  or  by  placing  a  bulls  of  folded 
blanket  or  of  pillow  over  or  under  the  mattrass  in  certain  situations,  any 
desired  position  of  the  body  may  be  easily  obtained.  If  the  water  be  about 
six  inches  deep,  which  in  general  will  suffice,  the  person  standing  upon  any 
part  of  the  bed,  or  sitting  with  the  knees  raised,  will  cause  the  part  of  the 
mattrass  on  which  he  rests  gently  to  touch  the  bottom,  because  a  narrow  end 
of  the  body  cannot  displace  water  equal  to  the  bulk  of  the  whole,  but  even 
then  the  person  is  as  if  standing  or  sitting  on  a  soft  sofa.  If  it  be  desired 
to  prevent  the  mattrass,  when  used  as  a  seat,  from  touching  the  bottom,  the 
object  may  be  attained  by  having  under  its  middle  a  broad  band  or  strap 
fixed  to  one  edge  of  the  trough,  and  connected  with  the  other  by  buttons  or 
otherwise,  so  as  to  be  tightened  to  allow  the  mattrass  to  descend  just  so  far, 
and  no  farther. 

This  bed  is  a  warm  bed,  owing  to  water  being  nearly  an  absolute  non- 
conductor of  heat  from  above  downwards,  and  owing  to  its  allowing  no  pas- 
sage of  cold  air  from  below.  From  this  last  fact,  however,  less  of  the  per- 
spiration, sensible  and  insensible,  is  carried  off  by  the  air  than  in  a  common 
bed,  and  unless  the  patient  can  leave  the  bed  daily  to  let  it  be  aired  like  a 
common  bed,  there  will  be  a  necessity  for  ventilation  to  prevent  the  perspi- 
ration from  being  condensed  on  the  water-sheet  below.  This  ventilation  is 
perfectly  obtained  by  placing  under  the  mattrass,  arranged  like  the  bars  of  a 
gridiron,  small  flexible  tubes  of  tinned  wire,  wound  spirally,  with  their  ends 
open  to  the  atmosphere,  either  directly  or  through  two  larger  tubes  crossing 
and  connecting  their  extremities  near  the  ends  of  the  mattrass,  and  then 
issuing  at  the  corners  of  the  bed  from  under  the  clothes.  This  bed  is  in 
itself  as  dry  as  a  bed  can  be,  for  the  India-rubber  cloth  (of  which  bottles  can 
be  made)  is  quite  impermeable  to  water,  and  the  maker  is  now  preparing 

28 


434  FLUIDITY    IN    RELATION    TO    ANIMALS. 

cloth  expressly  for  this  purpose.  Then,  as  Sir  Humphrey  Davy  recommended 
that  his  safety  lamp  should  be  double,  some  persons  may  prefer  a  double 
sheet,  to  obviate  the  possibility  of  accident.  Unlike  any  other  bed  that  ever 
was  contrived,  it  allows  the  patient,  when  capable  of  only  feeble  efforts,  to 
change  his  position,  almost  like  a  person  swimming,  and  so  to  take  a  de- 
gree of  exercise,  affording  the  kind  of  relief  which,  in  constrained  positions, 
is  obtained  by  occasional  stretching,  or  which  an  invalid  seeks  by  driving  out 
in  a  soft-springed  carriage.  It  exceedingly  facilitates  turning  for  the  pur- 
pose of  dressing  wounds,  for  by  raising  one  side  of  the  mattrass  or  depress- 
ing the  other,  or  merely  by  the  patient's  extending  a  limb  to  one  side,  he  is 
gently  rolled  over,  nearly  as  if  he  were  simply  suspended  in  water;  and  it  is 
possible  even  to  dress  wounds,  apply  poultcies,  or  place  vessels  under  any 
part  of  the  body,  without  moving  the  body  at  all :  for  there  are  some  inches 
of  yielding  water  under  the  body,  and  the  elastic  mattrass  may  at  any  part 
be  pushed  down,  leaving  vacant  space  there,  without  the  support  being  less- 
ened for  the  other  parts.  Then,  with  all  the  advantages  which  other  inva- 
lid beds  posses,  and  with  those  which  are  entirely  its  own,  it  may  yet  be 
made  so  cheaply,  that  even  in  hospitals,  where  economy  must  prevail,  it  may 
at  once  be  adopted  for  many  of  the  bed-ridden.  Mr.  Earle  within  a  few  days 
of  seeing  the  first  one,  had  others  made  for  patients  in  St.  Bartholomew's 
Hospital,  and  has  been  as  much  pleased  with  the  results  of  them  as  of  the 
first.  The  bed  has  since  been  introduced  into  St.  George's  Hospital  by  Mr. 
Keate,  and  elsewhere,. — The  author  has  now  seen  enough  of  the  effects  of 
this  bed  to  make  him  feel  it  a  duty  at  once  to  publish  a  notice  of  it.  With 
it,  evidently, *the  fatal  termination  called  sloughing,  now  so  common,  of  fe- 
vers, and  other  diseases  need  never  occur  again.  And  not  only  will  it  pre- 
vent that  'termination,  but  by  alleviating  the  distress  through  the  earlier 
stages,  it  may  prevent  many  cases  from  even  reaching  the  degree  of  danger. 
Then  it  is  peculiarly  applicable  to  cases  of  fractured  bones,  and  other  surgical 
injuries;  to  palsies,  diseases  of  the  hip  joint,  and  spine;  and  universally, 
where  persons  are  obliged  to  pass  much  time  in  bed.  And  in  all  cases  of 
curvature  of  the  spine,  either  actually  existing  or  threatened,  it  affords  a 
means  of  laying  a  patient  in  any  desired  position,  and  with  any  degree  of 
pressure  incessantly  urging  any  part  of  the  spine  back  to  its  place.  If  used 
without  the  mattrass,  it  becomes  a  warm  or  a  cold  bath,  not  allowing  the 
body,  however,  to  be  touched  by  the  water,  and  in  India,  it  might  be  made 
a  cool  bed  for  persons  sick  or  sound,  during  the  heats  which  there  prevent 
sleep  and  endanger  health.  There  are  numerous  other  professional  adaptions 
and  modifications  of  it,  which  will  readily  occur  to  practitioners  sufficiently 
versed  in  the  department  of  natural  philosophy  (hydrostatics)  to  which  it  be- 
longs. Before  reflection,  a  person  might  suppose  a  resemblance  between  it 
and  an  air-bed  or  pillow,  calling  this  a  water-bed  or  pillow;  but  the  prin- 
ciples of  the  two  are  perfectly  distinct  or  opposite.  An  air-pillow  supports 
by  the  tension  of  the  surface  which  encloses  the  air,  and  is  therefore  like  a 
hammock  or  the  tight  sacking  under  the  straw  mattrass  of  a  common  bed, 
and  really  is  a  hard  pillow;  but  in  the  hydrostatic  bed,  there  is  no  tense 
surface  or  web  at  all ;  the  patient  is  floating  upon  the  water,  on  which  a 
loose  sheet  is  lying,  merely  to  keep  the  mattrass  dry,  and  every  point  of  his 
body  is  supported  by  the  water  immediately  beneath  it.  To  recall  the  dif- 
ference here  described,  and  which  is  of  g'reat  importance,  the  bed  is  better 
described  by  the  appellations  of  hydrostatic-bed,  or  floating -led,  than  of 
water-bed. 

The  author  has  given  no  exclusive  right  or  privilege  to  any  person  to 


THE    CIRCULATION.  435 

make  this  bed.  He  has  hitherto  employed  the  carpenter  nearest  to  him, 
Mr.  Smith,  253  Tottenham-court  Road,  at  the  back  of  Bedford  Square; 
Mackintosh  &  Co.,  58  Charing  Cross,  the  manufacturers  of  the  cloth;  and 
Mr.  Williams,  25  Cleveland  Street,  Fitzroy  Square;  but  any  carpenter  or 
upholsterer  may  learn  to  supply  them,  and  he  gives  free  permission  to  all. 
He  hopes,  for  the  sake  of  the  poor,  that  a  trough,  without  metallic  lining, 
and  with  a  cheaper  water-proof  cloth,  may  be  found  to  answer  satisfactorily.. 

The  principle  of  the  hydrostatic  bed,  is  applicable,  also,  to  couches  for 
invalids,  and  with  certain  considerable  modifications,  to  the  construction, 
also,  of  chairs ;  and  there  are  other  means  than  the  water-proof  sheet  of 
adapting  the  hydrostatic  principle  for  all — but  the  subject  has  already 
occupied  its  full  share  of  this  volume. 

The  preceding  paragraphs  are  intended  as  much  to  direct  in  the  choice  and 
use  of  common  beds  for  the  sick,  as  to  announce  and  describe  the  hydro- 
static bed  for  the  cases  in  which  it  may  be  required.  At  present,  the  medi- 
cal attendant  generally  leaves  whatever  regards  the  bed  to  the  judgment  of 
friends  or  nurses ;  but  evidently,  he  who  has  been  led  to  reflect  how  much 
the  course  and  event  of  a  maladay  may  depend  on  the  patient's  being  sup- 
ported, so  that  no  pain  shall  arise  from  local  pressure,  and  as  little  muscular 
weariness  as  possible  from  constrained  position,  will  deem  the  bed-manage- 
ment worthy  of  his  own  attention,  and  will  be  able  more  judiciously  both  to 
choose  and  to  use  beds.  There  is  a  bed  constructed  of  spiral  springs,  which 
may  be  made  so  as  to  diffuse  the  support  more  equally  than  any  except  the 
hydrostatic  bed ;  and  bad  professional  men  generally  been  acquainted  with 
it,  it  would  have  been  more  used  than  it  is,  and  would  have  received  various 
modifications,  of  which  it  is  susceptible,  for  medical  purposes.  It  has  long 
been  known,  chiefly,  however,  as  a  mechanical  curiosity,  or  an  object  of 
luxury,  and  was  introduced  into  this  country  about  seventy  years  ago  by 
Mr.  Merlin ;  but  it  has  been  so  little  known,  that  a  few  years  ago  an  English 
tradesman  thought  he  might  appropriate  the  manufacture  by  taking  a  patent 
for  it.  It  is  now  made  by  upholsterers  generally,  and  the  same  principle  is 
applied  in  the  construction  of  sofas,  chairs,  and  carriage  cushions. 

The  velocity  of  the  circulating  blood. 

This  has  been  much  overrated.  1st.  By  assuming  that  the  ventricles  of 
the  heart  are  both  completely  filled  from  the  auricles  and  emptied  towards 
the  arteries  at  each  pulsation  : — an  assumption  disproved  by  inspections  of 
the  exposed  heart  of  a  living  body,  and  by  the  fact  of  the  valves  between  the 
auricles  and  ventricles  not  closing  so  perfectly  as  quite  to  prevent  regurgi- 
tation.  2d.  By  supposing  the  issue  of  blood  from  a  wounded  artery  or  vein 
to  be  the  measure  of  the  usual  velocity.  Now  it  would  be  as  reasonable  to 
suppose  the  issue  of  water  from  a  wounded  pipe  connected  with  any  reser- 
voir to  be  the  measure  of  a  continued  current  in  that  pipe,  although,  in  truth, 
the  issue  would  be  the  same  even  if  the  water  in  the  pipe  were  usually  at 
rest.  3d.  By  supposing  the  frequency  of  the  pulse  to  be  a  measure.  Now 
we  know,  that  in  diseases  of  debility,  and  in  animals  bleeding  to  death,  the 
pulse  usually  becomes  more  frequent  as  it  becomes  more  feeble,  and  as  there 
is  less  blood  moving  :  viz.,  the  heart  very  partially  discharging  its  contents 
at  each  contraction.  4th,  and  lastly.  By  supposing  the  strength  of  the 
pulse  to  be  the  measure.  Now  we  find  that  the  pulse  in  an  artery  just  tied, 
and  in  which,  consequently,  there  is  no  current  at  all,  is  scarcely  weaker 
than  in  an  open  artery.  The  common  fact  of  a  person's  feet  remaining 
stone-cold  for  hours,  although  the  arteries  leading  to  them  pulsate  nearly 


436  FLUIDITY    IN    RELATION    TO    ANIMALS. 

as  usual,  is  a  proof  that  exceedingly  little  blood  is  passing  through  the 
capillaries  at  the  time,  and  that  the  strength  of  the  pulse,  therefore,  is  no 
measure  of  the  speed  of  the  blood. 

The  ventricles  of  the  heart  appear,  under  common  circumstances,  to  throw 
out  about  an  ounce  and  a  half  of  blood  at  every  contraction — or  about  seven 
pounds  per  minute.  Now  if  the  body  contain  about  twenty  pounds  alto- 
gether, as  seems  to  be  the  case,  the  whole  would  circulate  twenty  times  in 
an  hour.  This  would  give  an  average  velocity  of  about  eight  inches  per 
second  in  the  aorta,  but  gradually  less  in  the  smaller  arteries,  because  when- 
ever a  vascular  channel  subdivides,  the  branches  taken  collectively  have 
considerably  greater  area  than  the  trunk  from  which  they  arise,  and  the 
current  diminishes  in  a  corresponding  proportion,  just  as  the  speed  of  a 
river  is  always  less  in  the  parts  of  the  channel  which  are  deeper  and  broader. 
The  velocity  in  the  extreme  capillaries  is  found  to  be  often  less  than  one 
inch  per  minute.  In  the  veins,  the  blood  must  move  more  slowly  than  in 
corresponding  arteries,  in  proportion  as  the  veins  are  more  capacious  than 
the  arteries. 

The  pulse. 

The  opinion  which  the  ancients  held,  that  the  arteries  contained  vital 
spirits  or  air  and  not  blood,  rendered  the  pulse,  to  them,  a  very  mysterious 
phenomenon;  and  many  curious  hypotheses  were  framed  to  explain  it. 
These  it  would  now  be  unprofitable  to  detail.  Even  Harvey's  grand  dis- 
covery of  the  circulation,  however,  has  not  rendered  the  subject  so  simple 
as  might  have  been  anticipated.  The  following  opinions  now  exist,  or  have 
lately  existed  with  respect  to  the  pulse. 

1st.  The  great  majority  of  physiologists  have  believed  that  a  tumefaction 
is  produced  in  the  aorta  by  each  jet  of  blood  from  the  heart,  and  spreads 
afterwards  as  a  wave  into  all  the  arterial  branches.  2d.  Many  have  supposed 
an  extensive  contractile  action  of  the  arteries  themselves,  corresponding  to 
that  of  the  heart.  3d.  Bichat,  unable  by  any  means  to  detect  the  slightest 
change  of  diameter  in  the  arteries  during  pulsation,  but  perceiving  that  in 
many  situations  they  were  at  the  time  somewhat  lengthened,  so  that  straight 
portions  became  bent,  and  portions  originally  bent,  were  bent  still  more,  held 
that  this  locomotion  or  changing  of  place  in  the  arteries  was  the  cause. 
4th.  Others  have  supposed  the  impulse  of  the  heart's  contraction  to  be  trans- 
mitted through  the  fluid  blood,  somewhat  as  sound  is  transmitted  through 
bodies  generally,  or  as  a  blow  struck  on  one  end  of  a  log  of  wood,  is  felt 
dfstinctly  by  a  hand  applied  to  the  other,  although  there  be  no7 visible  loco- 
motion. 5th.  Dr.  Young,  in  the  paper  in  the  Philosophical  Transactions 
already  alluded  to,  has  shown  that  a  sudden  rush  forward  of  the  blood  in 
the  artery,  such  as  might  be  produced  by  injection  at  one  end  of  a  rigid 
tube,  would  be  felt  by  a  finger  applied  to  the  artery,  quite  as  distinctly  as 
a  tumefaction ;  and  he  deems  this  occurrence  to  be  a  chief  cause  of  the 
pulse.  Dr.  Parry,  in  his  work  on  the  pulse,  points  to  this  almost  as  exclusively 
the  cause. 

Now  the  truth  is,  that  the  pulse  in  the  living  body  does  not  depend 
exclusively  upon  any  one  of  the  particulars  just  noticed,  but  has.  all  of  them 
as  elements ;  and  its  fluctuations  and  varieties  depend  upon  proportions  in 
which  these  elements  are  combined.  We  shall  review  them  again  to  prove 
this. 

1st.  At  each  jet  of  blood  thrown  into  the  aorta,  a  tumefaction  or  wave 
must  spread  from  the  heart  to  the  extremities ;  for  it  is  evident,  that  if  blood 


THE    CIRCULATION.  437 

be  at  all  pushed  into  the  arterial  system,  it  either  must  dilate  it,  or  cause  an 
equal  quantity  to  be  expelled  at  the  same  instant  from  the  distant  extremities  : 
now  as  the  passage  of  blood  through  the  capillaries  appears  perfectly  uniform, 
there  must  be  an  intermediate  dilatation.  Dr.  Parry  and  others  should  not 
have  denied  this  dilatation  because  they  could  not  see  it:  for  even  if  its  ad- 
vancing front  were  more  considerable  than  it  is,  it  passes  with  such  velocity 
that  like  a  cannon-ball  crossing  before  the  face,  it  would  not  be  perceived. 

2d.  Contraction  of  the  arterial  coats  certainly  does  not  take  place  in  the 
manner  and  to  the  extent  supposed  by  some,  who  have  spoken  of  it  as  resem- 
bling the  contraction  of  the  heart  itself,  and  as  what  might  be  a  substitute  for 
the  action  of  the  heart  in  propelling  the  blood;  but  as  shown  at  page  414, 
the  rigidity  of  tube  which  in  all  degrees  of  arterial  dilatation  causes  the  pulse 
to  be  transmitted  so  quickly,  can  depend  on  nothing  but  a  contractile  action 
of  the  fibres.  There  are  some  reasons  for  doubting  whether  this  rigidity 
may  not  increase  at  the  moment  of  the  pulse. 

od.  Unless  the  arterial  tubes  were  absolutely  inelastic,  which  they  are  far 
from  being,  they  must  be  lengthened  a  little  by  a  sudden  injection  of  blood, 
and,  therefore,  at  all  the  curvatures  particularly,  there  must  be  a  degree  of 
the  locomotion  described  by  Bi^hat,  often  sensible  to  a  finger  applied. 

4th.  That  a  tangible  shock  is  conveyed  through  a  fluid  without  any  appa- 
rent accumulation  of  the  fluid  or  change  of  velocity,  and  much  in  the  manner 
of  sound,  is  proved,  by  the  facts,  that  we  may  discover  the  working  of  a 
water-pump  at  very  great  distances,  through  iron  pipes  connected  with  it,  and 
even  through  elastic  pipes  of  leathe^,  as  those  of  a  common  fire-engine,  from 
which  the  water  is  spouting,  nevertheless,  in  a  uniform  stream.  The  pulse 
in  a  tied  artery,  in  which  there  is  no  current  or  rushing  wave,  must  be  chiefly 
from  this  cause  and  from  the  locomotion  of  the  artery. 

5th.  That  any  additional  quantity  of  fluid  injected  into  elastic  vessels 
already  full,  must  spread  all  over  with  &  forward  rush,  affecting  the  finger 
of  an  examiner,  as  described  above,  is  also  most  certain.  As  the  heart,  how- 
ever, often  beats  without  discharging  much  of  its  blood,  and  as  in  many 
arteries  from  inaction  of  the  capillaries,  or  pressure  of  the  blood  for  a  time 
makes  little  or  no  progress,  while  the  pulse,  however,  remains  very  distinct, 
the  pulse  in  such  cases  must  be  produced  independently  of  the  forward  rush. 
An  animal  intestine  prepared,  and  filled  with  water  or  air,  and  laid  upon  a 
table — or  a  full  vein  in  the  living  body,  carries  a  rapid  and  distinct  pulse  to 
a  great  distance  when  gently  tapped  by  the  finger.  The  cause  of  the  sensa- 
tion, then,  cannot  be  the  simple  forward  rush  without  tumefaction,  described 
by  Dr.  Young  and  Dr.  Parry. 

In  whatever  proportions  these  particulars  combine  to  form  the  pulse,  its 
force  will  be  proportioned  to  the  size  of  the  artery.  Hence  as  an  artery  lead- 
ing to  an  inflamed  part  becomes  of  greater  calibre,  its  pulse  also  becomes 
stronger. 

It  is  a  remark  respecting  the  pulse,  appearing  to  the  author  worthy  of  deep 
consideration,  that  if  the  purpose  of  the  heart  and  arteries  were  merely  the 
propulsion  and  conveyance  of  the  blood,  their  structure  and  action  would 
form  most  signal  deviations  from  the  ascertained  rules  of  fitness  in  mechanics. 
In  machines  of  human  contrivance,  it  is  one  of  the  most  important  maxims 
uto  avoid  shocks,  or  jerking  motions  j"  and  in  former  parts  of  this  work,  we 
have  described  fly-wheels,  air-vessels,  springs,  &c.,  as  means  of  accomplishing 
this  object,  and  thereby  of  preventing  the  tearing  and  straining  of  parts  which 
would  else  happen.  In  the  human  body,  also  we  have  to  describe  the  ad- 
mirable elasticity  of  the  spine,  of  the  arch  of  the  foot,  of  the  cartilages  of  joints, 


438  FLUIDITY    IN    RELATION    TO    ANIMALS. 

&c.,  as  contrivances  answering  the  same  end ;  and  to  remark  that,  in  other 
cavities  than  the  heart,  which  are  alternately  filled  and  emptied  like  it,  as  the 
stomach,  bladder,  uterus,  &c.,  there  is  a  smooth  and  gradual  action.  The 
heart  alone  is  the  rugged  anomaly,  which,  from  before  birth  unto  the  dying 
moment,  throbs  unceasingly,  and  sends  the  bounding  pulse  of  life  to  every 
part;  and  which,  moreover,  instead  of  being  secured  and  tied  down  to  its 
place,  is  attached  at  the  extremity  of  the  aorta,  like  a  weight  at  the  end  of 
an  elastic  branch  of  a  tree,  and  every  time  that  it  fills  the  aorta,  is  thrown 
with  violence,  by  the  consequent  sudden  tendency  of  that  vessel  to  become 
straighter,  against  the  ribs,  in  the  place  where  the  hand  applied,  feels  it  so 
distinctly  beating. 

Now  one  use  of  the  pulsation  of  the  heart  probably  is,  by  the  agitation  and 
churning  which  the  blood  suffers  in  passing  through  it,  to  keep  in  complete 
mixture  all  the  heterogeneous  parts  of  the  blood,  which  so  readily  separate 
when  left  to  repose: — but  this  cannot  be  the  only  use,  for  the  object  might 
have  been  more  simply  detained;  and  we  may  conclude  that  the  phenomenon 
has  relation  to  some  important  laws  of  life  still  hidden  from  us.  The  cause 
commonly  assigned  for  the  heart' scon  traction  is  the  peculiar  stimulus  of  the 
blood ;  yet  if  we  reflect  that  the  heart  will  beat  after  removal  from  the  body 
and  when  it  contains  only  air,  and  that  during  life  it  beats  with  extraordinary 
regularity,  whether  the  state  of  the  circulation  allow  it  to  empty  itself  at  each 
beat  or  not,  we  perceive  that  the  case  is  more  obscure.  We  cannot  con- 
template this  subject  attentively  without  perceiving  a  strong  analogy  between 
the  action  of  the  heart  and  some  electrical  phenomena  in  which  there  are 
successive  accumulations  and  exhaustion  of  power ;  and,  recollecting  the 
important  relations  which  late  researches  have  shown  to  exist  between  elec- 
tricity and  certain  other  actions  of  life,  the  inquiry  becomes  very  interesting. 
Galvanism  can  excite  the  muscles  to  their  usual  actions;  it  powerfully  affects 
the  secretions  and  the  digestive  function;  and  the  breathing  in  asthma;  strong 
animal  passion  seems  to  produce  electrical  excitement :  and  certain  animals 
have  the  faculty  of  stunning  their  enemies  by  an  electrical  discharge.  The 
pulse,  then,  in  its  sudden,  strong,  and  regular  recurrence,  may  be  a  kindred 
phenomenon.  In  this  view,  there  would  be  less  difficulty  in  supposing  a 
momentary  stiffening  or  slight  contraction  of  the  whole  arterial  system,  such 
as  the  sudden  rising  of  the  mesenteric  arterial  tree  so  readily  suggests  :  if 
there  be  such,  however,  it  is  still  closely  connected  with,  and  proportioned 
to,  the  action  of  the  heart;  for  it  occurs  only  with  that  action,  it  indicates 
any  disturbance  in  the  action,  and  as  death  approaches,  it  ceases  in  the 
remote  extremities  first. 

The  preceding  considerations  exhibit  the  pulse  as  a  complex  subject,  and 
one  on  which  professional  opinions  are  not  yet  settled.  By  showing  its  close 
relation  to  the  powers  of  life,  they  also  prove  it  to  be  an  object  of  high  im- 
portance to  the  medical  practitioner.  This  last  truth  has  scarcely  been  ques- 
tioned but  by  persons  either  utterly  uninformed  or  singularly  deficient  in  the 
power  of  tactile  discernment ;  yet,  because  no  simple  and  good  analysis  of 
the  pulse,  and  detail  of  its  relation  to  morbid  states,  has  been  made  and  pub- 
lished, the  degrees  of  skill  acquired  by  individual  practitioners  with  respect 
to  it  at'fi  very  various,  and  in  a  great  measure  accidental.  Some  practitioners 
try  the  pulse  merely  for  form's  sake,  because  patients  expect  it ;  many  ex- 
amine it  only  to  count  its  frequency;  but  others  read  in  it,  with  confidence, 
much  of  the  history  and  probabilities  of  the  disorder,  and  decide  on  the  treat- 
ment accordingly.  Few  who  have  attended  to  the  subject  at  all,  can  confound 
the  pulses  of  such  diseases,  as  acute  rheumatism,  gastric  inflammation,  the 


THE    CIRCULATION.  '439 

fits  of  ague,  &c.  The  author  remembers  to  have  conversed  with  a  Chinese 
practitioner  who  had  only  the  scanty  medical  information  of  his  countrymen, 
but  who  judged  by  the  pulse  with  a  singular  penetration. 

The  changing  circumstances  in  the  state  of  the  circulatory  system,  con- 
nected with  health  and  disease,  and  discoverable  by  a  finger  watching  the 
pulse,  seem  to  be  chiefly  the  following ;  and  the  epithets  added  in  italics, 
are  those  which  seem  best  to  indicate  the  sensations  perceived.  The  artery 
at  the  wrist  is  that  generally  chosen  for  examination,  because  it  is  not  like 
others  imbedded  in  soft  parts,  having  only  the  skin  over  it,  and  nothing  be- 
tween it  and  the  bone  below. 

1st.  The  number  of  the  contractions  of  the  heart  in  a  given  time,  and 
the  regularity  of  their  recurrence. — Pulse,  frequent,  slow,  intermittent,  equal, 
regular,  of  varying  force. 

2d.  The  degree  of  the  heart's  contraction,  or  the  quantity  of  blood  ejected 
.at  each  time ;  and  the  corresponding  state  of  the  capillaries  as  to  the  quan- 
tity of  blood  passing  through  them. — Pulse,  full,  long,  labouring,  bound- 
ing, feeble.. 

3d.  The  force  of  the  heart's  action,  with  the  correspondent  arterial  tension 
or  rigidity. — Pulse,  hard,  sharp,  strong,  wiry,  weak,  soft,  yielding. 

4th.  The  suddenness  of  the  individual  contractions  of  the  heart,  and  the 
rigidity  of  the  vessels  in  conveying  the  shock. — Pulse,  quick,  tardy. 

5th.  The  size  of  the  artery  for  the  time,  whether  larger  or  smaller  than 
usual. — Pulse,  large,  small. 

Superficial  as  is  this  sketch,  it  may  show  that  a  good  treatise  on  the  sub- 
ject of  the  pulse,  as  connected  with  disease,  is  yet  a  desideratum  in  medi- 
cine. The  sort  of  empirical  but  useful  tact  which  many  persons  acquire, 
is  not  fitted  to  satisfy  the  physician  who  reasons  deeply,  and  whose  mind 
should  have  always  present  to  it  the  various  constituents  of  the  pulse,  and 
all  the  important  circumstances  of  health  or  disease  relategl  to  its  indications. 
The  laboured  treatise  of  Solano,  Bordue,  Boerhaave,  &c.,  may  treat  of 
what  were  clear  ideas  to  their  authors,  but  by  not  referring  the  physical 
causes  of  many  varieties,  they  become  so  obscure  to  others,  that  many  of 
the  divisions  and  denominations  appear  altogether  fanciful.  Dr.  Young's 
excellent  paper  in  the  Phiosophical  Transactions,  details  important  facts,  as 
far  as  it  goes,  but  it  was  not  intended  to  point  out  all  the  pathological  rela- 
tions. Dr.  Y.,  guided  by  general  principle,  asserted  a  progressive  motion 
of  the  pulse,  while  other  authorities  were  holding  it  to  be  quite  simultaneous 
over  the  whole  system.  He  might  have  mentioned  in  proof,  that  careful 
examination  can  practically  detect  a  succession  of  beats  at  different  distances, 
particularly  at  the  four  stations;  1st,  of  the  heart;  2d,  in  the  lip;  3d,  at 
the  wrist;  4th,  at  the  ankle: — but  the  interval  of  time,  even  between  the 
extremes,  being  only  a  small  part  of  a  second,  persons  will  often  fail  -to 
make  their  first  experiment  satisfactorily.  Dr.  Parry's  treatise  on  the  pulse 
which  is  the  last  one  of  note,  although  having  excellencies,  errs — in  attri- 
buting the  phenomenon  to  one  cause  so  exclusively — in  denying  arterial 
dilatation,  because  it  was  not  discovered  by  his  mode  of  searching  for  it,  in 
supposing  that  a  liquid  column  in  an  elastic  tube,  can  be  made  to  advance 
like  a  solid  rod,  or  line  of  billiard-balls.  The  too  common  neglect  of  me- 
chanical philosophy  by  medical,  men  is  signally  proved,  by  our  finding  in 
works  of  authority,  published  at  the  present  day,  such  statements  as  that  the 
arterial  pulse  may  be  more  frequent  or  less  frequent  than  the  beatings  of  the 
heart.  Dr.  Good  (Study  of  Medicine'}  says,  that  there  may  be  various  fre- 
quency of  pulse  in  various  parts  of  the  body  at  the  same  time :  Richerand 


440   "        FLUIDITY    IN    RELATION    TO    ANIMALS. 

(Physiologic)  says,  the  pulse  is  more  frequent  in  the  artery  leading  to  a 
\vhitlow  than  at  the  same  time  elsewhere;  and  many  practitioners  share 
these  notions.  What  a  satire  on  the  medical  profession  is  this  disagreement 
on  a  point  which,  to  common  observers,  seems,  obove  all  others,  to  occupy 
the  attention  of  the  attendant  on  the  sick  ! 

Having  now  explained  the  circulation  of  the  blood  in  general,  we  proceed  to 
consider  some  cases  where  mechanical  circumstances  modify  it. 

Circulation  in  the  head. 

The  head  may  be  considered  as  an  air-tight  vessel  or  cavity  of  bone,  con- 
taining chiefly  brain  arid  blood,  and  having  openings  occupied  by  blood- 
vessels, leading  to  and  from  the  heart.  The  atmospheric  pressure,  there- 
fore, always  keeps  the  head  full,  as  it  keeps  the  top  of  a  syphon  full ;  and 
because  the  substance  of  the  brain  itself  does  not  more  than  water,  sensibly 
change  in  bulk  by  any  ordinary  degrees  of  pressure,  there  must  always  be 
the  same  quantity  of  blood  in  the  head,  how  much  soever  the  quantity  may 
vary  in  the  body  generally.  Regard  to  this  important  truth,  a  knowledge  of 
which  has  followed  the  discovery  of  the  true  nature  of  atmospheric  pressure, 
enables  us  to  explain  many  hitherto  obscure  facts,  both  in  health  and  disease ; 
— as  the  following  instances  will  show. 

If,  from  any  cause,  the  arteries  in  the  he-ad  become  too  full  of  blood,  in 
the  same  proportion  the  veins  must  become  too  empty  •  or,  if  the  veins 
become  too  full,  the  arteries  must  be  too  empty ;  and  in  either  case,  the  cir- 
culation in  the  head  will  be  in  a  corresponding  degree  impeded,  because, 
when  one  part  of  a  channel  is  narrowed  or  diminished,  the  current  through- 
out the  whole  is  slackened.  Now,  as  insensibility  supervenes  when  the 
supply  of  fresh  blood  to  the  brain  is  interrupted,  and  death  follows  if  the 
interruption  continue  long,  it  seems  evident  that  in  many  of  the  cases  of 
apoplexy,  where,  on  inspection,  there  is  found  nothing  but  a  fulness  of  the 
arterial  or  of  the  venous  system  of  the  head,  death  has  happened  merely 
because  the  circulation  was  arrested  in  this  way.  In  other  parts  of  the 
body,  not  circumstanced  like  the  brain,  an  excess  of  blood  in  one  set  of 
vessels  may  happen  without  inducing  deficiency  in  another,  and  therefore 
with  perfect  impunity  to  the  individual. 

Simple  increase  of  pressure  produced  by  the  blood  on  the  brain,  provided 
the  proper  balance  exist  between  the  quantity  in  veins  and  arteries,  has  no 
injurious  effect.  This  is  proved  by  the  safe  descent  of  a  person  in  a  diving- 
bell,  where,  at  thirty-four  feet  under  the  surface  of  the  water,  the  body  is 
bearing  an  additional  pressure  of  fifteen  pounds  on  a  square  inch  (see  page 
163,)  which  pressure  through  the  blood-vessels  affects  the  brain  as  much  as 
any  other  part.  On  the  other  hand,  when  a  man  climbs  a  mountain,  or  is 
lifted  up  in  a  balloon,  the  brain  is  less  pressed  than  usual ;  but  the  proper 
balance  in  artery  and  vein  being  maintained,  no  inconvenience  is  felt.  The 
inhabitants  of  some  of  the  valleys  among  the  Andes  are  as  far  above  the  sea 
as  they  would  be  at  the  top  of  Mont  Blanc,  where  the  atmosphere  presses 
only  half  as  much  as  on  the  sea-shore;  but  they  enjoy  good  health. 

As  the  box  of  the  cranium  encloses  the  brain  so  as  to  leave  no  vacant 
space,  it  is  evident,  that  when  the  heart  injects  blood  with  unusual  violence, 
the  strain  at  first  is  borne  chiefly  by  the  cranium,  and  not  by  the  coats  of 
the  blood-vessels.  Hence,  the  arteries  of  the  brain  need  not  to  be,  and  are 
not,  nearly  so  strong  as  those  of  other  parts  of  the  body. 


THE    CIRCULATION.  441 

The  veins  of  the  brain  are  also  peculiar.  Common  veins  in  the  head 
would,  for  the  reasons  above  given,  collapse  by  any  sudden  tension  of  the 
arteries  there,  and  if  they  did,  insensibility  or  death  would  ensue,  on  account 
of  the  consequent  stoppage  of  the  circulation.  The  chief  channels,  there- 
fore, for  the  refluent  blood,  instead  of  being  common  compressilile  veins,  are 
what  have  been  called  sinuses,  or  grooves  in  the  bone  itself,  with  -exceed- 
ingly strong  membranous  coverings,  supported  so  that  the  channels  become 
in  strength,  and  as  to  maintenance  of  their  capacity,  a  little  inferior  to  com- 
plete channels  of  bone.  This  singular  deviation  in  the  structure  of  the 
cerebral  veins  from  what  is  found  elsewhere,  and  without  which  deviation, 
animal  existence  could  not  be  continued,  is  one  of  those  particulars  which 
powerfully  affect  the  contemplative  mind,  as  proofs  of  the  designing  intelli- 
gence which  has  planned  this  glorious  universe. 

From  not  adverting  sufficiently  to  the  fact  now  explained,  of  the  cranium 
being  a  vessel  always  full,  and  which  will  hold  only  a  certain  quantity, 
misconception  has  prevailed  among  medical  men  with  respect  to  many  of 
the  affections  of  the  brain. 

It  has  been  said^for  instance,  that  the  substance  of  the  brain  cannot  bear 
pressure  with  impunity,  for  that  stupor  immediately  follows  pressure,  how- 
ever produced.  Now  the  truth  is,  that  pressure  produces  stupor  only  when 
it  interferes  with  the  circulation.  In  wounds  with  loss  of  a  large  piece  of 
the  cranium,  the  brain  will  bear  very  rough  handling,  because  if  compressed 
at  one  part,  it  may  bulge  at  another,  and  leave  the  circulation  free ;  but  if  the 
wound  be  small,  pressure  made  through  it  instantly  affects  the  whole  brain, 
and  the  blood  is  prevented  from  entering  from  the  heart.  Let  one  reflect,  for 
an  instant,  on  what  happens  to  the  foetal  head  during  parturition — how  often 
it  escapes  elongated  and  bent,  almost  as  if  it  were  of  soft  clay — yet  the  child 
lives  and  thrives,  and  the  natural  form  is  soon  recovered.  The  reason  is, 
that  the  fretal  skull  is  soft,  and  pressure  in  one  part  is  compensated  for  by 
a  bulging  or  extension  in  another,  and  the  blood  is  not  expelled. 

Water  in  the  head,  again,  is  said  to  kill  by  this  fatal  pressure  on  the  tender 
brain ;  but,  in  reality,  it  kills  by  keeping  out  of  the  blood,  and  so  mechanically 
arresting  the  circulation.  Accordingly  we  see,  that  where  the  fontanelle  still 
remains  open,  or  where  the  sutures  or  joinings  of  the  skull  will  yield,  water 
may  accumulate  to  a  great  degree  without  causing  much  disturbance. 

A  tumour  in  the  brain,  which  would  be  of  no  consequence  if  the  brain 
were  unconfined,  soon  becomes  fatal  by  occupying  room  in  the  skull,  and  to 
the  extent  of  its  size  excluding  or  checking  the  supply  of  blood. 

If  the  substance  of  the  brain  at  all  increase  and  diminish  in  bulk,  as  muscles, 
&c.,  under  certain  circumstances,  do,  in  the  body  below,  all  such  changes 
must  produce  a  considerable  effect  on  the  cerebral  circulation  and  functions. 

Effects  of  position  on  the  circulation. 

While  a  man  is  in  a  standing  attitude,  the  heart  and  arteries  have  to 
send  the  blood  up  the  head  against  gravity;  but  in  the  horizontal  position, 
the  blood,  if  equally  propelled,  must  arrive  with  greater  force,  because  gravity 
then  does  not  resist.  Hence  headache,  no  other  symptom  arising  from  ful- 
ness of  blood  in  the  arteries  of  the  head,  is  often  relieved  by  the  upright 
position,  and  is  increased  by  lying  down. 

Many  people  who  have  had  a  slight  degree  of  toothache  during  the  day, 
find  it  intolerable  when  they  lie  down  at  night,  and  are  relieved  again  by 
rising  and  walking  about.  Commonly  they  suppose  that  it  is  the  cold  of  the 


442  FLUIDITY    IN    RELATION    TO    ANIMALS. 

night  which  then  lulls  the  pain ;  but  it  is  in  fact  the  change  of  position. 
The  authoKknew  a  lady  who  was  obliged  to  sleep  for  months  in  the  sitting 
posture,  because  she  had  a  tic  doloureux  in  the  face  whenever  she  lay  down; 
and  another  who  was  under  the  same  necessity  for  a  considerable  period  after 
an  inflammatory  affection  of  the  brain,  because  if  her  head  fell  low  during 
sleep  she  was  immediately  assailed  by  a  terrific  dream  of  swords  driven  into 
the  brain. 

Delirium  in  fever  is  sometimes  checked  at  once  by  elevating  the  head. 
On  account  of  the  great  relief  thus  obtained,  some  continental  practitioners 
had  proposed  to  support  the  patients  occasionally  in  an  upright  posture. 

Apoplexy  has  often  been  brought  on  by  a  man  bending  his  head  down  in 
the  act  of  tying  his  shoe,  or  of  pulling  on  his  boot. 

Children  and  professed  tumblers  being  much  in  the  habit  of  placing  their 
bodies  in  all  positions,  feel  no  inconvenience  from  having  the  head  down- 
wards ;  apparently,  because  arteries  and  veins  usually  become  strong  enough 
to  bear  the  pressure  to  which  they  are  habitually  exposed;  but  to  many  old 
people,  accustomed  to  keep  the  head  always  up,  the  attempt  would  be  fatal. 

Ulcers  on  the  legs  are  often  obstinate  and  will  bleed,  because  the  veins 
about  them  are  too  weak  to  support  the  lofty  columns  of  blood  above.  Hence 
the  frequent  counsel  given  in  such  cases  to  keep  the  feet  raised  upon  a 
chair,  and  the  utility  of  certain  modes  of  bandaging. 

Many  inflammations  of  the  legs  and  feet  become  exceedingly  painful  when 
the  limbs  are  in  a  hanging  position,  and  the  pain  is  relieved  by  laying  them 
horizontally. 

Many  anasarcous  or  dropsical  affections  of  the  legs  increase  towards  night, 
because,  during  the  dependent  position  of  the  legs  through  the  day,  the 
absorbents  have  not  power  to  lift  the  fluid.  The  swelling  disappears  again 
before  morning. 

When  the  heart  has  to  send  blood  upwards,  it  requires  to  act  more  strongly 
than  when  the  body  is  horizontal,  and  the  pulse  increases  five  or  six  beats 
in  the  minute;  hence  the  common  rule  to  make  a  patient  with  haemorrhage 
lie  in  the  horizontal  position,  that  the  heart  may  become  tranquil  and  allow 
the  bleeding  to  cease. 

Fainting  from  diminished  arterial  tension. 

Painting,  which  is  a  temporary  cessation  of  the  action  of  the  heart,  and 
hence,  as  explained  above,  of  the  action  of  the  brain  for  want  of  blood,  is 
produced  by  several  causes,  and  among  others,  by  any  occurrence  which 
renders  the  blood-vessels  about  the  heart  suddenly  less  full  of  tense  than 
usual.  It  would  appear  that  the  heart  being  accustomed,  when  it  contracts, 
to  a  certain  degree  of  resistance,  has  its  action  disturbed  when  the  resistance 
is  much  diminished. 

Thus  haemorrhage,  from  any  cause,  by  lessening  the  general  tension  of  the 
sanguiferous  system,  often  causes  fainting.  The  state  is  relieved  by  lying 
down ;  probably  because  the  still  remaining  weaker  action  of  the  heart  is 
sufficient  to  send  blood  to  the  head  along  a  horizontal  course,  until  the  gradual 
contraction  of  the  whole  vascular  system  reproduces  the  tension  necessary  to 
perfect  action.  A  small  quantity  of  blood  taken  away  suddenly,  affects  the 
circulation  as  much  as  a  larger  quantity  taken  gradually,  apparently  because 
a  certain  space  of  time  i?  required  for  the  gradual  lessening  of  the  vessels. 

The  operation  of  tapping  for  dropsy  in  the  abdomen  would  often  bring  on 
fainting,  but  for  the  precaution  of  tightening  a  broad  bandage  upon  the  body 


THE    CIRCULATION.  443 

as  the  water  flows.  The  reason  is,  that  the  sudden  removal  of  a  large 
quantity  of  fluid  which  had  been  compressing  all  the  abdominal  vessels,  and 
keeping  them  perhaps  only  half  full  of  blood,  allows  them  again  suddenly 
to  receive  their  natural  quantity,  arid  thus  produces  a  relaxation  of  the  other 
parts  of  the  vascular  system. 

Sudden  parturition  often  causes  faintness  for  the  same  reasons. 

Even  rising  up  suddenly  from  a  horizontal  position  will  cause  an  approach 
to  fainting  in  weak  people,  or  in  those  who  have  been  long  bed-ridden  :  pro- 
bably because  the  heart  having  for  a  time  been  accustomed  to  send  blood 
only  in  a  horizontal  direction  to  the  head,  does  not  at  an  instant  exert  the 
additional  power  required  to  lift  an  upright  column  with  equal  force  ;— 
besides,  that  the  blood  does  not  then  return  to  the  heart  by  the  veins,  from 
the  inferior  parts  of  the  body  so  readily  as  before. 

These  various  facts,  now  easily  understood,  form  the  reason  of  a  rule 
which  is  a  great  modern  improvement  in  the  practice  of  the  healing  art,  viz., 
in  bleeding  for  the  cure  of  inflammation,  to  take  the  blood  away  as  quickly 
as  possible.  This  subject  deserves  a  little  farther  consideration. 

A  great  proportion  of  dangerous  diseases  involve  inflammation  of  some 
vital  organ  ;  an  inflammation  consists  chiefly,  as  already  stated  at  page  422, 
of  a  gorging  or  over-distension  of  the  capillary  vessels  in  the  part.  The 
nature  of  the  capillaries,  again,  is  such, (page  422)  that  when  not  maintained 
constantly  full  by  the  pressure  of  the  heart  behind  them,  they  gradually  by 
their  own  action,  empty  themselves  towards  the  veins — as  is  seen  in  the 
disappearance  of  a  local  inflammation  soon  after  the  death  of  the  person,  or 
in  the  fact  of  the  arteries  being  emptied  of  blood  after  breathing  ceases,  &c. 
Now  ever  since  medicine  deserved  the  name  of  an  art,  practitioners  have 
accounted  the  lancet  their  sheet-anchor  in  inflammatory  disease  ;  but  it  is 
only  in  late  times,  since  the  circulation  of  the  blood  was  understood,  that 
they  have  known  the  rationale  of  the  remedy,  viz.,  that  it  acts  by  diminishing 
vascular  tension,  and  hence  the  action  of  the  heart,  and  so  allowing  the  small 
vessels  to  empty  themselves  by  their  own  force,  and  to  recover  sufficiently 
to  resist  the  return  of  an  excessive  load.  It  is  still  more  lately  that  they 
have  understood  how  much  more  suddenly  and  completely  the  disease  is 
cured  by  abstraction  of  a  small  quantity  of  blood  so  rapidly  as  to  produce 
fainting,  than  of  a  much  larger  quantity  so  slowly  that  only  weakness  fol- 
lows. Judicious  treatment  now  cures  inflammation  much  more  certainly 
and  completely  than  was  done  formerly,  yet  with  much  smaller  loss  of  the 
precious  blood,  and  with  less  danger  of  those  diseases  of  weakness,  or  of 
that  complete  breaking  up  of  the  constitution,  which  often  follow  great 
depletion.  To  induce  faintness,  large  openings  are  to  be  made  into  the 
veins — sometimes  into  two  veins  at  once,  and  the  patient  is  kept  in  the 
upright  attitude,  Often  thus  an  inflamed  eye,  which  was  as  red  as  scarlet 
before  bleeding,  in  a  few  minutes  is  rendered  nearly  of  the  natural  appear- 
ance ;  and  intense  internal  inflammations,  as  of  the  brain,  lungs,  bowels, 
&c.,  which,  if  neglected,  would  be  shortly  fatal,  are  removed  in  the  same 
manner.  In  all  these  cases,  the  faintness  seems  to  be  almost  equally  effica- 
cious, whether  it  happens  after  the  loss  of  ten  ounces  of  blood,  or  of  fifty; 
or  even,  as  sometimes  occurs,  when  it  happens  without  bleeding  at  all,  after 
merely  tying  the  arm  in  preparation. 

Reflection  upon  these  circumstances  led  the  author  to  think  that,  in  certain 
cases,  the  beneficial  effects  of  blood-letting  might  be  attainable  by  the  simple 
means  of  extensive  dry  cupping,  alluded  to  at  page  176  ;  that  is  to  say,  by 
diminishing  the  atmospherical  pressure  on  a  considerable  part  of  the  body, 


444  FLUIDITY    IN    RELATION    TO    ANIMALS. 

on  the  principle  of  the  cupping-glass  used  very  gently,  and  thus  suddenly 
removing  for  a  time  from  about  the  heart,  a  quantity  of  blood,  sufficient  by 
its  absence  to  produce  faintness.  The  results  of  trial  have  been  such  as  to 
give  great  interest  to  the  inquiry,  and  the  author's  leisure  will  be  devoted 
to  the  prosecution  of  it. — An  air-tight  case  of  copper  or  tin-plate,  or  of  air- 
tight cloth  kept  extended  by  hoops,  being  put  upon  a  limb,  and  made  close 
by  a  suitable  collar  tied  at  the  same  time  round  its  mouth  and  the  limb, — 
on  part  of  the  air  being  then  extracted  by  a  suitable  syringe,  in  an  instant 
the  vessels  all  over  the  limb  become  gently  distended  with  blood;  and  as  the 
blood  is  suddenly  taken  from  the  centre  of  the  body,  faintness  is  produced, 
just  as  by  bleeding  from  a  vein.  The  excess  of  blood  may  be  retained  in 
the  limb  as  long  as  desired  ;  for  the  circulation  is  not  impeded.  To  produce 
a  powerful  effect  with  a  slight  diminution  of  pressure,  more  than  one  limb 
must  be  operated  upon  at  the  same  time. 

An  instrument  resembling  the  contrivance  now  described,  was  proposed 
about  twenty  years  ago  by  a  non-professional  person,  as  a  means  of  drawing 
all  sorts  of  diseases  out  of  the  body  through  the  pores  of  the  skin.  He 
enclosed  a  leg  in  an  air-tight  case ;  he  then  admitted  steam  to  heat  the  limb, 
and  relax  the  pores  of  the  skin,  as  he  said,  and  then  he  worked  an  air-pump 
to  draw  out  the  disease.  He  called  the  engine  the  air-pump  vapour  bath. 
In  various  cases  where  its  true  action  was  desirable,  although  not  understood 
by  the  proposer,  nor  judiciously  managed,  it  proved  beneficial. 

The  operation  of  applying  tourniquets  or  bandages  round  the  limbs,  so  as 
to  influence  the  transmission  of  the  blood,  affects  the  action  of  the  heart.  It 
is  said  sometimes  to  have  prevented  the  accession  of  the  ague.  It  is  a  means 
akin  to  those  above  described. 

Because  arteries  are  stronger  and  more  tense  than  veins,  a  bandage  may 
be  put  round  a  limb  tight  enough  to  close  the  veins  but  not  the  arteries,  and 
the  limb  will  then  swell  beyond  the  ligature.  By  thus  putting  tight  elastic 
bandages  round  all  the  limbs  at  once,  and  immersing  them  in  warm  water  to 
favour  the  dilatation  of  their  vessels,  so  much  blood  may  be  suddenly  detained 
in  them  as  to  cause  the  person  to  faint.  Such  a  means,  therefore,  might 
also  be  used  remedially. 

In  the  same  way,  a  tight  handkerchief,  or  stock  round  the  neck,  will  often 
retain  the  venous  blood  in  the  head,  and  cause  apoplexy. — Strong  pressure 
made  on  a  jugular  vein  kills  as  certainly  as  if  made  on  the  windpipe. 

When  a  hernia,  or  other  tumor,  is  strangulated,  it  swells  in  the  manner 
above  described,  and  if  not  relieved,  soon  mortifies. 

Diffused  pressure,  like  that  made  by  rolling  a  bandage  round  a  whole  limb, 
or  by  immersing  the  limb  in  fluid,  must  affect  the  circulation.  The  veins 
will  be  more  compressed  than  the  arteries,  by  reason  of  the  distending  force 
in  them  being  less.  Varicose  veins,  therefore,  are  usefully  supported  by  a 
bandage  or  laced  stocking.  The  reason  why  this  manner  of  supporting 
assists  so  powerfully  in  the  healing  of  ulcers  on  the  legs,  may  be,  that  the 
support  affects  the  capillaries  and  absorbents,  as  well  as  the  larger  vessels. 

Poultices,  by  their  weight,  produce  a  soft  compression  of  the  parts  on 
which  they  are  applied  ;  and  in  certain  cases,  may  benefit  by  mechanically 
squeezing  the  excess  of  blood  out  of  the  weakened  vessels. 

The  author  has  relieved  the  chronic  inflammation  of  a  sprained  ankle,  by 
ordering  the  foot  and  leg,  covered  with  an  oiled-silk  stocking,  to  be  enclosed 
in  a  boot  strong  enough  to  support  the  pressure  of  quicksilver,  which  was 
then  poured  into  the  boot.  The  effect  is  a  pressure  by  the  fluid  metal  on 
the  weak  vessels,  of  one  pound  to  the  square  inch,  for  every  two  inches  of  the 


RESPIRATION    AND    VOICE.  445 

depth  of  metal  above  the  part. — A  height  of  four  or  five  inches  gives  the 
relief  expected.  A  much  greater  elevation  would  stop  the  circulation  alto- 
gether. No  bandage  can  press  with  uniformity  approaching  to  this  action 
of  a  fluid. 

The  effect  of  continued  pressure,  in  removing  tumours  of  various  kinds, 
is  explicable  on  the  same  principle.  The  author  doubts  not  that  in  such 
cases,  pressure  properly  managed,  would  prove  a  more  valuable  remedy  than 
is  at  present  generally  supposed.  The  elastic  steel  half-hoop  with  one 
cushion  before  and  another  behind,  lately  introduced  for  the  relief  of  hernia, 
affords  an  admirable  mode,  in  certain  cases,  of  producing  a  uniform  pressure 
of  the  nature  spoken  of,  and  of  any  desired  force. 

When  a  man  stands  in  a  bath,  with  the  water  up  to  his  chin,  there  is  a 
pressure  of  the  water  upon  his  body,  proportioned  everywhere  to  the  depth, 
(see  page  130.)  This  pressure  must  produce  a  considerable  effect  on  the 
blood-vessels  of  the  lower  parts  of  the  body.  We  see  in  this  that  a  bath 
must  propel  the  blood  from  all  other  parts  of  the  body,  towards  the  cavity 
of  the  chest,  which  the  pressure  cannot  reach.  It  is  this  effect  which  in 
part  causes  the  feeling  of  thoracic  oppression  experienced  by  persons  on 
first  plunging  into  water,  which  feeling  is  usually  attributed  altogether  to 
the  cold. 

The  old  practice  of  placing  a  patient  in  a  pit,  and  surrounding  his  body 
with  earth  and  sand,  must  have  had  a  mechanical  action  of  the  kind  now 
contemplated,  in  addition  to  any  other  influence. 

Transfusion  of  blood  from  a  vein  of  a  healthy  person  into  that  of  one 
fainting  or  dying  from  haemorrhage,  is  an  operation  the  converse  of  some  of 
those  mentioned  above.  It  has  been  frequently  performed  with  success. 
The  cases  to  which  it  seems  best  fitted,  are  those  of  flooding  after  parturition, 
and  of  wound ;  and  there  can  be  no  doubt  that  many  of  the  lives  lost  from 
these  so  frequently  recurring  causes,  might  be  saved  by  its  adoption.  The 
blood  to  be  injected  is  received  into  a  vessel,  as  in  common  bleeding,  from 
which  vessel,  by  a  fit  syringe,  (to  be  described  in  a  future  page,)  it  is  trans- 
ferred, as  it  flows,  into  an  opened  vein  of  the  patient.  The  admission  of  air 
with  the  blood  would  be  fatal,  and  has  therefore  to  be  most  carefully  guarded 
against.  The  last  interesting  report  upon  this  subject  is  that  of  Dr.  Blundell, 
in  his  Physiological  Essays. 


RESPIRATION   AND   VOICE, 

Tlie  doctrines  of  fluidity ',  illustrating  and  illustrated  by  the  animal  reppi- 

ration  and  voice. 

As  the  motion  of  a  windmill  depends  altogether  on  the  breeze  to  which  its 
vanes  are  exposed,  so  does  the  motion  and  the  life  of  that  most  wonderful  of 
structures,  the  animal  body,  depend  on  the  supply  of  air  for  its  breathing. 
If  this  supply  be  withheld  but  for  a  few  moments,  painful  convulsions  ensue; 
and  if  for  a  still  longer  period,  the  body,  however  perfect  and  beautiful,  is 
made  a  lifeless  corpse,  soon  to  putrify  and  be  decomposed. 

The  mechanical  nature  of  air,  as  to  its  lightness,  elasticity,  &c.,  and  the 
fact  of  its  forming  an  ocean  around  the  earth  of  about  fifty  miles  high,  are 
now  well  understood,  and  have  been  fully  explained  under  pneumatics  ;  but 
the  precise  nature  of  its  life-sustaining  action  has  yet  to  be  elucidated  by 
farther  research  of  chemists  and  physiologists.  Thus  far,  however,  we 


446  FLUIDITY    IN    RELATION    TO    ANIMALS. 

know— that  the  ingredient  called  oxygen,  constituting  a  fifth  of  the  atmo- 
sphere, is  the  most  essential  part — that  air,  by  being  breathed  once  is  rendered 
unfit  for  farther  respiration  at  the  time — and  that  a  man  requires  about  a 
gallon  per  minute.  The  enterprising  Mr.  Spalding,  who  introduced  the  use 
of  the  diving-bell,  descended  for  the  last  time  with  a  companion  on  the  coast 
of  Ireland,  when  owing  to  the  signal  cord  becoming  entangled  round  the 
great  rope  of  the  bell,  which  had  turned  in  descending,  he  could  not  make 
their  want  of  air  known  above,  and  both  were  found  dead  when  the  bell  was 
drawn  up  soon  after,  although  the  water  had  not  touched  them.  Of  a  hun- 
dred and  forty-six  Englishmen,  who,  in  the  year,  1750,  were  made  prisoners 
at  Calcutta,  and  where  thrown  into  the  close  dungeon,  since  called  the  black- 
hole,  only  twenty-three  survived  the  few  hours  of  their  confinement,  and  one 
of  the  most  appalling  recitals  of  human  suffering  existing  on  record,  is  what 
these  persons  had  afterwards  to  make. 

We  know  generally  of  the  life-supporting  action  of  air,  that  it  consists  in 
some  change  operated  by  the  air  on  the  blood;  and  we  know  that  the  func- 
tion of  respiration  has  merely  to  bring  air  and  blood  together  in  the  cavity  of 
the  chest,  that  this  change  may  take  place.  The  blood  while  in  the  chest  is 
moving  along  a  part  of  its  circle,  in  vessels  of  extreme  minuteness  and  thin- 
ness, and  the  air  at  each  inspiration  rushes  in  among  these,  so  that  every 
globule  of  blood  passes  within  its  influence.  And  the  blood,  which,  after 
having  served  the  purposes  of  the  body,  arrives  at  this  part  of  its  course, 
black  and  impure,  immediately  after  its  exposure  to  the  air  enters  the  left 
chamber  of  the  heart,  of  a  beautiful  scarlet  colour,  and  thence  departs  to 
carry  new  life  to  the  general  system.  . 

The  minute  vessels  through  which  the  circulating  blood  is  strained  in  the 
chest,  do  not  hang  loose  in  the  cavity,  but  are  supported  by  running  through 
spungy  masses,  called  the  lungs,  which  consist  chiefly  of  these  vessels  and 
of  thin  membrane  formed  into  cells.  The  cells  at  every  inspiration,  receive 
fresh  air  through  the  cartilaginous  windpipe  which  branches  into  them,  and 
at  every  inspiration  they  return  the  changed  air  by  the  same  channels  to  the 
atmosphere.  The  lungs  of  a  child,  before  birth,  are  perfectly  collapsed,  or 
without  the  least  air  in  their  structure,  and  hence  are  dense  enough  to  sink 
in  water ;  but  after  breathing,  they  retain  a  portion  of  air,  and  will  float. 
This  fact  has  been  accounted  a  test  of  whether  a  child  had  been  born  dead 
or  alive;  but  because  putrefaction,  &c.,  will  cause  air  to  be  in  lungs  which 
have  never  breathed,  the  criterion  may  be  fallacious. 

The  chest  is  a  large  cavity,  of  form  approaching  to  that  of  a  common  bee- 
hive, bounded  laterally  by  the  encircling  ribs,  behind  the  spine,  and  before 
by  the  sternum,  and  divided  below,  from  the  abdomen  or  belly,  by  a  strong 
membranous  and  muscular  expansion,  called  the  diaphragm.  The  ribs,  in 
the  natural  state,  hang  obliquely  downwards  from  their  posterior  attachments, 
and  on  being  raised  in  front,  they  widen  or  increase  the  size  of  the  cavity,  as 
already  explained  at  p.  403.  The  cavity  is  farther  enlarged  by  the  descent  of 
the  diaphragm,  which  may  be  regarded  as  both  the  floor  of  the  chest  and  the 
roof  of  the  abdomen,  and  which  being  convex  upwards  like  a  dome,  by  con- 
tracting itself  to  a  more  flat  condition,  sinks  out  of,  and  enlarges  the  chest, 
while  it  descends  into,  and  diminishes  the  abdomen,  or  at  least  causes 
protrusion  of  its  sides. 

Now  on  the  chest  being  enlarged  by  the  rising  of  the  ribs  and  descent  of 
the  diaphragm,  or  by  either  singly,  the  air  rushes  into  it  through  the  mouth 
and  windpipe,  exactly  as  air  rushes  into  a  common  bellows  through  its  pipe, 
when  the  valve  is  shut  and  the  two  boards  are  drawn  apart;  and  air  is  again 


•RESPIRATION    AND    VOICE.,  447 

expelled  from  the  lungs  by  the  contraction  of  the  chest,  as  from  the  bellows 
by  the  approximation  of  the  boards.  Into  both  cavities  air  enter,  because 
with  the  enlarging  dimensions,  the  air  which  was  within  dilates,  and 
becomes  less  powerfully  tense  or  resisting  against  the  external  pressure  of 
the  atmosphere,  and  so  allows  more  air  to  rush  in  to  restore  the  equilibrium. 
The  air  is  expelled  again  by  the  contraction  of  the  cavities,  because  by  being 
compressed,  its  elastic  force  or  tension  becomes  greater  than  that  of  the  ex- 
ternal air,  which  it  therefore  easily  repels,  and  so  in  part  escapes.  By  im- 
mersing in  water  an  india  rubber  bottle,  and  then  opening  and  shutting  it, 
the  entrance  and  exit  of  fluid  in  this  manner  may  be  rendered  very  apparent. 

That  the  air  admitted  into  the  chest  should  have  the  fullest  action  on  the 
blood  passing  there,  it  was  necessary  that  the  spongy  mass  of  lungs  in  which 
the  blood-vessels  ramify,  should  occupy  the  whole  of  the  cavity,  and  be 
equally  distributed.  Now  while  the  equable  distribution  is  effected  by  the 
uniform  elasticity  or  resilience  which  belongs  to  the  structure  of  the  lung  the 
complete  filling  of  the  cavity  is  obtained,  not  by  general  attachment  between 
the  lungs  and  the  ribs  or  sides  of  the  chest,  as  might  be  expected,  but  by  the 
following  means,  equally  simple,  and  still  more  perfect.  The  spongy  mass 
of  the  lungs,  is  completely  covered  by  a  strong  adherent  membrane,  called  the 
pleura,  through  which  air  cannot  pass  ;  between  this  membrane  and  a  similar 
lining  of  the  chest  there  is  no  air  or  empty  space,  and  therefore  in  the  raising 
and  falling  of  the  ribs  during  respiration,  this  membrane  remains  always  in 
contact  with  the  lining  of  the  ribs,  just  as  a  bladder  put  into  a  bellows  as  a 
lining,  with  its  mouth  secured  around  the  nozzel,  is  filled  and  emptied,  and 
remains  in  contact  with  the  interior  of  the  bellows,  in  all  the  states  of  dilata- 
tion, as  if  there  were  attachments  in  a  thousand  places.  This  construction 
allows  the  lungs  to  have  a  singular  freedom  of  play  during  all  the  motions 
of  the  body  ;  a  freedom  further  provided  for  by  their  being  divided  into  five 
portions  or  lobes,  which  slide  upon  one  another :  of  these,  three  occupy  the 
right  side  of  the  chest,  and  two  with  the  heart  occupy  the  left.  The  right 
and  left  sides  of  the  chest  are  rendered  cavities  quite  distinct  from  each  other 
by  the  mediastinum,  a  strong  membranous  partition.  The  mechanical  dis- 
position of  the  contents  of  the  chest,  as  now  described,  is  productive  «of 
certain  consequences  which  it  is  important  to  understand; — for  instance, 

If  a  wound  be  made  in  one  side  of  the  chest  so  as  to  admit  air,  the  lungs  of 
that  side  collapse  in  obedience  to  their  weight  and  elasticity ;  and  as  the  chest 
afterwards  enlarges  and  diminishes  in  respiration,  air  more  easily  enters  and 
leaves  the  space  around  the  collapsed  lung,  through  the  wound,  than  it  can 
enter  or  leave  the  lung  itself  through  the  windpipe  \  because,  in  the  first  case, 
it  has  no  force  to  overcome,  and  in  the  second,  the  elasticity,  weight  and 
inertia  of  the  lung  oppose  Thus  the  lungs  of  the  wounded  side  become  col- 
lapsed and  useless.  If  such  a  wound,  therefore,  were  made  in  both  sides  of 
the  chest  at  once,  even  without  hurting  any  part  within,  the  person,  unless 
assisted,  would  die  of  suffocation.  The  kind  of  resistance  required  in  such  a 
case,  is  first  to  press  the  ribs  down  so  as  to  empty  the  chest  of  air  as  much 
as  possible,  and  then  to  keep  the  wounds  close  or  covered  while  the  ribs  rise 
again  ;  the  air,  of  course,  will  then  enter  by  the  natural  road,  the  only  one  left, 
to  fill  the  chest,  and  will  distend  the  lung,  and  reach  the  blood  in  the  pulmo- 
nary vessels  as  usual.  Then  by  straining  with  the  muscles  or  the  chest,  as 
in  the  action  of  blowing,  and  at  the  same  time  preventing  the  breath  from 
escaping  by  the  mouth  or  nose,  all  the  air  which  had  entered  by  any  wound 
in  the  chest  may  be  expelled.  In  Benjamin  Bell's  system  of  surgery,  which 
was  long  the  manual  practitioners,  counsel  on  this  head  was  given  the  very 


448  FLUIP1TY    IN    RELATION    TO     ANIMALS. 

contrary  of  that  required,  and,  of  course,  any  patient  treated  according  to  it 
must  have  been  lost. 

In  cases  of  dangerous  haemorrhage  from  a  lung,  caused  by  a  wound  in  the 
side,  the  proper  practice  is  to  allow  the  lung  to  collapse,  as  now  explained, 
that  the  haemorrhage  may  be  checked  :  and  when  the  danger  is  past,  the 
treatment  above  described  is  to  be  adopted  to  restore  the  natural  play  of  the 
lung.  Life  may  be  supported  for  a  long  time  by  the  lung  in  one  side  of  the 
chest. 

In  cases  of  haemoptysis,  or  spontaneous  bleeding  from  the  lungs,  a  disease 
so  often  fatal,  life  might  sometime  be  "saved  or  prolonged  by  making  an 
opening  between  two  of  the  ribs,  and  allowing  the  lung  to  collapse.  The 
affected  lung  is  often  pointed  out  by  the  circumstances ;  and  the  opening, 
when  properly  made,  would  be  no  more  dangerous  than  in  the  case  where, 
by  a  similar  opening,  water  or  pus  is  discharged  from  the  chest. 

The  same  operation  has  been  tried  as  a  forlorn  hope  in  pulmonary  con- 
sumption. This  disease  is  often  limited  to  the  lung  of  one  side,  and  as  the 
alternate  stretching  and  collapse  of  the  deceased  lung  during  respiration, 
together  with  the  contact  of  the  air,  powerfully  prevent  an  ulcer  there  from 
healing,  or  inflammation  from  subsiding,  a  new  chance  of  recovery  is  given 
by  allowing  the  deceased  lung  to  collapse  and  remain  at  rest.  Some  cases 
are  recorded  where  cure  is  said  to  have  followed  this  operation,  and  certainly 
where  the  circumstances  are  favourable  for  it,  and  where  death  must  ensue 
unless  it  can  save,  it  is  worth  trying. 

When  ribs  are  fractured,  it  is  the  practice  to  put  a  bandage  round  the  chest, 
so  as  for  the  time  to  prevent  almost  entirely  the  respiratory  motion  of  the 
ribs,  and  the  breathing  is  then  performed  chiefly  by  the  rising  and  fulling  of 
the  diaphragm  or  floor  of  the  chest  as  above  described.  Although  a  person 
with  broken  ribs  is  wisely  for  a  time  subjected  to  the  unnatural  restraint,  it 
is  surely  the  height  of  folly  to  inflict  the  same  on  healthy  beings,  as  is  yet, 
however,  so  commonly  done  among  young  women,  and  often  to  the  destruc- 
tion of  their  health,  by  the  fashion  of  bracing  the  body  in  tight  stays. 

The  force  of  a  healthy  chest's  action  in  blowing  is  equal,  as  stated  in  last 
section,  to  be  about  one  pound  on  the  inch  of  its  surface  ;  that  is  to  say,  the 
chest  can  condense  its  contained  air  with  that  force,  and  can,  therefore,  blow, 
through  a  tube,  the  mouth  of  which  is  two  feet  under  the  surface  of  the 
water.  In  the  opposite  action  of  sucking  or  drawing  in  air,  the  power,  is 
nearly  the  same.  In  both  actions  it  is  possible  to  use  the  cavity  of  the 
mouth  separately  from  that  of  the  chest ;  and  the  mouth,  being  smaller,  with 
stronger  muscles  about  it  in  proportion  to  its  size,  it  can  act  more  strongly. 
Some  men  can  suck  with  the  mouth  so  as  to  make  nearly  a  perfect  vacuum, 
or  to  lift  water  nearly  thirty  feet.  An  expert  operator  with  the  blow-pipe 
can  keep  up  an  uninterrupted  blast  by  shutting  the  mouth  behind,  while  he 
inhales,  and  replenishing  it  as  is  required  in  the  intervals. 

In  coughing,  the  glottis  or  top  of  the  windpipe,  by  a  curious  sympathy 
of  parts,  is  first  closed  for  an  instant,  during  which  the  chest  is  compressing 
and  condensing  its  contained  air,  and  on  the  glottis  being  then  opened,  a 
slight  explosion  as  it  were,  of  the  compressed  air  takes  place,  and  blows  out 
any  irritating  matter  that  may  be  in  the  air-passages ;  just,  only  with 
inferior  force,  as  the  burst  from  the  chamber  of  an  air-gun  discharges  its 
bullet.  This  shutting  of  the  glottis  to  allow  the  compression  of  the  air,  and 
the  subsequent  opening  to  allow  the  discharge,  may  recur  at  every  minute 
intervals,  and  many  times  for  one  fill  of  the  chest,  as  is  instanced  in  hooping- 
cough.  The  action  of  coughing  is  often  produced  by  irritation  from  a  cause 


RESPIRATION    AND    VOICE.  449 

which  cannot  be  removed  by  coughing,  as  inflammation  of  the  chest,  or 
tubercles ;  or  even  by  irritation  in  a  distant  part,  as  when  children  are 
teething,  or  when  the  stomach  is  overloaded. 

Sneezing  is  a  phenomenon  resembling  cough,  only  the  chest  empties  itself 
at  one  throe,  and  chiefly  through  the  nose,  instead  of  through  the  mouth, 
as  in  coughing.  The  irritation  that  produces  sneezing  is  generally  in  the 
nose;  but,  as  in  the  case  of  cough,  sneezing  may  occur  from  distant  sympa- 
thies :  witness  that  from  worms  in  the  bowels. 

Laughing  consists  of  quickly  repeated  expulsions  of  air  from  the  chest, 
the  glottis  being  at  the  time  in  a  condition  to  produce  voice ;  but  there  is  not 
between  the  gusts,  as  in  coughing,  complete  closure  of  the  glottis. 

Crying  differs  from  laughing  almost  solely  in  the  circumstances  of  the 
intervals  between  the  gusts  of  air  being  longer.  Children  laugh  and  cry 
in  the  same  breath,  and  it  is  often  difficult  to  mark  the  moment  of  change. 

Hiccup  is  the  sudden  stopping,  by  a  closure  of  the  glottis,  of  a  strong 
inspiration  at  its  commencement.  If  the  inspiratory  effort  be  afterwards 
continued,  it  may  cause  air  from  the  atmosphere,  or  half-digested  food  from 
the  stomach,  to  enter  the  oesophagus. 

In  straining  to  lift  weights,  or  to  make  any  powerful  effort,  the  air  is  shut 
up  in  the  lungs,  that  there  may  be  steadiness  and  firmness  of  the  person. 
At  such  a  time,  by  the  compression  and  condensation  of  air  around  the  heart 
and  large  blood-vessels,  the  blood  is  determined  violently  outwards  from  the 
chest,  and  often  rises  to  the  head,  with  force  that  produces  giddiness,  or  even 
apoplexy, — and  the  eye  will  sometimes  become  suddenly  bloodshot  from  a 
small  vessel  giving  way;  and  leech-bites  will  break  out  afresh. — The  force 
of  this  pressure  outwards  is  measured,  as  already  stated,  by  a  column  of 
about  two  feet  of  blood ;  and  this  is,  therefore,  the  measure  of  the  additional 
arterial  and  venous  tension  in  the  body  generally. 

Suffocation  is  the  name  given  to  what  happens  when  the  supply  of  air  to 
the  lungs  is,  any  way,  prevented.  The  blood,  not  then  refreshed  by  the 
approach  of  the  air,  rises  in  the  brain  unfit  for  its  purpose,  and  confusion  of 
thought  is  immediately  produced,  soon  followed  by  convulsion  and  death. 

When  this  happens  from  mechanical  obstruction  at  the  narrow  entrance 
of  the  windpipe,  as  in  croup,  by  the  tenacious  films  thrown  off  from  the 
inflamed  lining  of  the  air-passages,  life  may  be  saved  by  making  a  new 
entrance  for  air  through  the  windpipe,  lower  down  in  the  neck,  and  keeping 
it  free  by  a  little  tube  inserted,  until  the  obstruction  above  be  removed. 
Where  children  die  with  croup,  it  is  frequently  not  from  the  violence  of  the 
constitutional  disease,  but  from  detached  films  thus  accidentally  sticking  in 
the  narrow  entrance  of  the  air-passage. 

In  the  cases  of  strangling  and  hanging,  the  tight  binding  of  the  rope  or 
ligature  crushes  inwards  the  cartilaginous  rings  of  the  windpipe,  and  shuts 
the  air-passage.  It  may  also  cause  apoplexy,  by  arresting  the  passage  of 
blood  to  and  from  the  head :  and  there  may  be  dislocation  of  the  cervical 
vertebrae  of  the  spine. 

In  drowning,  communication  with  the  atmosphere  is  cut  off  altogether 
by  the  supernatent  water.  If,  during  submersion,  the  chest  expands,  it  can 
receive  water  only,  instead  of  air.  The  nerves  and  muscles,  however,  at  the 
entrance  of  the  windpipe,  are  so  irritable,  as  to  be  immediately  excited  by 
the  contact  of  any  unusual  matter,  and,  for  a  considerable  time,  they  keep 
the  passage  shut  against  the  liquid  seeking  entrance.  It  is  partly  on  this 
account  that  the  body  of  a  person,  after  submersion  in  water  and  apparent 
death,  may,  often,  if  recovered  within  a  moderate  time,  be  restored  to  life. 

29 


450     FLUIDITY  IN  RELATION  TO  ANIMALS. 

The  apparatus  of  the  Humane  Society  for  the  recovery  of  persons  appa- 
rently drowned,  includes  a  bellows  for  producing  artificial  respiration.  This 
bellows  resembles  a  common  bellows,  except  that  its  flap  or  valve,  instead 
of  being  internal,  is  external  like  a  large  flute-key,  and  has  a  spring  to  close 
it,  obedient  to  the  finger  of  the  operator.  The  bellows  receives  its  charge 
of  fresh  air  on  being  expanded,  while  the  valve  is  open ;  it  sends  the  charge 
into  the  lungs  on  being  compressed  while  the  valve  is  shut ;  it  withdraws 
the  charge  again  on  being  expanded  with  the  valve  shut ;  and  the  impure 
air  is  thrown  out  to  the  atmosphere  on  its  being  compressed  with  the  valve 
open.  These  changes,  repeated  and  continued,  produce  the  artificial  respi- 
ration required.  It  is  most  important  to  remark  here,  that  if  air  be  injected 
into  the  lungs,  either  in  too  large  quantity  or  very  suddenly,  instead  of 
recalling  or  sustaining  life,  it  is  as  certain  a  means  of  killing  as  a  dagger 
driven  through  the  heart.  This  truth  has  been  but  lately  known,  and  igno- 
rance of  it  has  probably  decided  the  fate  of  many  persons,  treated  with  a 
view  to  recovery  after  submersion.  The  operator  should  reflect  that  he  is 
dilating  the  delicate  air-cells  of  the  lungs  with  the  force  of  an  hydraulic 
press;  and  that  if  he  does  so  very  suddenly,  although  to  a  small  extent,  he 
still  may  rupture  many  small  blood-vessels,  before  they  can  empty  them- 
selves so  as  to  yield.  In  a  bellows  for  the  purpose  of  artificial  respiration, 
there  should  be  the  means  of  checking  its  opening  to  suit  the  capacity  of  the 
patient's  chest,  and  there  should  be  a  cock  in  the  pipe  or  nozzle  to  regulate 
the  speed  of  the  passing  air. 

In  addition  to  the  artificial  breathing  for  the  recovery  of  suspended  ani- 
mation, it  is  often  necessary  to  restore  natural  warmth  of  the  body,  to  rub 
the  limbs  in  aid  of  the  circulation,  to  administer  stimulants  by  the  mouth, 
to  excite  by  galvanism,  &c. 

It  seems  to  be  an  error,  and  probably  often  a  fatal  error,  in  the  present 
mode  of  treating  persons  apparently  drowned,  to  use  cold  instead  of  warm 
air  for  the  artificial  respiration.  Thus  while  the  important  object  of  restoring 
the  temperature  of  life  is  sought  by  all  external  means,  the  great  inconsis- 
tency is  committed  of  blowing  cold  air  upon  an  internal  surface  of  the  body 
more  extensive  than  the  external ;  and  until  that  reciprocal  action  of  the 
air  and  blood  begins,  which  constitutes  the  slow  combustion  of  natural 
respiration,  every  bellows-full  of  cold  air  admitted,  brings  back  with  it  a 
portion  of  the  remaining  central  warmth,  and  may  thus  chill  so  as  to  make 
the  recovery  impossible  : — as  a  fire  which  has  fallen  very  low  may  be  imme- 
diately extinguished  by  the  same  action  of  a  bellows,  which  a  little  before 
would  have  made  it  blaze.  Air  might  easily  be  heated  for  the  purpose  of 
respiration  by  pouring  boiling  water  into  a  vessel  containing  it,  and  then 
connecting  the  bellows  with  that  vessel  by  a  fit  pipe,  or  by  making  the 
bellows  draw  through  a  pipe  partially  immersed  in  hot  water/: — a  quart  of 
boiling  water  has  heat  enough  in  it  to  warm  many  gallons  of  air  to  blood- 
heat.  This  plan  would  not  only  avoid  the  mischiefs  arising  from  the  cold 
air,  but  by  affording  the  means  of  applying  warmth  even  higher  than  that 
of  life,  it  might  probably  furnish  the  most  useful  of  all  stimulants  to  the 
parts  about  the  heart.  A  healthy  man  can  breathe  with  impunity  air  that 
is  much  hotter  than  boiling  water. 

Late  physiological  investigations  have  shown  that  the  breathing,  or  me- 
chanical action  of  the  chest  in  respiration,  is  so  dependent  upon  the  influence 
of  the  brain,  as  to  be  disturbed  and  even  stopped  when  the  brain  is  embar- 
rassed ]  they  have  shown,  farther,  that  the  action  of  the  heart  is  dependent  on 
the  breathing,  but  not  on  the  brain,  except  as  the  cause  of  the  breathing — 


THE    VOICE    AND    SPEECH.  451 

for  that  respiration  kept  up  artificially,  will  preserve  the  circulation  and  the 
life  for  a  considerable  time  after  the  brain  has  altogether  ceased  to  act,  or 
even  has  been  removed  from  the  body.  Now,  some  interesting  experiments 
of  Mr.  Brodie  have  proved,  that  certain  poisons  are  fatal,  merely  because 
they  suspend  for  a  time  the  action  of  the  brain — through  which  suspension, 
the  actions,  first  of  the  chest,  and  then  of  the  heart,  cease,  and  death  ensues  : 
but  that  in  such  cases,  if  the  action  of  the  chest  be  maintained  artificially, 
tho  circulation  and  life  of  the  body  will  be  for  a  time  continued,  and  the 
brain  may  gradually  recover  from  the  effect  of  the  poison,  so  as  to  resume 
its  office.  Thus  certain  cases  of  poisoning,  which  formerly  would  have  been 
fatal  may  now  end  in  recovery. 

An  important  application  of  this  discovery  is  to  the  treatment  of  cases  of 
convulsion,  particularly  those  occurring  from  teething  or  other  irritations  in 
infancy.  The  respiration  ceases  in  these  cases  often  only  because  the  action 
of  the  brain  is  suspended ;  and  if  the  respiration  be  continued  artificially,  the 
circulation  and  life  will  also  continue  for  a  time,  during  which  the  brain  may 
recover  itself,  either  spontaneously,  or  in  consequence  of  remedies  employed, 
and  life  may  be  saved. — The  chest  of  an  infant  is  comparatively  so  small, 
that  it  may  be  filled  from  the  mouth  and  windpipe  of  a  grown  person,  with 
air  which  has  not  descended  to  that  person's  lungs,  and  therefore  has  not  been 
rendered  unfit  by  respiration;  and  on  the  little  chest  being  afterwards  com- 
pressed by  the  hand,  the  air  will  return.  The  air  may  be  blown  directly  in- 
to the  child's  mouth  through  a  thin  handkerchief  laid  over  the  mouth,  or  it 
may  pass  through  a  tube  inserted  into  the  nostril  or  treachea : — to  prevent  it 
from  passing  into  the  stomach,  the  larynx  must  be  pressed  against  the  oeso- 
phagus during  its  entrance.  Let  all  who  try  this  remedy,  keep  present  to 
their  minds  the  danger  of  inflating  too  much. 

Any  medicated  air  is  generally  inhaled  by  a  patient  from  an  oiled-silk, 
or  other  air-tight  bag,  or  from  a  light  gasometer  (see  page  211.)  When  the 
compound  nature  of  our  atmosphere  was  first  discovered,  great  advantages 
were  anticipated  to  medicine  from  the  use  of  pneumatic  or  aerial  mixtures. 
These  expectations  have  not  been  realized,  but  the  subject  still  remains 
highly  deserving  of  research. 


THE   VOICE   AND    SPEECH. 

The  chest  and  air-passages,  with  certain  additional  parts,  constitute  the 
organs  of  voice  and  speech. 

An  inquirer  into  the  constitution  of  the  universe  around  him,  meets  with 
few  things  calculated  more  to  surprise  him  than  th^t  faculty  in  the  human 
mind  by  which  it  can  associate  the  ideas  of  objects  with  any  arbitrary  signs, 
so  closely  that  the  ideas  are  afterwards  excited  by  the  signs  almost  as  vividly 
as  by  the  objects  themselves.  The  inhabitants  of  China,  for  instance,  have 
contrived  many  thousand  grotesque  characters,  and  determined  what  object 
each  should  recall ;  when  a  Chinese  by  study  becomes  familiar  with  these, 
he  may  have  his  bodily  eye  poring  over  pages  of  crooked  and  unseemly 
scratches,  while  his  mental  eye  through  them  sees  only  a  pleasing  succession 
of  the  most  beautiful  imagery  of  nature;  and  the  characters  may  be  rendered 
intelligible  to  the  deaf  and  dumb  man,  as  well  as  to  him  who  speaks ;  and 
they  serve  as  a  media  of  thought  and  communication  through  many  provinces 
and  countries  of  which  the  spoken  languages  have  no  common  resemblance. 


452  FLUIDITY    IN    RELATION    TO    ANIMALS. 

But  if  the  ready  resemblance  of  visible  signs  be  wonderful,  which  have 
a  permanent  existence,  and  which  often  may  have  some  resemblance  in  form 
to  the  things  signified,  how  much  more  wonderful  is  it  that  an  audible  sign, 
that  is,  a  passing  sound  or  fugitive  breath,  called  by  man  a  word,  should 
serve  as  well;  and  that  by  a  succession  of  mere  sounds,  having  so  little 
natural  connection  with  the  things  signified,  that  they  are  totally  different  in 
different  countries,  and  are  changing  with  fashion  from  age  to  age,  any  train 
of  thought  may  be  made  to  pass  through  the  minds  of  an  audience,  so  as  to 
excite  and  to  leave  impressions  almost  as  vivid  as  from  realities  !  Such, 
however,  is  the  fact,  and  it  is  greatly  owing  to  this  and  to  a  correspondent 
faculty  of  producing  at  will  a  sufficient  number  of  distinguishable  sounds, 
that  man  owes  his  elevation  above  the  brutes  of  the  field.  His  godlike 
powers  of  intellect  would  have  remained  dormant  and  unknown,  had  he 
wanted  the  faculty  of  comparing  invisible  thoughts  with  those  of  his  fellow 
men,  and  of  arranging  and  recording  them  by  means  of  signs. — Written 
language  is  a  double  remove  from  the  objects  themselves,  being  visible  signs 
not  of  things,  but  of  the  audible  signs. 

The  admirable  apparatus  by  which  man  is  enabled  to  produce  a  sufficient 
variety  of  sounds  to  answer  his  purposes,  passes  generally  under  the  denomi- 
nation of  the  organs  of  speech;  because  the  act  of  using  sounds  which  have 
meanings  assigned  to  them  is  called  speech.  It  consists  of  the  chest  for  con- 
taining air;  of  the  larynx  or  cartilaginous  box,  with  its  narrow  aperture  called 
the  glottis,  at  the  top  of  the  windpipe,  for  producing  the  voice,  and  varying 
its  pitch;  and  of  the  short  tube  of  the  mouth,  with  the  tongue  and  lips,  for 
farther  modifying  the  voice. 

In  the  chapter  on  acoustics,  we  explained  that  sound  is  the  name  given  to 
the  effect  produced  upon  the  ear  by  certain  tremblings  conveyed  to  it  gener- 
ally through  the  medium  of  the  air;  and  we  explained  how  air,  forced  from 
the  human  lungs  through  the  opening  at  the  top  of  the  windpipe,  causes  the 
elastic  lips  of  that  opening  to  vibrate,  and  to  excite  the  tremblings.  We  have 
now  to  show  that  this  sound,  in  passing  forward  from  the  top  of  the  wind- 
pipe, may  be  modified  at  the  will  of  the  individual,  in  a  great  variety  of  ways 
— a  variety,  however,  which  is  still  very  simple. 

The  modifications  of  voice  easily  made,  and  easily  distinguishable  by  the 
ear,  and  therefore  fit  elements  of  language,  are  about  fifty  in  number;  but 
no  single  language  contains  more  than  about  half  of  them.  They  are  divisi- 
ble into  two  very  distinct  and  nearly  equal  classes,  called,  for  reasons  now  to 
be  explained,  vowels  and  consonants. 

Those  of  the  first  class  are  the  simple  voices  issuing  through  the  open  mouth, 
and  influenced  only  by  the  degrees  in  which  the  mouth  is  opened  and  elon- 
gated. They  may  be  continued  as  long  as  there  is  breath  to  issue  from  the 
chest,  and  it  is  for  this  reason  that  they  are  named  vowels  or  calling  sounds, 
The  Roman  letters,  A,  E,  I,  0,  U,  as  generally  pronounced  on  the  continent 
of  Europe,  and  of  which  the  sounds  correspond  nearly  to  those  aw,  a,  e,  oy 
and  oo}  of  English  writing,  indicate  the  most  easily  distinguishable  vowels. 
Sounds  passing  through  the  mouth  in  its  most  natural  state  of  relaxation,  is 
heard  as  the  modifications  expressed  by  the  Italian  E,  (or  the  a  of  the  English 
wdrd  care  ;)  if  the  mouth  be  then  widened,  the  sound  becomes  A  (as  in  the 
English  word  bar  ;)  if  the  mouth  be  narrowed,  we  hear  the  I  (or  the  e  of  the 
English  tedious  ;)  if  the  mouth  be  elongated  and  at  the  same  time  widened, 
we  hear  the  0  (as  in  the  English  word  bore;]  and  if  more  elongated  but 
narrowed,  we  hear  the  U  (as  in  the  English  rude.)  The  possible  number  of 
vowels,  however,  is  as  great  as  the  possible  degrees  in  which  the  dimensions 


THE    VOICE    AND    SPEECH.  453 

of  the  mouth  may  be  altered.  About  twenty  of  them  are  sufficiently  distin- 
guishable, but  few  languages  comprehend  more  than  twelve.  Modern  art 
can  produce  the  vowel  sounds  mechanically  by  means  of  tubes  of  certain 
dimensions. 

The  alphabets  of  Europe  are  very  faulty  in  not  all  using  the  same  charac- 
ters for  the  same  sounds,  and  in  not  having,  according  to  the  true  intent  of 
an  alphabet,  a  character  for  each  distinct  sound.  In  English,  one  letter  is 
used  for  several  sounds,  as  A,  in  water,  far,  fat,  fate,  where  it  indicates  four 
sounds  perfectly  distinct.  In  repeating  the  English  alphabet,  the  A  is  pro- 
nounced as  the  broad  E  of  the  Italians  and  of  continental  Europe,  and  the  E 
as  the  I;  and  the  I  (in  tide,  for  instance,)  as  the  dipthong  AI  of  more  cor- 
rect alphabets ;  and  the  U  (in  mw.se,)  as  the  dipthong  IU.  In  consequence 
of  the  changes  which  the  English  have  made  in  the  meaning  of  the  Roman 
letters,  they  now  experience  increased  difficulty  in  learning  modern  conti- 
nental languages;  and  their  own  pronunciation  of  the  ancient  languages,  to 
all  but  themselves,  is  ridiculous,  and  almost  unintelligible.  The  same  cause 
renders  the  pronunciation  of  English  difficult  to  foreigners,  and  thus  restricts 
much  in  other  countries,  the  cultivation  of  English  literature. 

To  explain  the  second  class  of  the  modifications  of  sound,  called  conso- 
nants, we  may  remark,  that  while  any  continued  or  vowel  sound'is  passing 
through  the  mouth,  if  it  be  interrupted,  whether  by  a  complete  closure  of 
the  mouth,  or  only  by  an  approximation  of  parts,  the  effect  on  the  ear  of  a 
listener  is  very  remarkable,  and  is  so  exceedingly  different,  according  to  the 
situation  in  the  mouth  where  the  interruption  occurs,  and  to  the  manner  in 
which  it  occurs,  that  many  most  distinct  modifications  thence  arise.  Thus 
any  continued  sound,  as  A,  if  arrested  by  a  closure  of  the  mouth  at  the  ex- 
ternal confine  or  lips,  is  heard  to  terminate  with  the  modification  which  we 
choose  to  express  by  the  letter  P,  that  is,  the  syllable  AP  has  been  pro- 
nounced ;  but  if,  under  similar  circumstances,  the  closure  be  made  towards 
the  back  of  the  mouth  by  the  tongue  rising  against  the  palate,  we  hear  the 
modification  expressed  by  the  letter  K,  and  the  syllable  AK  has  been  pro- 
nounced ;  and  if  the  closure  be  made  in  the  middle  of  the  mouth  by  the  tip 
of  the  tongue  rising  against  the  roof,  the  sound  expressed  by  T,  is  produced 
and  the  syllable  AT  is  heard — and  so  of  others.  It  is  to  be  remarked,  also, 
that  the  ear  is  equally  sensible  of  the  peculiarities  whether  the  closure  pre- 
cedes the  continued  sound  or  follows  it :  that  is  to  say,  whether  the  syllables 
pronounced  are  as  above,  AP,  AT,  AK,  or  on  the  contrary,  PA,  TA,  KA. 
— The  modifications  of  which  we  are  now  speaking,  appear,  then,  not  so 
much  to  be  sounds,  as  distinguishable  manners  of  beginning  and  ending 
sounds  j  and  it  is  because  they  are  thus  only  perceivable  in  connection  with 
.vocal  sounds  that  they  are  called  consonants. 

Now  in  the  mouth,  considered  as  a  vocal  tube,  there  are  three  situations, 
in  which  interruption  of  voice  or  breath  may  most  conveniently  be  made, 
and  there  are  six  modes  of  making  it  at  each  ,  so  that  eighteen  distinct  inter- 
ruptive  modifications  or  consonants  hence  arise.  These  we  shall  now  describe. 

The  three  great  oral  positions,  as  they  may  be  called,  are, 

1st.  At  the  external  confine  of  the  mouth,  or  lips,  giving  the  labial  arti- 
culations, of  which  is  an  example. 

2d.  In  the  middle  of  the  mouth,  where  the  tip  of  the  tongue  approaches 
the  palate  behind  the  teeth,  producing  the  palatal  articulations,  of  which 
T  is  an  example. 


454  FLUIDITY    IN    RELATION    TO    ANIMALS. 

3d.  Near  the  back  of  the  mouth,  where  the  body  of  the  mouth  approaches 
the  palate,  giving  th  guttural  articulations,  of  which  K  is  an  example. 

And  the  six  modes  in  which  the  voice  or  breath  may  be  affected  in  passing 
through  each  of  the  three  positions  of  the  mouth,  are, 

1st.  A  sudden  and  complete  stoppage,  producing  what  may  be  called  a 
mute  articulation  :  viz.,  P,  in  the  labial  position;  T,  in  the  palatal ;  and  K, 
in  the  guttural.  (See  here  the  general  table  of  articulations  next  page 
which  table  may  be  considered  as  representing  the  tube  of  the  mouth,  with 
the  letters  so  placed  as  to  show  in  what  situations  in  the  mouth  the  sounds 
represented  by  them  are  severally  produced.)  A  mute  may  also  be  made  by 
stopping  the  breath  exactly  at  the  teeth,  viz.,  a  dental-mute;  but  it  is 
hardly  distinguishable  from  the  palatal  mute,  produced  just  behind  it,  and 
being  less  perfect  is  not  used. — Some  awkward  speakers,  substituting  it  for 
the  proper  mute,  are  said  to  speak  thick. 

2d.  A  sudden  shutting,  as  in  the  last  case,  but  the  voice  being  allowed 
to  continue  until  the  part  of  the  mouth  behind  the  closure  be  distended  with 
air. — This  produces  the  semi-mules,  B,  D,  and  G,  (as  heard  in  the  syllables 
AB,  AD,  AG,)  for  the  three  positions.  There  might  be  a  dental  half-mute, 
but  it  is  not  more  used  than  the  dental-mute,  and  for  the  same  reasons.  If 
the  sides  of  the  tongue  be  depressed,  after  it  has  taken  the  position  required 
for  T  or  D,  the  sound  L  is  produced  j  and  the  letter  is,  in  the  table,  placed 
below  D,  although  the  sound,  from  being  continuable,  is  not  in  any  sense  a 
mute, 

3d.  The  positions  closed  as  for  the  mutes,  while  sound  is  allowed  to  pass 
by  the  nose. — Thus  arise  the  semi-voiocls  or  nasals,  M,  N,  NO,  for  the 
three  positions, — NG  (as  in  Icing)  is  a  simple  sound,  although  our  imperfect 
alphabet  has  no  single  letter  for  it.  The  nasal  sound  of  the  French  language, 
which  gives  it  so  great  a  peculiarity,  approximates  to  the  English  NG,  but 
differs  from  it  in  sound  being  allowed  to  pass  by  the  mouth,  as  well  as  by 
the  nose.  It  is  pointed  at  by  the  small  n  in  the  table,  and  like  the  other 
sounds  which  do  not  occur  in  the  English  language,  is  here  pointed  in  the 
Italic  character. 

4th.  Breath  only  (or  whisper)  allowed  to  pass  at  the  three  oral  positions, 
nearly  closed — Hence  come  the  sounds  which  we  call  aspirates,  viz.,  F,  for 
the  labial  position,  TH  and  S,  for  the  palatal,  and  CH  (heard  in  the  Scottish 
word  loch,)  for  the  guttural ;  the  TH  and  CH,  are  simple  sounds,  although 
each  expressed  in  Britain  by  two  letters.  The  TH  is  heard  in  the  word 
lath,  and  is  the  sound  expressed  by  the  single  letter  6  of  the  Greeks.  The 
CH  is  heard  in  the  German  icli,  and  is  the  %  of  the  Greeks.  The  soft  aspi- 
rate TH  is  more  easily  made  by  pressing  the  tongue  gently  against  the 
teeth,  and  allowing  the  breath  to  pass  all  round,  than  by  the  true  palatal 
approximation  of  parts,  and  the  soft  dental  aspirate,  therefore,  is  used  in 
preference  to  the  palatal.  The  letter  S  is(the  hard  palatal  aspirate,  and 
differs  from  the  soft  palatal  aspirate  TH,  in  the  breath  being  made  to 
issue  with  greater  force,  and  only  by  a  narrow  space  over  the  centre  of  a 
rigid  tongue,  instead  of  on  all  sides  of  a  soft  tongue,  as  for  TH.  French 
people  on  first  attempting  to  pronounce  TH,  substitute  for  it  the  D,  or  the 
S,  or  the  Z  (which  is  nearly  related  to  S,  as  explained  below.)  The  author 
has  found  it  easy  to  enable  them  to  pronounce  the  TH  at  once,  and  perfectly, 
by  explaining  its  nature  as  above.  If  we  depress  the  sides  of  the  tongue  while 
pronouncing  S,  we  produce  the  simple  sound  expressed  by  the  English  double 


THE    VOICE    AND    SPEECH. 


455 


letter  S  H,  just  as  by  depressing  the  sides,  of  the  tongue  while  making  D, 
we  produce  L. 

5th.  Using  voice  in  the  same  manner  as  breath  or  whisper  is  used  for  the 
aspirates. — This  produces  the  sound  called  vocal  aspirates,  viz.,  V,  TH,  Z, 
J,  and  GIL  TH  vocal  aspirate,  is  heard  in  lathe,  as  contrasted  with  the 
simple  aspirate  in  bath;  Z  comes  from  the  S  position,  only  with  sound 
instead  of  breath  ;  SH  pronounced  with  voice,  becomes  the  J  of  the  French 
in  the  word  je,  or  the  sound  heard  in  the  middle  of  the  English  word  vision. 
GH  is  a  simple  sound  in  German,  but  not  in  English. 


Table  of  Articulations. 


i 
1     '  S 


P 

B 

M 

F 

Y 

pr 

. 

. 

. 

th 

th 

, 

T 

D 

N 

S 

Z 

R 

• 

L 

• 

sh 

J 

• 

K 

G 

ng 

ch 

ah 

ghr 

n 

H 

LABIAL. 

Dental. 
PALATAL. 

{with  the  edges  of 
the  tongue  depressed. 

GUTTURAL. 


6th.  Shaking  the  approaching  parts  in  the  three  positions. — We  thus 
make  vibratory  sounds,  of  which  the  middle  position  gives  the  common  R, 
the  only  one  of  them  used  in  England.  Some  bad  .speakers  of  English,  how- 
ever, make  the  labial  vibratory  by  shaking  the  P  in  such  words  i\§ property ; 
and  many  use  the  guttural,  which  is  the  burr  of  Northumberland,  and  the 
common  affectation  in  Parisian  speech,  termed  parler  gras,  or  yrasscyer. 

Additional  Remarks. 

The  sound  of  H  is  an  aspirate  produced  even  behind  the  situation  of  the 
guttural  aspirate  ch  ;  it  is,  indeed,  merely  a  forcible  passing  of  the  breath 
through  the  very  back  part  of  the  mouth  or  throat. 


456     FLUIDITY  IN  RELATION  TO  ANIMALS. 

CH,  in  such  words  fts  chain,  means  T  before  sh. 

J,  as  heard  in  the  English  name  John,  is  a  compound  sound,  viz.,  D  be- 
fore the  simple  J  of  the  table,  which  is  the  S  of  vision. 

LL.  The  liquid  or  double  LL  of  the  French  as  heard  in  the  word  jo«?7fc, 
is  merely  L  with  the  letter  Y  begun  to  be  pronounced  after  it.  It  is  heard 
in  the  English  words  billiard  and  halyard,  and  would  be  their  terminating 
liquid  were  the  syllable  ard  not  pronounced.  The  double  LL  of  the  Welch, 
as  in  the  name  of  Lloyd,  has  the  first  L  pronounced  as  an  aspirate,  that  is, 
as  a  whisper,  and  the  second  in  the  ordinary  way. 

GN.  The  soft  G  N  of  the  Italians  and  French,  is  the  English  N  with  Y 
begun  to  be  pronounced  after  it.  It  is  heard  in  our  word  lanyard;  and  in 
the  Italian  words  pegnio  bagnio  ;  and  in  the  French  word  craignent. 

C,  in  English,  stands  always  either  for"  S  or  K,  as  in  the  words  certain 
and  car,  and  has  no  sound  proper  to  itself. 

Q,  in  English,  expresses  the  sound  of  the  letter  K,  with  U  following  it  : 
and  yet,  uselessly,  U  is  always  written  after  Q. 

X,  in  English,  means  either  KS,  as  in  the  word  axle,  or  GZ  as  in  the  word 
example. 

The  consonants  are  best  heard  by  sounding  them  with  voice  before  them : 
'that  is  to  say,  by  making  them  rather  terminate  a  syllable  than  begin  it : 
pronouncing  B,  I),  G,  thus  eb,  ed,  eg,  rather  than  their  common  alphabetical 
names  be,  de,  ge. 

The  labial  sounds  may  be  made  either  by  the  two  lips,  or  by  one  lip  and 
the  opposite  teeth.  F  may  be  pronounced,  for  instance,  by  the  lips  only,  or 
by  the  lips  and  teeth :  and  some  persons  awkwardly  make"  it  by  the  under 
teeth  and  upper  lip. 

The  letters  Y  and  I,  in  most  modern  languages,  stand  for  nearly  the  same 
sound.  In  English,  for  instance,  bullion  and  minion  might  be  written 
bullyon  and  mint/on,  without  suggesting  a  change  of  pronunciation.  In  the 
words,  yard,  you,  yes,  &c.,  the  Y  is  a  short  I,  very  closely  joined  to  the 
following  sound. — W  is  also  thus  a  short  U,  as  perceived  in  the  words  icar, 

W€i  &C. 

The  author  believes  the  analysis  of  articulation  to  be  the  best  basis  for  a 
system  of  short-hand  written  characters.  He  has  tried  such  a  system,  and 
found  it  exceedingly  convenient. 

Lisping  is  chiefly  the  habitual  substitution  of  the  aspirate  Til  for  the  S 
and  SH. 

Whispering  is  articulation  without  voice;  that  is  to  say,  articulation  while 
breath  only  is  passing. 

Stuttering,  stammering  or  hesitation  of  speech,  are  terms  implying  an 
interrupted  articulation,  accompanied  generally  with  more  or  less  of  strain- 
ing and  distortion  of  feature.  It  is  remarkable  with  respect  to  this  defect, 
that  when  the  present  work  was  first  published,  scientific  or  regular  medicine 
had  taught  as  yet  no  certain  cure  for  it,  although  the  frequent  success  of  non- 
professional,  and  often  ignorant  individuals,  by  a  mode  of  treatment  which 
they  solemnly  bound  their  patients  not  to  divulge,  proved  the  cure,  in  certain 
cases,  to  be  both  possible  and  not  difficult. — The  author's  attention  had  been 
drawn  to  the  subject  some  years  before,  by  an  interesting  case  submitted  to 
him  of  stuttering  connected  with  other  disease;  and  it  was  in  analysing  the 
subject  with  a  view  to  the  treatment  of  that  case,  that  he  framed  the  analysis 
of  articulation  contained  in  the  preceding  pages,  and  drew  up  a  part  of  the 
additional  observations  which  are  now  to  follow.  A  cure  was  obtained; 
but  as  the  case  possessed  a  favourable  peculiarity  in  the  powerful  mind  of 


THE    VOICE    AND    SPEECH.  457 

the  individual,  to  which  the  author  attributed  great  importance,  as  he  had 
little  leisure  from  his  ordinary  professional  duties,  to  pursue  the  subject,  or 
to  ascertain  in  what  respects  his  plan  might  differ  from  that  employed  by 
the  most  sudcessful  of  the  practitioners  who  concealed  their  proceedings,  he 
gave  his  remarks  in  former  editions  of  this  work,  merely  as  continued  elu- 
cidation of  the  subject  of  speech.  He  is  now,  however,  enabled  to  state,  that 
his  analysis  has  completely  detected  the  nature  of  the  morbid  affection,  and 
that  it  directs  simple  and  effectual  means  of  relief.  He  declined  meddling 
with  many  cases  offered  to  him  after  the  original  publication  of  his  work, 
from  the  impression  that  the  cure  in  the  instance  mentioned  above,  was 
owitg  at  least  as  much  to  the  ingenuity  and  perseverance  of  the  patient,  as 
to  his  suggestions,  and,  therefore,  that  his  professional  superintendence  of 
the  discipline  required  for  ordinary  cases  would  demand  care  and  attention 
which  he  could  not  spare ;  but  subsequent  experience  has  proved  to  him 
that  the  business  is  altogether  very  simple  and  easy,  and  as  regards  children, 
may  be  managed  by  any  intelligent  instructor  of  youth  who  chooses  to  de- 
vote attention  to  it,  while  grown  individuals  will  often  be  able  to  relieve 
themselves  by  the  study  of  the  present  section ;  and  he  hopes  that  in  very 
few  cases  will  the  counsel  of  a  person  familiar  with  the  anatomy  and  dictions 
of  the  organs  be  found  to  fail. 

Command  over  the  organs  of  speech  is  acquired  in  the  same  way  as  over 
all  the  other  muscular  organs  of  the  body ;  those,  for  instance,  used  in  walk- 
ing, skating,  fencing,  performing  on  musical  instruments,  &c.  :  that  is  to  say, 
at  first,  a  distinct  act  of  volition  is  required  for  every  individual  movement ; 
but  the  law  of  association  or  habit  of  rendering  the  actions  easier  with  each 
successive  repetition,  they  are  at  last  formed -into  connected  tribes  or  trains, 
which  appear  as  obedient  to  a  single  wish  as  the  separate  elements  originally 
were.  A  child  at  first  exerts  as  distinct  and  powerful  a  volition  to  pro- 
nounce the  syllable  pa,  as  after  some  practice  to  double  the  syllable  and 
make  it  papa ;  or  after  still  more  practice,  to  pronounce  the  longest  and 
hardest  word  of  the  language : — nay,  at  last,  where  there  is  strong  and 
healthy  power  of  association,  complete  sentences,  and  even  rounded  periods 
of  eloquence,  are  poured  out  like  single  words,  the  mind  of  the  speaker  seem- 
ing at  liberty,  after  each  sentence  or  period  is  begun,  to  meditate  and  prepare 
that  which  is  next  to  follow.  As  the  faculties  of  locomotion  and  of  speech 
are  acquired  in  infancy  and  early  childhood,  persons  no  more  recollect  how 
they  gradually  acquired  them  than  how  their  limbs  grew;  but  the  progress, 
described  above,  may  be  watched  by  any  individual  of  mature  years  in  his 
own  person,  while  he  is  learning  such  an  art  as  that  of  playing  on  a  musical 
instrument.  He  will  find,  that  at  first,  every  finger  which  is  moved  to  pro- 
duce a  note,  obeys  a  distinct  thought  and  volition  ;  that  soon  short  trains  of 
connected  notes  become  obedient  to  the  will  almost  like  a  single  note ;  that 
then  by  degrees,  longer  arid  longer  trains  or  passages  become  familiar,  until 
at  last  the  instrument  is  obedient  to  the  practised  player,  as  voice  is  to  the 
singer  or  speech  to  the  orator.' 

There  is  a  great  original  diversity  among  individuals  as  to  their  powers  of 
muscular  association,  and  therefore,  also,  as  to  their  aptitude  for  acquiring 
the  various  faculties  of  which  we  have  been  speaking.  Thus  some  children 
walk  well  before  a  year,  others  require  a  much  longer  time,  and  some  never 
succeed  perfectly  until  they  have  had  lessons  from  the  dancing-master  or 
drill  serjeant.  So,  again,  many  people,  by  ear  and  imitation  alone,  learn 
easily  to  play  on  musical  instruments ;  but  others  must  begin  by  studying 
the  written  notes,  and  the  precise  fingering  by  which  each  note  is  produced 


458  FLUIDITY    IN    RELATION    TO    ANIMALS. 

on  the  instrument;  and  many,  unless  the  notes  be  constantly  before  thein 
cannot  play  at  all.  So,  again,  all  persons  may  be  said  to  learn  to  speak  at 
first  by  ear  and  imitation  ;  ^ut  many  grow  up  to  a  certain  age  with  defects, 
which  judicious  lessons  from  parents  or  other  tutors  are  required  to  remove; 
and  there  are  some  as  stutterers,  who,  owing  to  a  naturally  weak  or  irregu- 
lar association,  or  to  some  accident  in  early  life,  which  has  strongly  affected 
their  nervous  system,  retain  defects  which  no  ordinary  teaching  can  correct. 
It  appears,  then,  that  an  analysis  and  scale  of  articulate  sounds,  with  minute 
description  of  the  organic  actions  required  to  produce  them,  like  the  scale 
which  we  possess  for  music,  in  the  gamut  and  rules  for  fingering,  should 
give  nearly  the  same  assistance  to  the  speaker  which  the  gamut  gives  \&  the 
player.  The  table  and  analysis  contained  in  the  preceding  pages  is  intended 
to  supply  this  information.  It  is  constructed  from  minute  consideration  of 
the  organs  of  speech  while  in  action.  It  agrees  in  many  respects  with  the 
common  grammatical  divisions  of  elementary  sounds,  but  in  others  it  pursues 
the  analysis  in  a  different  way,  and  considerably  farther.  A  person  who 
understands  it  well,  will  have,  while,  he  speaks,  an  intelligent  perception  of 
what  he  is  doing,  in  addition  to  the  parrot-like  faculty  of  habit,  or  of  repeat- 
ing by  rote,  and  will  thus  command  any  desired  sound  by  two  powers  instead 
of  one.  And,  as  a  musician,  when  his  musical  memory  fails  him,  finds 
help  by  thinking  of  his  written  notes  and  their  relation  to  his  instrument, 
so  may  a  stutterer,  when  hesitating  at  any  sound,  receive  benefit  by  thinking 
of  the  letter  which  represents  it,  and  of  the  position  of  the  organs  required 
for  that  letter.  Then,  by  frequent  practice  in  making  the  particular  combi- 
nations of  sound  which  are  difficult  to  him,  he  may  strengthen  the  useful 
habit,  and  ultimately  overcome  his  defect. 

The  most  common  case  of  stuttering,  however,  is  not,  as  has  been  almost 
universally  believed,  where  the  individual  has  a  difficulty  in  respect  to  some 
particular  letter  or  articulation,  by  the  disobedience  of  the  parts  of  the  mouth 
which  should  form  it  to  the  will  or  power  of  association,  but  where  the  spas- 
modic interruption  occurs  altogether  behind  or  beyond  the  mouth,  viz.,  in 
the  glottis,  so  as  to  affect  all  the  articulations  equally.  To  a  person  ignorant 
of  anatomy,  and  therefore  knowing  not  what  or  where  the  glottis  is,  it  may 
be  sufficient  explanation  to  say,  that  it  is  the  slit  or  narrow  opening  at  the 
top  of  the  windpipe,  by  which  the  air  passes,  to  and  from  the  lungs,  being 
situated  just  behind  the  root  of  the  tongue.  It  is  that  which  is  felt  to  close 
suddenly  in  hiccup,  arresting  the  ingress  of  air,  and  that  which  closes,  to 
prevent  the  egress  of  air  from  the  chest  of  a  person  lifting  a  heavy  weight  or 
making  any  straining  exertion ;  it  is  that,  also,  by  the  repeated  shutting  of 
which  a  person  divides  the  sound  in  pronouncing  several  times,  in  distinct 
and  rapid  succession,  any  vowel,  as  o,  o,  o,  o.  Now  the  glottis,  during 
common  speech,  needs  never  to  be  closed,  and  an  ordinary  stutterer  is  instant- 
ly cured,  if,  by  having  his  attention  properly  directed  to  it,  he  can  keep  it  open. 
Had  the  edges  or  thin  lips  of  the  glottis  been  visible,  like  the  external  lips  of 
the  mouth,  the  nature  of  stuttering  would  not  so  long  have  remained  a  mystery, 
and  the  effort  necessary  to  the  cure  would  have  been  suggested  to  the  most 
careless  observer  :  but  because  they  were  hidden,  and  professional  men  had 
not  detected  in  how  far  they  were  concerned,  and  the  patient  himself  had 
only  a  vague  feeling  of  some  difficulty,  which,  after  straining,  grimace,  gesti- 
culation, and  sometimes  almost  general  convulsions  of  the  body,  gave  way, 
the  uncertainty  with  respect  to  the  subject  has  remained.  Even  many  per- 
sons who,  by  attention  and  much  labor,  had  overcome  the  defect  in  themselves, 
as  Demosthenes  did,  have  not  been  able  to  describe  to  others  the  nature  of 


THE    VOICE    AND    SPEECH.  459 

their  efforts,  so  as  to  ensure  imitation  ;  and  evidently  the  quacks  who  have 
succeeded  in  relieving  many  cases,  but  in  many  also  have  failed,  or  have 
given  only  temporary  relief,  have  not  really  understood  what  precise  end  in 
the  action  of  the  organs  their  imperfect  directions  were  accomplishing. 

Now,  a  stutterer,  understanding  of  anatomy  only  what  is  stated  above,  will 
comprehend  what  he  is  to  aim  at,  by  being  farther  told,  that  when  any  con- 
tinued sound  is  issuing  from  his  mouth,  as  when  he  is  humming  a  single  note 
or  tune  the  glottis  is  necessarily  open,  and  therefore,  that  when  he  chooses 
to  begin  pronouncing  or  droning  what  we  have  already  described  to  be  the 
simplest  of  vocal  sounds,  namely  the  vowel  e,  and  in  its  less  distinct  modifi- 
cation, as  heard  in  the  English  word  certain  or  in  the  French  word  que  (to 
do  what  at  once  no  stutterer  has  difficulty,)  he  thereby  opens  the  glottis,  and 
renders  the  pronunciation  of  any  other  sound  easy: — or  if,  when  speaking  or 
reading,  he  joins  his  words  together,  nearly  as  a  person  joins  them  in  singing, 
(and  this  may  be  done  without  its  being  at  all  noted  as  a  peculiarity  of  speech, 
for  many  persons  do  it  in  their  ordinary  conversation,)  the  voice  never  stops, 
the  glottis  never  closes,  and  there  is  of  course  no  stutter.  The  author  has 
given  merely  this  explanation  or  lesson,  with  examples,  to  persons  who  be- 
fore would  have  required  half  an  hour  to  read  a  page,  but  who  immediately 
afterwards  read  it  quite  smoothly;  and  who  then,  on  transferring  the  lesson 
to  the  speech,  by  continued  practice  and  attention,  obtained  the  same  facility 
with  respect  to  it.  There  are  many  persons  not  accounted  peculiar  in  their 
speech,  who  in  seeking  words  to  express  themselves,  or  while  coming  to  a 
decision,  often  rest  between  their  words  on  the  simple  sound  of  e  mentioned 

above,  saying,  for  instance,  hesitatingly,  "  e.  .  .  .  .  .  I  e think 

e I  shall,"  the  sound  never  ceasing  until  the  end  of  a  sentence, 

however  long  it  may  be  delayed.  Now  a  stutterer,  who,  to  open  his  glottis 
at  the  begiuing  of  a  phrase,  or 'in  the  middle  after  any  interruption,  uses 
such  a  sound,  would  not  even  at  first  be  more  remarkable  than  a  drawling 
speaker,  and  he  would  only  require  to  drawl  for  a  little  while,  until  practice 
facilitated  his  command  of  the  other  sounds.  Although  producing  the  simple 
sound  mentioned  is  a  means  of  opening  the  glottis,  which  by  stutterers  is 
found  very  generally  to  answer,  there  are  cases  in  which  such  other  means 
may  be  more  suitable,  as  the  intelligent  preceptor  will  soon  discover. — Were 
it  possible  to  divide  the  nerves  of  the  muscles  which  close  the  glottis,  with- 
out at  the  same  time  destroying  the  faculty  of  producing  voice,  such  an 
operation  would  be  an  immediate  and  certain  cure  of  stuttering. 

While  the  spasmodic  closure  of  the  glottis,  as  above  described,  is  the  com- 
mon cause  of  stuttering,  there  are  also  cases  in  which  the  cause  is  a  spasmodic 
prolongation  of  some  of  the  aspirates  or  semivowel  sounds,  as  of  s  m  I,  &c., 
Fortunately,  however,  the  substitution  of  the  simple  sound  is  equally  the 
cure  for  all. 

While  the  cure  of  many  stutterers  has  been  accomplished  by  their  own 
efforts,  after  the  study  of  what  is  written  in  this  section,  for  others,  and  par- 
ticularly for  young  people,  the  following  have  been  found  to  be  farther  useful 
rules  or  forms  of  direction  ;  and  a  commentary  upon  them  making  them 
perfectly  intelligible,  would  seem  to  comprehend  all  that  can  be  communicated 
upon  the  subject — 1.  Familiarize  yourself  with  the  idea  of  a  continued  sound 
as  of  the  roar  of  the  sea  or  waterfall,  or  the  note  of  an  organ-pipe,  and  feel 
that  your  speech  is  to  be  as  uninterrupted. — 2d.  Then  never  stutter  more,  but 
substitute  always  the  simple  continued  sound  for  any  threatened  defect,  and 
rest  upon  it  until  power  be  felt  to  overcome  the  difficulty. — 3d.  Never  repeat 
words  or  syllables. — 4.  The  simple  sound  must  become  the  first  syllable 


460  FLUIDITY    IN    RELATION    TO    ANIMALS. 

(closely  joined)  of  every  difficult  word,  until  the  morbid  habit  be  weakened. 
The  object  of  all  these  directions  is  to  enable  the  patient,  first  to  substitute 
universally  the  drawl  for  the  stutter,  and  then;  as  soon  as  possible,  to  discard 
the  drawl  too. 

The  view  given  above  of  the  nature  of  stuttering  and  its  cure,  explains  the 
following  facts^  which  to  many  persons  have  hitherto  appeared  extraordinary. 
— Stutterers  often  can  sing  well,  and  without  the  least  interruption  j  for  the 
tune  being  continued  the  glottis  does  not  close. — Many  stutterers  also  can 
read  poetry  well,  or  any  declamatory  composition,  in  which  the  uninterrupted 
tone  is  almost  as  remarkable  as  jn  singing. — A  person  who  draws  a  deep 
breath  before  beginning  to  speak,  as  he  cannot  long  retain  the  air,  and  the 
glottis  must  be  open  to  let  it  escape,  is  to  a  degree  secured  against  the  occur- 
rence of  stuttering.  The  secret  remedy  of  an  American  quack,  who  years 
ago  got  much  money  from  Englishmen,  was  the  direction  thus  to  fill  the  chest 
before  beginning  to  speak.  A  Dr.  McCormac,  also,  who  published  a  work 
on  this  subject,  founded  on  the  erroneous  idea,  that  stuttering  was  an  effort 
to  speak  while  inhaling  air,  instead  of  while  exhaling,  gave  the  same  direc- 
tion.— The  cause  of  stuttering  being  a  weak  and  easily  disturbed  association 
of  certain  muscular  actions,  we  have  the  reason  why  any  degree  of  anxiety 
or  dread  as  to  speaking  well,  exceedingly  increases  the  defect ;  and  why  many 
stutterers,  who  cannot  make  themselves  intelligible  in  society,  still,  when 
alone  can  speak  and  read  as  perfectly  as  any  other  person.  This  explains 
also  why  many  stutterers,  who  have  gone  to  live  for  a  time  at  the  houses  of 
pretended  curers  of  their  defect,  have  felt  themselves  singularly  relieved  from 
the  moment  of  enteringthe  house;  because  knowing  that  they  were  expected 
to  speak  ill,  they  had  no  fear  of  disagreeably  attracting  attention,  and  therefore 
had  their  powers  much  more  at  command.  These  persons,  on  returning  to 
the  world,  have  generally  stuttered  as  badly  as  ever,  but  many  of  the  asserted 
cures  of  stuttering,  with  certificates  obtained  from  the  parties  at  the  time  have 
been  of  the  nature  now  described. — The  cause  of  stuttering  being  so  simple, 
as  above  described,  one  rule  given  and  explained  may,  in  certain  cases,  in- 
stantly cure  the  defect,  however  aggravated,  as  has  been  observed  in  not  a 
few  instances;  and  this  explains  also  why  an  ignorant  pretender  may  occa- 
sionally succeed  in  curing,  by  a  given  rule  of  which  he  knows  not  the  reason, 
and  which  he  cannot  modify  to  the  peculiarities  of  other  cases. — The  same 
view  of  the  subject  explains  why  the  speech  of  a  stutterer  has  been  correctly 
compared  to  the  escape  of  liquid  from  a  bottle  with  a  long,  narrow  neck, 
coming — "  either  by  hurried  gushes  or  not  at  all :"  for  when  the  glottis  -is 
once  opened,  and  the  stutterer  feels  that  he  has  the  power  of  utterance,  he  is 
glad  to  hurry  out  as  many  words  as  he  can,  before  the  interruption  recurs. 

The  study  of  the  table  of  articulations  lead  to  the  immediate  correction  of 
many  minor  defects  in  utterance,  and  is  calculated  to  facilitate  the  acquire- 
ment of  foreign  languages.  A  lisping  person,  for  instance,  is  cured  at  once, 
by  being  told  that  the  tongue  must  not  touch  the  teeth  in  pronouncing  the 
letter  S;  and  a  Frenchman  who  deems  it  impossible  for  him  to  pronounce 
the  English  sound  of  TH,  discovers  that  he  cannot  avoid  doing  so  if  he  rests 
his  tongue  softly  against  his  teeth,  opened  a  little,  and  then  forces  breath  or 
sound  to  pass  between  the  tongue  and  teeth. 

Several  of  the  modern  languages  of  Europe  consist  of  nearly  the  same  ele- 
mentary or  radical  words,  and  differ  among  themselves  chiefly  by  the  preva- 
lence in  each  of  certain  terminations  and  of  one  or  other  of  the  related  and 
convertible  sounds  classified  in  the  analysis  given  above.  A  student,  there- 
fore, who,  by  analytical  investigation,  or  considerable  practice,  has  become 


THE    VOICE    AND    SPEECH.  461 

impressed  with  the  peculiar  genius  of  a  language,  may  invent,  or  determine 
by  analogy,  even  "before  minute  study,  the  majority  of  those  words  belonging 
to  each  which  have  sprung  from  a  common  origin.  This  remark  is  so  true 
with  respect  to  the  languages  of  Italy,  Spain,  Portugal,  and  even  France,  that 
to  persons  familiar  with  them,  they  are  at  last  listened  to  rather  as  the  same 
language  spoken  by  different  individuals,  than  as  languages  in  themselves 
different. 

Ventriloquism  is  the  name  commonly  given  to  the  art  by  which  an  indi- 
vidual can  assume  characters  of  voice  and  speech  which  are  not  natural  to 
him,  and  thus,  although  alone,  can  imitate  closely  a  conversation  hfila 
between  two  or  more  persons. 

The  most  remarkable  diversity  is  obtained  by  speaking  during  inspiration, 
instead  of,  as  usual,  during  expiration.  The  voice  so  produced  is  more  feeble 
than  the  ordinary  voice,  and  when  accompanied  by  other  circumstances  fa- 
vouring the  illusion,  it  may  suggest  very  completely  the  idea  of  a  boy  calling 
from  the  bottom  of  a  pit,  or  from  the  interior  of  a  chimney,  &c.  An  unsus- 
pecting peasant  may  be  tricked  into  unloading  his  hay-wagon  by  an  expert 
ventriloquist,  who  makes  him  believe  that  there  is  a  poor  child  packed  under 
the  heap  and  ready  to  be  smothered  there. 

A  person,  by  a  little  practice,  may  acquire  the  power  of  producing,  with- 
out the  slightest  apparent  motion  of  the  lips  or  countenance,  all  the  articu- 
lations except  the  labial,  and  of  them  the  F,  V  and  M  may  be  tolerably  imi- 
tated by  parts  behind ;  hence  by  avoiding  words  in  which  P  and  B  occur, 
such  person  may  speak  without  visible  movements  of  the  organs,  and  if  he 
assume  the  attitude  of  a  listener,  he  may  make  the  deception  of  ventriloquism 
complete.  The  idea  which  some  authors  have  had  (see  Good's  Study  of 
Medicine,  <£c.)  that  the  articulations  of  the  ventriloquist  are  not  produced  by 
the  tongue  and  mouth,  as  in  common  speech,  is  altogether  an  error.  The  art, 
carried  to  a  certain  degree,  is  not  very  difficult,  as  any  person  may  ascertain 
who  tries  it  after  considering  minutely  the  nature  of  common  speech. 

There  are  also  striking  varieties  of  voice  producable  by  speaking  with  a 
more  acute  or  grave  pitch  than  usual,  and  with  different  degrees  of  contrac- 
tion of  the  mouth  ;  but  these  may  be  more  properly  called  imitations  than 
ventriloquism. 

The  variety  of  effect  in  sound  which  the  human  organs  are  capable  of  pro- 
ducing is  truly  surprising.  There  are  adepts  in  the  art  of  imitations,  who 
not  only  mimic  the  speech  of  all  ages  and  condition  of  the  human  race,  but 
the  songs  of  birds,  the  cries  of  animals,  and  even  not  a  few  of  the  sounds  pro- 
ducing among  inanimate  things.  Many  of  these  performances  become  in  the 
highest  degree  ludicrous,  and  furnish  favourite  amusements  in  our  theatres. 
A  Mr.  Henderson,  of  London,  about  the  end  of  the  eighteenth  century,  used 
to  kill  his  calf,  as  he  called  it,  to  crowded  houses  every  night.  After  drop- 
ping a  screen  between  him  and  the  audience,  he  caused  to  issue  from  behind 
it  all  the  sound,  even  to  the  minutest  particular,  which  may  be  heard  while 
a  calf  is  falling  a  victim  in  the  slaughter-house; — the  conversation  of  the 
butchers,  the  struggling  and  bellowing  and  quick  breathing  of  the  frightened 
animal,  the  whetting  of  the  knife,  the  plunge,  the  gush,  the  agony ; — and, 
revolting  as  the  occasion  is  in  itself,  the  imitation  was  so  true  to  nature,  that 
thousands  eagerly  went  to  see  the  art  of  the  mimic. 

The  following  cases  of  inanimate  sound  may  be  closely  imitated  by  the 
mouth :  The  working  of  a  grindstone,  including  the  noise  of  the  water  into 
which  it  dips,  the  rough  attrition  of  the  steel  upon  it,  and  the  various  changes 
occuring  with  the  change  of  pressure; — the  working  of  a  saw  cutting  wood; 


462  FLUIDITY    IX    RELATION    TO    ANIMALS. 

— the  uncorking  of  a  bottle,  and  the  gurgling  noise  of  decanting  its  contents; 
— the  sound  of  air  rushing  into  a  room  in  a  winter  night  by  a  crevice  or 
key-hole — and  many  others. 

It  has  already  been  explained,  that  voice  depends  on  the  vibration  of  tho 
two  edges  or  lips  of  the  slit-like  opening  of  the  glottis,  by  which  the  air 
passes  to  and  from  the  chest.  The  number  of  vibrations  in  a  given  time,  or 
the  pitch  of  voice  depends,  of  course,  on  the  length  and  tension  of  these 
edges.  The  length  is  varied  by  the  position  sof  the  arytrenoid  cartilages,  and 
the  tension  by  the  action  of  small  muscles  which  act  on  these ;  and  the 
cavity  of  the  mouth  is  enlarged  or  lessened  to  accord  with  the  number  of 
vibrations,  by  the  rising  or  falling  of  the  tongue  and  larynx  which  form  its 
bottom.  The  peculiarities  of  individual  voices  must  depend  chiefly  on  the 
size  and  firmness  of  the  cartilaginious  box  of  the  larynx,  the  strength  of  the 
muscles  of  the  chest  which  force  the  air  through  the  glottis,  and  the  pliancy 
of  the  moving  parts. 

The  glottis  is  smaller  in  women  than  in  men,  and  hence  their  pitch  of 
voice  is  higher : — with  reference  to  music,  the  difference  is  generally  of  an 
octave,  or  eight  notes. 

The  voice  of  a  boy,  in  regard  to  pitch,  is  generally  the  same  as  that  of  a 
woman  ;  but  at  the  age  of  puberty,  the  sounding  organs  in  the  male  enlarge 
suddenly,  and  render  the  voice  stronger  than  before,  and  by  nearly  an  octave 
graver.  The  voice  of  a  eunuch  is  the  voi-cc  of  the  boy  continued,  because 
the  change  called  puberty  does  not  take  place  in  him. 

Complete  loss  of  voice,  for  longer  or  shorter  periods,  is  often  experienced 
by  persons  while  in  feeble  states  of  health.  The  vibrating,  and,  therefore, 
sounding  edges  of  the  glottis,  which  are  usually  kept  tense  by  the  operation 
of  certain  muscles,  on  these  ceasing  to  act,  owing  to  the  state  of  their  nerves, 
will  not  vibrate  as  required,  and  the  voice  is  lost.  Slight  colds  suffice  in 
many  people  to  produce  this  effect :  in  others  of  morbidly  sensitive  or  deli- 
cate nervous  temperament,  it  follows  fatigue,  or  any  other,  cause  of  debility. 
Articulation  is  not  destroyed  by  loss  of  voice;  and  whispering  answers 
passably  the  end  of  vocal  speech. 

No  intelligent  mind  can  meditate  on  human  speech  and  its  influence  in  the 
world,  without  being  roused  to  vivid  admiration.  But  for  speech,  the  most 
gifted  individuals  who  have  lived,  had  they  existed  at  all,  could  have  been 
little  superior  in  their  worldly  state  to  the  leading  oxen  of  our  herds,  or  to 
leading  monkeys  in  the  woods.  As  regarded  the  rest  of  mankind,  Homer 
and  Newton  would  have  lived  in  vain.  At  the  present  day,  among  the 
natives  of  Australasia,  where  language  may  be  said  scarcely  yet  to  exist, 
human  nature  is  seen  thus  brutishly  debased;  while,  on  the  other  hand,  in 
the  history  of  the  world,  we  may  trace,  as  a  consequence  of  more  perfect 
speech,  all  the  progress  which  has  been  made  in  arts  and  civilization.  By 
language,  fathers  have  communicated  their  gathered  experience  and  reflec- 
tions to  their  children,  who  in  their  turn  become  fathers,  have  transmitted 
them  to  succeeding  children,  with  new  accumulation;  and  when,  in  the 
course  of  ages,  the  precious  store  had  increased,  until  mere  memory  could 
retain  no  more,  the  art  of  writing  arose,  making  language  visible  and  per- 
manent, and  enlarging  without  limit  the  receptacles  of  knowledge;  and  then 
the  art  of  printing  came,  which  now  rolls  the  still  swelling  flood  into  every 
hamlet  and  every  hut.  Language  thus,  at  the  present  moment  of  the  world's 
existence,  may  be  said  to  bind  the  whole  human  race  of  uncounted  millions 
into  one  gigantic  rational  being,  whose  memory  reaches  to  the  beginnings 
of  written  record,  and  retains  imperishably  the  important  events  that  have 


THE    DIGESTION.  463 

occurred ;  wholl  judgment,  analysing  the  treasures  of  memory,  has  already 
discovered  many  of  the  sublime  and  unchanging  laws  of  nature,  and  has 
built  on  them  the  arts  of  life,  and  through  them,  piercing  far  into  futurity, 
sees  distinctly  many  events  that  are  to  come;  and  whose  eyes,  and  ears,  and 
observant  mind  are,  at  this  moment,  in  every  corner  of  the  earth,  watching 
and  recording  new  phenomena,  for  the  purpose  of  still  better  comprehending 
the  magnificence  and  simplicity  and  beauty  of  creation. 


THE   DIGESTION. 

The  doctrines  of  fluidity,  illustrating  and  illustrated  by  certain  phenomena 

of  digestion. 

The  animal  body  may  be  seen  at  first,  in  the  maternal  ovary,  as  a  single, 
speck  of  mucus ;  but  from  possessing  life — wonderful  life — the  little  nucleus, 
placed  in  new  circumstances,  begins  to  gather  itself  substance  from  around, 
and  it  increases  in  bulk.  For  a  certain  time  it  remains  attached  to  the  body 
of  its  parent,  and  draws  the  material  of  its  increase  from  its  parent's  blood ; 
but  after  that  time  it  is  alone  and  entirely  dependent  on  its  own  resources. 
Then  we  see  brought  into  play  that  extraordinary  apparatus  now  to  be 
described  under  the  name  of  the  digestive  or  assimilating  organs  ;  which, 
under  the  direction  of  a  nervous  energy,  can,  out  of  almost  any  kind's  of  dead 
animal  or  vegetable  matter  build  up  the  beautiful  living  body  to  perfect 
maturity  of  size,  and  form,  and  faculty.  And  it  is  not  only  while  their 
bodies  are  growing  that  animals  require  to  take  in  and  assimilate  new  matter, 
but  also  after  maturity,  in  order  to  repair  the  waste  of  constant  action. 
Supply  of  fuel  and  water  to  the  steam-engine  is  not  more  necessary  than  of 
aliments  to  the  living  body. 

Some  of  the  less  perfect  animals  take  in  sustenance  almost  like  vegetables, 
by  absorbent  tubes  that  open  on  their  surface;  but  by  far  the  greater  part 
receive  it  first  into  an  interior  cavity,  where  it  undergoes  certain  preparation, 
and  is  then  offered  to  internal  absorbents,  which  drink  up  what  is  required 
and  carry  it  into  the  circulating  blood.  This  internal  cavity  is  called  a 
stomach.  Its  form  and  appendages  differ  exceedingly  in  different  animals, 
according  to  the  nature  of  the  substances  which  serve  for  their  sustenance, 
and  to  various  other  circumstances. 

In  man,  the  process  of  digestion  has  the  following  steps.  The  food  is 
first  received  by  the  mouth.  It  is  there  broken  or  torn  into  small  portions 
by  the  cutting  and  grinding  wedges,  called  teeth,  with  which  the  Jaws  are 
armed;  at  the  same  time  a  fluid  called  saliva  is  mixed  with  it,  poured  out 
from  glands  around,  so  as  to  reduce  it  into  a  pulpy  mass :  this  mass  is  then 
pushed  backwards  by  the  tongue  to  enter  the  long  tube  called  the  gullet  or 
oesophagus,  which  by  successive  contraction  of  circular  fibres,  propels  it 
down  to  the  pouch  of  the  stomach,  placed  under  the  edge  of  the  left  ribs. 
From  the  internal  surface  of  the  stomach  a  liquor  oozes,  called  the  gastric 
juice,  the  most  general  solvent  in  nature,  and  which,  attacking  the  received 
food,  soon  reduces  it,  of  whatever  kind,  to  the  state  of  a  pultaceous  mass, 
named  cliyme ;  in  this  state  it  enters  the  narrow  intestinal  canal  which  is 
continued  from  the  stomach,  where  it  almost  immediately  receives  a  mixture 
of  bile  and  pancreatic  juice  poured  out  from  the  liver  and  pancreas.  After 
this  mixture,  as  it  gradually  passes  on,  a  chemical  decomposition  and  separa- 
tion of  parts  takes  place,  and  the  pure  nutriment  of  the  body  assumes  the 


464  FLUIDITY    IN    RELATION     TO    ANIMALS. 

state  of  a  milky  fluid  floating  among  refuse.  This  milky  fliHd,  called  chyle, 
is  taken  up  all  along  the  canal  by  the  numberless  absorbent  mouths  of  the 
vessels  called  lacteah,  and  is  then  carried  to  the  thoracic  duct,  and  by  it  into 
the  blood  to  supply  the  waste.  The  intestinal  canal  is  about  six  times  as 
long  as  the  body,  affording,  therefore,  a  very  extensive  surface  from  which 
absorbtion  may  take  place.  That  remnant  of  the  chyme  which  the  absorb- 
ents refuse,  mixed  with  various  depositions  or  secretions,  continues  its 
journey  onwards,  and  is  periodically  discharged. 

Much  of  the  process  which  we  have  now  described  is  mechanical,  as  will 
appear  immediately;  other  parts  of  it  are  chemical,  such  as  the  solution  of 
the  food  by  the  gastric  juice,  the  separation  of  the  milky  chyle,  &c.;  and 
parts  are  vital,  such  as  the  afflux,  just  when  wanted,  of  saliva,  gastric  juice, 
bile,  &c.,  and  the  muscular  and  absorbent  actions.  He  who  neglects  the 
study  of  any  one  of  these  three  classes  of  particulars,  must  have  a  very 
incomplete  acquaintance  with  the  function. — We  proceed  now  to  explain  the 
mechanical  or  physical  circumstances  connected  with  digestion. 

The  abdomen  may  be  considered  as  a  vessel  full  of  liquid,  in  which,  there- 
fore, there  is  pressure  in  all  directions,  increasing  with  the  depth,  (see 
hydrostatics,)  and  increased  also  by^the  action  of  the  surrounding  muscles 
which  form  the  sides  of  the  cavity. 

The  justness  of  this  view  of  the  abdomen  becomes  evident,  when  we  con- 
sider that  only  moistened  or  semifluid  food  descends  into  the  stomach,  that 
drink  follows,  and  that  gastric  and  other  juices  are  poured  out  to  mix  with 
the  food  as  it  passes  on  to  occupy  the  long  intestinal  canal;  and  that  then 
the  intestines  externally  are  perfectly  smooth,  and  are  moistened  by  the 
constant  secretion  of  lubricating  serum,  so  that  they  slide  among  each  other, 
without  sensible  impediment  from  friction.  The  abdomen,  therefore,  is  in 
fact  a  roundish  smooth  vessel  filled  with  a  thick  fluid,  which  is  farther  con- 
tained in  a  perfectly  pliant  and  smooth-coated  tube. 

Thus  any  part  of  the  contents  of  the  stomach  and  bowels,  in  a  living  man. 
is  supported  like  water  in  surrounding  water,  and  therefore,  if  the  whole 
contents  be  of  equal  specific  gravity,  no  part  can  descend  or  advance  by  its 
weight.  Neither  can  any  general  pressure,  nor  contraction  of  the  surround- 
ing parietes,  hasten,  except  at  the  moment  of  expulsion,  the  motion  of  any 
contained  matter— as  has,  however,  often  been  supposed ;  nor  can  it  help  to 
empty  one  part  into  another — the  stomach,  for  instance,  or  the  gall-bladder, 
into  the  small  intestine. 

For  the  same  reason,  however,  the  very  slightest  contractile  action  of  any 
containing  part  is  sufficient  to  dislodge  its  contents — gravity  as  a  resistance 
being  neutralized  by  the  surrounding  fluid.  And  when  the  gall-bladder,  or 
stomach,  or  any  part  of  the  intestinal  tube,  becomes  so  full  as  to  put  the 
elasticity  of  the  coats  ever  so  little  upon  the  stretch,  this  circumstance  alone, 
unless  some  muscular  action  oppose,  will  cause  a  discharge  of  the  contents. 
— The  natural  action  of  the  intestinal  canal  is  a  successive  contraction  of  its 
circular  fibres  from  above  downwards  propelling  the  contents,  just  as  if  a 
small  ring  or  tube  were  put  round  the  canal  and  pushed  forwards. 

These  considerations  make  evident  the  common  error  of  supposing  that 
vomiting  can  by  the  sudden  compression  of  the  abdominal  viscera  mechani- 
cally emulge  or  clear  the  obstructed  biliary  ducts.  If  general  pressure  of  the 
abdomen  could  produce  this  and  similar  effects,  a  descent  in  the  diving-bell 
should  be  a  powerful  remedy  in  human  maladies;  for  nearly  fifteen  pounds 


THE    DIGESTION.  465 

on  the  inch  are  added  to  the  ordinary  abdominal  pressure,  at  a  depth  of 
thirty  feet  in  water. 

We  hence  see  also  the  kind  of  error  into  which  our  predecessors  fell  so 
generally,  when  they  attributed  much  of  the  digestive  power  of  the  stomach 
to  its  simple  pressure  upon  the  food.  The  idea  probably  arose  from  the 
contemplation  of  the  stomach  or  gizzard  of  a  fowl,  which  is  a  powerful  gristly 
substance,  answering  the  purpose  almost  of  a  mouth  and  teeth,  as  well  as  of 
a  stomach. 

It  is  an  error  also  to  suppose  that  quicksilver,  which  is  sometimes  swal- 
lowed to  remove  obstruction,  runs  through  the  bowels  simply  by  its  weight. 
On  first  entering  the  loose  small  intestine,  it  must  drag  the  part  containing 
it  to  the  bottom  of  the  abdomen,  and  in  that  situation,  the  whole  intestine 
must  pass  round  it,  nearly  as  a  rope  passes  through  a  ring  fixed  to  the  floor. 
"When  the  mercury  arrives  at  the  part  of  the  intestine  called  the  caecum, 
where  the  farther  course  lies  upward  along  the  fixed  arch  of  the  colon,  it  pro- 
bably can  be  dislodged  only  by  the  patient's  lying  down.  Any  useful  opera- 
tion of  quicksilver  in  such  cases,  may  be  in  its  stimulating  the  bowels,  by 
dragging  or  displacing  them,  in  the*manner  above  described. 

When  the  abdominal  muscles,  which  are  the  containing  sides  of  the  cavity, 
become  tense,  whether  from  unusual  fulness  of  the  cavity,  or  from  their 
own  action  in  any  of  the  straining  exertions,  a  variety  of  important  me- 
chanical effects  ensue.  Thus,. 

A  full  stomach  produces — tension  and  projection  of  the  belly — projection 
of  the  diaphragm  into  the  chest,  causing  hurried  breathing,  and  impeding 
speech  and  singing — expulsion  of  blood  from  the  abdominal  vessels,  aftd, 
therefore,  congestions  elsewhere,  as  in  the  arteries  of  the  head,  sometimes 
producing  apoplexy. 

Abdominal  fulness,  as  in  dropsy,  tympanitis,  corpulency,  pregnancy,&c., 
produces  most  of  the  effects  now  mentioned  in  an  aggravated  degree.  If 
dropsy  be  allowed  to  proceed  too  far  without  tapping,  the  patient  will  die  of 
suffocation  from  the  rise  of  the  diaphragm. — The  external  veins  of  the  legs 
and  abdomen  of  a  dropsical  person  are  generally  turgid,  because  the  blood  is 
pressed  into  them  out  of  the  abdominal  cavity,  and  because  the  passage  of 
blood  through  the  abdomen  is  impeded.  In  tympanitis,  or  windy  dropsy, 
as  it  has  been  called,  the  viscera  hang  down  in  the  abdominal  cavity,  while 
the  air  occupies  the  upper  part.  In  common  dropsy  the  viscera  float  about 
and  are  supported. 

Straining  or  strong  action  of  the  abdominal  muscle,  and  therefore,  also 
pressure  on  the  abdominal  contents,  occur  with  almost  every  considerable 
bodily  exertion  ;  for  the  abdominal  muscles  are  the  antagonists  of  the  great 
muscles  on  the  back  and  about  the  spine,  and  must  always  come  into  play 
with  them  to  give  firmness  and  rigidity  to  the  trunk  of  the  body..  This 
may  be  seen  remarkably  in  the  actions  of  lifting,  running,  wrestling,  &c. 
As  the  abdominal  muscles  cannot  act  in  a  continued  way  and  strongly,  unless 
the  ribs,  from  which  they  arise,  become  nearly  fixed,  the  ribs  are  supported 
during  exertion  by  the  intercostal  muscles,  and  by  the  air  in  the  chest,  then 
confined  by  the  closure  of  the  air-passages  :  hence  there  is  generally  compres- 
sion in  the  chest  also  when  the  abdomen  is  compressed,  and  the  blood  is 
squeezed  towards  the  extremities  from  both  cavities  at  once.  It  is  important 
to  remark  also,  that  in  what  are  called  the  strong  actions  of  the  chest,  as 
coughing,  sneezing,  blowing,  &c.,  the  abdominal  muscles  are  at  least  as 

30 


466  FLUIDITY    IN    RELATION    TO    ANIMALS. 

active  as  the  pectoral :  by  pulling  down  the  ribs  to  which  they  are  attached, 
they  narrow  the  chest,  and  by  compressing  the  abdominal  contents,  and  thus 
raising  up  the  diaphragm,  they  shorten  the  chest. 

The  following  cases  exemplify  the  effects  of  straining  — The  lifting  of  a 
great  weight,  or  making  any  great  exertion,  drives  the  blood  up  to  the  head  ; 
as  is  marked  by  the  sudden  redness  of  the  face. — Coughing  or  vomiting  will 
cause  closed  leech-bites  to  bleed  afresh,  and  sometimes  will  overcome  the 
action  of  the  sphincter  of  the  bladder  or  rectum  :  coughing  will  also  produce 
vomiting, — Straining  to  empty  the  bladder,  rectum,  or  womb,  or  the  effort 
of  vomiting,  will  cause  the  rupture  of  a  blood-vessel  in  the  white  of  the  eye, 
with  consequent  effusion  of  blood  there. — Apoplexy  often  happens  under 
the  same  circumstances,  from  the  breaking  of  a  vessel  in  the  brain. — The 
rupture  of  a  varicose  vein,  or  of  aneurism,  generally  happens  during  exer- 
tion.— And  during  exertion,  the  protrusion  is  likely  to  occur  at  any  weak 
part  of  the  abdominal  cavity,  of  some  portion  of  its  contents,  producing  what 
is  called  hernia  or  rupture. 

Vomiting  is  produced,  not  by  the  forcible  contraction  of  the  stomach,  as 
was  long  supposed,  but  chiefly  by  the  action  of  the  parietes  of  the  abdomen. 
This  is  proved  by  the  fact  that  the  stomach  has  been  removed  from  a  living 
animal,  and  a  sheep's  bladder  containing  liquid  has  been  substituted  for  it, 
in  connection  with  the  gullet  above  and  the  intestines  below ;  and  on  then 
injecting  an  emetic  drug^into  the  veins  of  the  animal  vomiting  has  taken 
place,  as  if  the  stomach  had  been  there  and  unhurt.*  From  this  we  see  why, 
to  prevent  regurgitation  of  the  food,  during  exertion,  the  upper  orfice  of  the 
stomach  requires  to  be  almost  as  strongly  closed  as  the  sphincters  below. 

A  small  pump — in  this  application  called  the  stomach-pump — has  lately 
been  used  in  medical  practice,  for  removing  poisons  from  the  stomach  in 
cases  where  the  action  of  vomiting  could  not  be  excited.  It  has  already  saved 
many  lives.  It  resembles  the  common  small  syringe,  except  that  there  are 
two  appertures  near  the  end,  instead  of  one,  which,  owing  to  the  valves  in 
them,  opening  different  ways,  become  what  are  called  a  sucking  and  a  forcing 
passage.  When  the  object  is  to  extract  from  the  stomach,  the  pump  is 
worked  while  its  sucking  orifice  is  in  connection  with  an  elastic  tube  passed 
into  the  stomach,  and  the  discharged  matter  passes  by  the  forcing  orifice. 
"When  it  is  desired,  on  the  contrary,  to  throw  the  cleansing  water  or  liquid 
into  the  stomach,  the  connection  of  the  tube  is  reversed. 

As  a  pump  may  not  be  always  procurable  when  the  occasion  for  it  arises, 
the  profession  should  be  aware  that  in  many  cases  a  simple  tube  will  answer 
the  purpose  as  well,  if  not  better.  Such  a  tube  being  introduced,  and  the 
body  of  the  patient  being  so  placed  that  the  tube  forms  a  downward  channel 
from  the  stomach,  all  fluid  matter  will  escape  from  the  stomach  by  the  tube, 
as  water  escapes  from  a  funnel  by  its  pipe  ;  and  if  the  outer  end  of  the  tube 

*  The  mechanism  of  vomiting  is  still  a  moot  point  in  physiology.  Mr.  Haighton,  a  cele- 
brated English  physiologist,  opened  several  animals  during  the  effort  of  vomiting,  and  he 
asserts  that  he  distinctly  saw  the  contractions  of  the  stomach.  The  more  recent  experiments 
of  M.  Magendie,  which  were  repeated  in  the  presence  of  a  committee  of  the  French  Insti- 
tute, are,  however,  entirely  contradictory  of  those  of  Mr.  Haighton,  and  seem  to  show  that 
the  stomach  is  entirely  quiescent  in  the  act  of  vomiting.  M.  Maingault,  nevertheless,  has 
been  led  to  results  opposed  to  those  of  M.  Magendie,  and  he  is  supported  by  Professor 
Portal  and  M.  Bourdon,  both  of  whom  appeal  to  experiments,  and  to  some  pathological 
facts,  which  are  very  imposing.  It  appears  to  us  probable  that  vomiting  is  usually  the 
joint  effect  of  the  contraction  of  the  stomach,  and  of  the  diaphragm  and  abdominal  muscles, 
though  either  is  of  itself  occasionally  sufficient  for  that  purpose. — Am.  Ed. 


THE    DIGESTION.  467 

be  kept  immersed  in  liquid,  there  will  be  during  the  discharge  a  syphon  action 
of  considerable  force.  On  then  changing  the  posture  of  the  body,  water  may 
be  poured  in  through  the  tube  to  wash  the  stomach,  and  may,  by  the  same 
channel,  be  again  discharged.  Such  a  tube,  made  long  enough,  might,  if 
desired,  be  rendered  a  complete  bent  syphon,  the  necessary  preliminary 
suction  being  produced  by  a  syringe,  or  by  an  assistant,  who  acts  through 
an  interposed  vessel. 

But  there  is  still  an  easier  mode  than  either  of  these  now  described,  of  dis- 
lodging poison  from  a  torpid  stomach,  viz.,  merely  to  place  the  patient  so 
that  the  mouth  shall  be  considerably  lower  than  the  stomach, — as  when  the 
body  lies  across  a  chair  or  a  sofa,  with  the  face  near  the  floor, — and  then,  if 
necessary,  to  press  on  the  stomach  with  the  hand.  The  cardiac  orifice  opens 
readily  in  such  a  case,  and  the  stomach  is  emptied  like  any  other  inverted 
vessel. 

Useful  as  the  pump  may  prove,  upon  occasions,  in  evacuating  the  stomach, 
its  more  ancient  office  of  injecting  the  enema  is  still  the  more  important — and 
recent  experience  seems  to  show  that  such  injection  may  become  a  remedy 
of  more  extensive  utility  than  had  yet  been  suspected.  From  an  erroneous 
opinion,  that  what  had  been  called  the  valve  of  the  csecum  acts  as  a  perfect 
valve,  allowing  passage  downwards  only,  few  practitioners  have  ventured  to 
order  much  liquid  to  be  injected,  for  fear  of  overstretching  the  lower  part  of 
the  intestine;  and  the  possibility  of  thus,  by  injection,  relieving  disease  situ- 
ated above  the  supposed  valve,  has  scarcely  been  contemplated.  It  is  now 
ascertained,  however,  that  fluid  may  be  safely  thrown  in  even  until  it  reach 
the  stomach. — Perhaps  few,  if  any  cases  of  obstruction  of  the  bowels,  could 
resist  the  gentle  force  of  penetrating  water,  so  that  a  mechanical  remedy  of 
certain  effect  may,  in  many  cases,  be  substituted  for  the  drastic  purgatives 
and  pernicious  bleedings  now  used,  and  often  used  in  vain. — From  what  has 
been  said  above  of  the  abdomen  and  the  intestinal  canal,  it  appears  that  an 
injection  tends  to  spread  itself  with  singular  uniformity  over  the  whole.  This 
tendency  may  be  rendered  obvious  to  sight  by  throwing  a  sheep's  intestine, 
recently  extracted,  into  a  bucket  of  water,  and  then  pumping  water  in  at  one 
end;  a  stream  will  issue  strongly  at  the  other  end,  although  several  feet  dis- 
tant, almost  immediately,  and  without  any  intermediate  part  having  become 
very  sensibly  tense. — Of  course,  in  the  living  body,  in  cases  of  spasms  or 
obstruction,  the  liquid  must  be  thrown  in  against  resistance  very  gradually. 

That  case  is  called  intro-susception  of  the  bowel,  in  which  an  upper  portion 
falls,  or  is  received  into  a  portion  below,  (as  one  part  of  the  finger  of  a  glove 
may  be  received  into  another  part,)  and  the  receiving  portion  of  the  bowel, 
mistaking  the  received  for  descending  food,  holds  it  fast.  This  occurrence 
forms  a  complete  obstruction,  and  generally  proves  fatal.  Many  infants  with 
irritable  bowels,  die  of  it. — Now  a  copious  enema,  such  as  we  have  described 
above,  is  almost  a  certain  cure.  The  liquid  advances  until  it  reaches  the 
part  where  the  portion  of  gut  has  been  swallowed  by  gut  below ;  and  as  it 
cannot  pass  without  pushing  the  intro-suscepted  portion  back  to  liberty,  it 
effects  the  cure.* 

The  perpetual  syringe  or  little  valved  pump,  of  wl^ch  we  have  been 
speaking  as  lately  used  in  applications  to  the  animal  body,  can  inject  or  with- 

*  It  should  be  remarked,  however,  that  this  measure  can  succeed  only  whilst  the  intro- 
susception  is  recent;  at  least  before  inflammation  has  occurred  and  adhesions  formed 
between  the  intro-suscepted  portion  and  that  portion  of  the  bowel  in  which  it  is  received. 
Common  or  atmospheric  air,  from  its  great  elasticity,  lightness,  &c.,  is  the  best  fluid  for 
the  injection. — Am.  Ed. 


468 


FLUIDITY  IN  RELATION  TO  ANIMALS. 


Fig.  175. 


Fig.  176. 


draw  any  quantity,  and  is  therefore  very  superior,  for  almost  every  purpose, 
to  the  large  old  syringes  which  had  no  valves,  and  which,  without  being 
removed  could  inject  only  once  their  fill.  With  well-adapted  additional 
apparatus,  the  same  instrument  will  answer  for  many  purposes,  as  for  throw- 
ing up  the  enema,  clearing  the  stomach,  transfusion  of  blood  ex- 
hausting cupping-glasses,  relieving  the  over-distended  breasts ; 
for  the  lotio  vesicae,  vaginae  vel  urethrse,  &c.  No  surgical 
apparatus  is  now  complete  without  one.  The  annexed  out- 
line represents  such  a  syringe.  The  aperature  c  is  rendered  a 
sucking  orifice,  by  a  valve  at  it,  opening  inwards ;  and  a  is 
the  forcing  orifice,  in  consequence  of  having  its  valve  opening 
outwards  :  b  is  the  piston,  with  its  handle.  The  valves  may 
be  variously  made,  or  a  single  double-way  cock  may  be  used 
instead  of  both.  Convenient  dimensions  for  the  syringe  are 
from  four  to  six  inches  for  the  length,  and  from  three-quarters 
of  an  inch  to  an  inch  and  a  quarter  for  the  diameter. 

For  a  case  of  diseased  rectum,  where  it  was  necessary  to 
use  an  enena  daily,  or  oftener,  the  enema  funnel,  represented 
here,  (from  the  funnel-shaped  mouth  a  downwards,  and  exclusive  of  the  part 
above  a)  was  contrived,  and  was  found  more  manageable  by  the  patient  than 
any  other  instrument.  If  the  tube  a  b  be  about  two  feet  long,  (it  may  be  of 
metal,  oiled  silk,  caoutchouc  cloth,  &c.,)  the  liquid  column  contained  in  it 
suffices  to  overcome  the  ordinary  abdominal  resistance ; 
but  if  a  very  short  tube  be  used,  the  open  funnel  a  must 
be  converted  into  a  close  vessel,  as  represented  here  by 
the  dotted  line  above  the  funnel,  having  a  bladder,  or 
other  air-tight  bag,  d,  connected  with  it,  and  a  bottle- 
neck and  cork,  or  acock, at  c,  for  admitting  the  enema. 
On  pouring  in  the  liquid  at  c,  the  air  in  the  vessel  at  c  a 
is  forced  into  the  bag,  and  on  then  closing  the  opening 
at  c,  and  compressing  the  bag,  it  is  evident,  that  any 
desired  degree  of  injecting  pressure  may  be  exerted  on 
the  enema.  This  apparatus  is  both  cheaper  and  more 
simple  than  a  syringe,  and  is  equally  effectual;  and 
the  bag  never  being  wetted,  lasts  long  :  b  is  a  cock  kept 
shut  until  the  moment  of  injection. — The  principle  of 
substituting,  in  an  injecting  apparatus,  the  pressure  of  a 
liquid  column  for  that  of  a  piston,  was  first  suggested  in 
this  work;  and  yet  since  the  publication  of  the  work, 
more  than  one  patent  has  been  taken  for  it,  for  parties 
seeking  to  convert  it  to  their  profit. 
By  viewing  the  abdomen  in  the  true  light  of  a  vessel  or  bag  filled  with 
liquid  which  is  seeking  to  escape  in  all  directions,  we  have  the  explanation 
of  several  circumstances  connected  with  hernia  or  rupture  ;  in  which  accident, 
the  containing  sides  of  the  abdomen  in  some  part  have  given  way,  allowing 
a  portion  of  the  viscera  to  escape,  so  as  to  form  a  tumour  under  the  skin. 

Hernia  may  be  produced  by  all  causes  which  strain  or  weaken  the  muscles ; 
as  by  leaping,  lifting,  great  weights,  coughing  and  sneezing,  lying  with  the 
belly  across  a  bench  or  yard,  as  sailors  do  on  ship-board,  over  distention  of 
the  belly  by  eating  or  drinking,  corpulency,  dropsy,  pregnancy;  debility  of 
muscle  from  dissipation,  &c. 

The  reason  that  a  rupture  increases  so  rapidly  after  it  has  once  begun,  is, 
that  the  protruding  part  is  truly  a  fluid  wedge,  of  which,  therefore,  the  opening 


'ci 


PELVIC    APPARATUS.  469 

force  increases  with  the  diameter.  (See  Hydrostatics.)  This  shows  the 
singular  importance  of  arresting  the  accident  at  its  very  commencement. 
The  truss  used  to  repress  rupture  were  described  at  page  412. 

In  attempting  to  return  any  part  of  the  abdominal  contents  which  may  have 
escaped  as  rupture,  we  should  recollect,  that  a  soft  uniform  compression  or 
squeezing  exerted  upon  the  tumour  by  the  hands  of  the  operator,  if  greater 
than  the  internal  pressure  of  the  abdomen,  is  slowly  pushing  back  again  any 
fluid  matter  that  can  ooze  inward  from  the  tumour ;  and  by  thus  gradually 
lessening  the  size  of  the  tumour,  may  effect  the  desired  object,  without  the 
adoption  of  the  last  resource,  of  cutting  parts  to  widen  the  inlet.  When,  in 
such  a  case,  the  operator  sees  clearly  with  the  mind's  eye  what  is  passing 
under  his  fingers,  his  efforts  may  often  be  successf^L  where  a  less  intelligent 
individual  would  fail.  No  man  practices  medicinelnong,  whatever  his  nomi- 
nal department,  without  having  opportunities  of  saving  life,  or  of  preventing 
a  serious  operation,  by  judicious  management  of  recent  hernia.  The  barba- 
rous old  fashion  of  lifting  the  patient  by  the  heels  and  shaking  him,  that  the 
weight  of  the  bowels  might  drag  back  again  the  part  which  had  escaped,  was 
founded  on  ignorance  of  the  fact,  that  the  weight  of  the  bowels  in  all  posi- 
tions of  the  body,  is  supported  almost  entirely,  not  by  their  attachments,  but 
by  the  surrounding  parts. 

The  function  of  digestion  or  assimilation  sketched  in  the  preceding  para- 
graphs, by  which  the  animal  body  assumes  foreign  matters  from  around,  and 
converts  them  into  his  own  substances,  is  a  subject  of  study  little  inviting  in 
some  of  its  details,  but  taken  altogether  is  one  of  the  most  wonderful  which 
can  engage  the  human  attention. «  It  points  directly  to  the  curious  and  yet 
unanswered  question — what  is  LIFE?  The  student  of  nature  may  analyze 
with  all  his  art  those  minute  portions  of  matter  called  seech  and  ova,  which 
he  knows  to  be  the  rudiments  of  future  creatures,  and  the  links  by  which 
endless  generations  of  living  creatures  hang  to  existence :  but  he  cannot  dis- 
entangle and  display  apart  their  mysterious  LIFE!  that  something  under  the 
influence  of  which  each  little  germ,  when  placed  in  due  circumstances,  swells 
out,  to  fill  an  invisible  mould  of  maturity  which  determines  its  forms  and  pro- 
portions. One  such  substance  thus  expands  into  a  beauteous  rose-bush  ; 
another  into  an  noble  oak;  a  third  into  an  eagle;  a  fourth  into  an  elephant — 
yea,  in  the  same  way,  out  of  the  rude  materials  of  broken  seeds  and  roots, 
and  leaves  of  plants,  and  bits  of  animal  flesh,  is  built  up  the  human  frame 
itself,  whether  of  the  active  man,  combining  gracefulness  with  strength  or  of 
the  gentler  woman,  with  beauty  around  her  as  light.  How  passing  strange, 
that  such  should  be  the  origin  of  the  bright  eye,  whose  glance  pierces  as  if 
the  invisible  soul  were  shot  with  it — of  the  lips  which  pour  forth  sweetest 
eloquence — of  the  larynx,  whose  vibrating  fills  the  surrounding  air  with 
music ;  and,  more  wonderful  than  all,  of  that  mass  shut  up  within  the  body 
fortress  of  the  skull,  whose  delicate  and  curious  texture  is  the  abode  of  the 
soul,  with  its  reason  which  contemplates,  and  its  sensibility  which  delights 
n  these  and  endless  other  miracles  of  creation. 

PELVIC  APPARATUS. 

The  Secretion  of  the  Kidneys,  &c. 

OF  the  large  quantity  of  fluid  daily  taken  into  the  human  body,  much 
escapes  with  the  breath,  as  is  provided  by  the  visible  condensation  of  it  in 
frosty  air;  or  on  any  cold  polished  surface  held  near  the  mouth ;  part  escapes 


470  FLUIDITY    IN    RELATION    TO    ANIMALS. 

by  the  skin  in  perspiration ;  but  the  greatest  part  after  having  answered  the 
purposes  of  the  constitution,  is  separated  from  the  blood  by  the  two  secreting 
organs,  called  the  kidneys,  and  from  them  by  fit  channels,  is  carried  off, 
holding  in  solution  various  other  matters,  which  the  system  does  not  require. 
The  kidneys  are  situated  in  the  loins,  one  on  each  side  of  the  spine ;  and 
the  constant  drain  of  liquid  from  them  passes  down  by  two  membranous 
canals  called  ureters  into  the  bladder,  from  which  the  liquid  is  again  expelled 
through  the  urethra,  at  considerable  intervals,  according  to  the  rapidity  of 
accumulation. 

The  bladder  is  a  curious  membranous  and  muscular  reservoir,  of  which 
the  fibres  can  contract  so  as  to  expel  the  last  drop,  and  yet  can  yield  so  as 
to  admit  a  quart  or  more. 

The  passage  of  fluid  downwards  through  the  ureters  from  the  kindeys  to 
the  bladder  resembles,  in  some  respects,  the  passage  of  blood  in  the  veins. 
Authors  have  erroneously  supposed  that  the  weight  of  the  fluid  suffices  to 
cause  its  descent :  but  the  bladder  and  ureters  are  enclosed  in  a  common 
cavity  with  the  intestinal  canal ;  and  while  this  is  full  of  a  semi-fluid  mass 
of  greater  specific  gravity  than  the  urine,  the  latter  is  not  only  supported  by 
the  surrounding  pressure,  as  water  would  be  supported  by  water,  but  is  forced 
upwards  ur  resisted,  as  water  would  be  in  honey  or  treacle  :  in  descending, 
therefore,  it  obeys  some  other  force  than  gravity,  namely,  that  of  the  secret- 
ing vessels  and  heart. 

The  ureters,  bladder  and  urethra  are  the  seats  of  some  of  the  most  distress- 
ing diseases,  to  which  the  human  frame  is  liable.  Two  classes  of  these 
being  relievable  chiefly  by  mechanical  means,  require  to  be  shortly  consi- 
dered here.  They  are,  obstructions  in  the  urethra  ;  and  concretions,  or  stones, 
as  they  are  called,  in  the  bladder. 

Obstructions  or  strictures  in  the  urethra  are  generally  consequences  of  an 
inflammation,  which  has  destroyed  the  dilatability  of  a  part  of  the  canal. 
They  appear  as  if  a  thread  or  a  bit  of  tape  were  tied  round  the  canal,  so  as 
to  narrow  its  caliber.  Constant  irritation,  which  destroys  the  general  health, 
fits  of  fever,  broken  rest,  and  even  death  from  total  suppression  of  urine, 
have  been  common  consequences  of  stricture. 

Until  within  a  recent  period,  the  treatment  of  such  obstructions  was  pur- 
sued very  generally  according  to  a  blind  routine.  The  attempt  was  made 
either  to  bore  them  open  by  wedges,  called  bougies,  often  of  doubtful  and 
tedious  operation,  or  to  destroy  them  by  caustic  passed  down  to  them  in  the 
end  of  a  bougie,  which  caustic  often  hurt  the  part  of  the  canal  anterior  to 
them,  or  eat  out  false  passages  about  the  stricture,  or  open  blood-vessels 
so  as  to  cause  dangerous  haemorrhage. 

Struck  by  the  defective  state  of  this  branch  of  the  healing  art,  the  author 
of  the  present  work,  while  abroad,  and  situated  where  he  had  interesting 
opportunities  of  observation,  had  bestowed  considerable  attention  upon  it, 
and  he  then  contrived  and  tried  several  new  means  of  relief.  These  were 
afterwards  brought  more  extensively  into  use  and  improved,  and  others  were 
added,  by  his  brother  Dr.  James  Arnott,  superintendent  surgeon  in  the  ser- 
vice of  the  Hon.  East  India  Company,  who  gave  a  minute  account  of  them 
in  a  treatise  on  urethral  diseases,  and  a  supplement  published  in  the  years 
1818  and  1820,  They  have  become,  perhaps,  still  better  known  in  Francee 
than  in  England,  through  the  work  of  Dr.  Ducamp,  which  describes  them, 
and  which,  having  been  submitted  to  the  French  Institute,  and  most  favoura- 
bly reported  upon  by  the  appointed  authorities,  soon  became  a  standard  trea- 
tise in  the  country; — in  France,  also,  the  philosophy  of  mechanics  had  been 


PELVIC    APPARATUS.  471 

studied  by  surgeons  more  generally  than  in  England.  It  is  painful  to  be 
obliged  to  add,  that  Dr  Ducamp  as  regarded  these  instruments  and  the 
views  of  disease  and  treatment  which  had  suggested  them,  concealed  the  fact 
of  his  being  only  a  translator.  The  imposition  was  not  discovered  at  the 
time  of  his  death,  which  happened  two  years  afterwards,  hastened  appa- 
rently by  the  fatigues  of  the  extensive  practice  which  the  report  of  the 
Acadamy  brought  upon  him.  The  auther  has  had  so  much  pleasing  inter- 
course with  enlightened  and  honourable  Frenchmen,  that  it  pains  him  to 
have  this  fact  to  relate. 

The  objects  aimed  at  by  the  new  means  were, — to  ascertain  the  exact  con- 
dition of  the  diseased  canal — to  facilitate  the  passing  of  instruments  in  cases 
of  difficulty — and  to  effect  a  permanent  cure.  The  following  seven  of  these 
means  may  here  be  particularized  : 

1st.  An  examining  sound  ;  being  a  bougie  with  the  point  formed  of  a  softer 
tenacious  material ;  in  which  fibres  of  cotton  or  silk  are  mixed  to  prevent 
any  portion  from  being  broken  off  or  detached  during  use.  This  sound, 
pressed  against  the  obstruction,  takes  a  correct  impression  of  its  anterior  face, 
and  shows  the  magnitude  and  exact  position  of  the  still  remaining  opening. 

2nd  An  expanding  or  dilator  sound  which  is  a  small  tube  with  a  dilatable 
Button  at  its  extremity.  The  button  consists  of  a  little  bag,  which  is  passed 
through  the  structure  empty,  and  is  filled  with  fluid  after  it  has  passed.  It 
readily  discovers  any  other  strictures  beyond  the  first;  and;  to  a  certain  degree, 
the  state  of  each. 

3d.  A  conducting  canula  or  tube,  open  at  both  ends.  It  is  passed  down 
to  the  stricture,  for  the  purpose  of  supporting  and  directing  small  bougies 
seeking  entrances  through  very  narrow  strictures,  or  of  guarding  the  caustic 
bougie  in  its  approach  to  the  place  of  its  action. 

4th.  In  cases  where  the  attempt  to  open  the  passage  has  failed  by  all  com- 
mon means,  a  conducting  tube  is  first  introduced,  and  through  it  six  or  more 
small  bougies  are  passed  side  by  side,  so  as  to  probe  the  whole  face  of  the 
stricture  at  the  same  time.  It  is  thus  scarcely  possible  that  the  opening 
should  not  be  found. 

5th.  Were  even  this  means  to  fail,  the  conducting  tube  may  be  filled  with 
water,  under  any  degree  of  pressure,  which  water  will  either  open  the  pass- 
age for  the  small  bougies,  or  will  itself  act  as  the  sharpest  and  most  insinu- 
ating of  all  instruments.  The  stricture,  by  which  ever  means  opened  will 
then  allow  the  urine  to  escape.  As  patients  might  fear  that  water  forced 
towards  a  bladder  already  too  full  would  ?only^increase  the  evil,  J.  Arnott 
waited  for  more  numerous  proofs  of  the  utility  and  safety  of  the  practice, 
before  strongly  recommending  it :  Dr.  Amussat,  of  Paris,  has  since  pub- 
lished.a  statement  of  numerous  cases  of  retention  thus  relieved. 

6th.  A  dilator  for  widening  the  stricture,  after  a  small  instrument  can  be 
passed  through  it.  It  is  intended  as  a  substitute  for  the  bougies  and  sounds 
of  former  times.  The  chief  objections  to  these  last  are,  the  painful  friction, 
the  danger  of  making  false  passages,  the  tediousness  and  imperfection  of  the 
cure,  and  that  they  cannot  dilate  any  part  of  the  canal  beyond  the  size  of  its 
orifice,  through  which  they  have  to  pass,  and  which  during  health,  is  the 
narrowest  part  of  the  canal.  * 

The  dilator  consists  of  a  tube  of  thin  membrane  introduced  while  empty 
into  the  stricture,  on  a  ball-pointed  wire  and  then  filled  with  fluid  by  a. 
syringe,  so  to  dilate  the  stricture,  with  any  degree  of  force,  from  the  mere 
filling  of  the  part  to  the  strain  of  the  hydrostatic  press,  sufficient  to  tear  the 
strongest  texture  that  disease  can  form.  The  dilating  tube  is  about  two  inches 


472  FLUIDITY    IN    KELATION    TO    ANIMALS. 

long,  and  its  end  next  to  the  operator  is  fixed  to  the  point  of  a  small  catheter, 
through  which  the  distending  fluid  is  injected.  The  tube  is  formed  of  thin 
silk  riband  of  various  sizes,  with  the  edges  joined.  It  is  lined  with  prepared 
gut  of  the  cat  or  dog,  which  is  almost  as  thin  as  gold-beater's  skin,  although 
very  strong  and  water-tight;  and  it  is  covered  with  the  same  to  give  the 
smoothest  and  softest  possible  external  surface.  When  complete  and  en- 
closing its  blunt  wire,  it  is  still  much  less  bulky  than  the  bougie  which  would 
be  required  for  the  same  case.  Thus,  it  passes  easily;  it  cannot  tear  the  canal 
or  make  false  passages ;  it  can  enter  through  a  small  orifice,  and  then  dilate 
to  any  desired  extent ;  and  its  greatest  advantage  is,  that  by  swelling  so  as 
to  follow  the  yielding  of  the  stricture,  it  can  eifect  at  one  application,  what 
only  a  succession  of  hard  bougies,  during  long  treatment,  could  accomplish. 
In  one  day  it  has  often  removed  disease  which  had  resisted  other  means  for 
months  or  even  years. 

Some  practitioners  and  critics,  not  understanding  the  law  of  fluid  pressure 
(explained  at  p.  128,)  objected  at  first  to  the  dilator,  that  a  little  water  or  air 
pressed  into  it  by  a  syringe,  would  be  unable  to  overcome  much  resistance. 
Had  they  seen  the  instrument  lifting  so  readily  as  it  does,  a  heavy  weight 
laid  upon  it,  or  snapping  a  strong  ligature  tied  round  it  they  would. not  have 
had  this  prejudice.  It  was  objected,  also,  that  the  instrument  would  do  mis- 
chief by  dilating  the  urethra  before  and  behind  the  stricture  more  than  th*e 
stricture  itself;  now  its  dimensions  being  determined  and  fixed  by  those  of 
its  silken  tunic,  it  never  can  distend  beyond  the  diameter  chosen,  and,  there- 
fore, if  of  the  proper  size,  it  can  only  press  on  the  stricture  itself.  It  was 
also  said,  that  this  instrument  requires,  in  the  operator,  greater  manual  dex- 
terity and  acquaintance  with  mechanical  philosophy  than  many  surgeons 
possess ;  but  this  is  merely  saying  that  the  arts  are  progressive,  and  that  the 
accomplished  surgeon  of  the  present  day  is  more  dexterous  and  intelligent 
than  his  predecessors  of  the  last  centuary.  It  is  not  accounted  a  reason  why 
the  delicate  apparatus  of  the  oculist  should  fall  into  disuse,  that  all  surgeons 
are  not  able  to  apply  it. 

Some  attempts  had  been  made  before,  to  construct  a  dilator  of  fluid  pres- 
sure, but  they  produced  nothing  of  value.  For  urethral  purposes,  a  simple 
gut  or  intestine  is  worse  than  useless,  for,  being  yielding  in  its  texture,  the 
surgeon  can  never  know  truly  the  size  of  his  instrument,  and  therefore  may 
do  much  mischief  by  it.  Dr.  Ducamp,  in  speaking  of  the  dilator,  allows  that 
he  did  not  first  invent  it,  but  then,  from  ignorance  of  what  constitutes  its  true 
value,  he  takes  praise  to  himself  for  simplifying  and  improving  it,  by  throw- 
ing away  the  silk,  and  using  the  gut  only. — A  variety  of  metallic  dilators 
have  been  contrived  and  used  by  English  surgeons  since  the  publication  of 
Arnott's  Treatise  on  Strictures,  but  although  manageable  with  less  trouble 
than  the  fluid  dilator,  they  want  its  chief  merits. 

The  dilator  is  applicable  to  many  other  purposes  in  surgery  besides  that 
now  mentioned, — as  for  removing  stricture  of  the  gullet,  and  of  the  rectum, 
for  checking  haemorrhage  in  deep  wound^,  for  dilating  wounds  as  a  tent,  &c. 
And  the  operation  of  lithotomy  was  saved  to  a  gentleman,  whom  Sir  Astley 
Cooper  and  the  auther  of  this  work  were  attending  together,  by  the  dilator 
opening  a  fistula  in perineq^  so  that  a  large  stone  was  extracted  without  cut- 
ting. The  dilator  has  also  served  in  removing  stones  from  the  female  bladder. 

7th.  Another  improved  means  for  the  treatment  of  stricture,  described  in 
the  treatise,  is  a  mode  of  apply  ing  caustic  for  its  entire  destruction,  but  so  as 
not  to  touch  any  other  part  of  the  canal.  Formerly  the  caustic  was  applied 
to  the  face  or  anterior  part  of  the  stricture,  and,  therefore,  had  almost  always 


PELVIC    APPARATUS.  473 

to  destroy  a  portion  of  the  healthy  canal  before  it  could  reach  the  narrowest 
fibres :  the  extent  of  such  portion  depending  on  the  distance  from  these 
fibres  of  the  part  where  the  lining  of  the  canal  began  to  be  drawn  inwards  by 
them.  This  explains  why  tfot  unfrequently  a  hundred  applications  of  caustic 
were  made  in  a  single  case,  and  why,  during  such  treatment,  false  passages 
were  often  bored,  and  other  mischiefs  produced.  Now  by  applying  the  caustic 
within  the  stricture  at  once  a  single  application  generally  suffices.  To  ac- 
complish this,  a  ring  of  caustic  is  placed  (as  described  in  the  Treatise,  and 
in  the  Cases,)  on  a  bougie  of  peculiar  construction,  about  an  inch  from  its 
extremity ;  and  the  bougie  being  then  passed  down  to  the  stricture  through 
a  tube  or  conductor,  and  the  point  being  passed  beyond  the  stricture,  the 
caustic  is  guided  to  the  very  spot  where  it  is  desired  to  act.* 

*  Dr.  Ducamp  incurred  a  singular  risk  in  giving  himself  out  as  the  first  proposer  of  the 
instruments  and  practice  described  above;  for  he  was  already  known  as  a  translator  of 
English  medical  books,  and  the  Treatise  on  Strictures  of  J.  Arnott  had  been  held  up  to 
public  attention  two  years  before  by  the  various  medical  reviews,  in  terms  such  as  the  fol- 
lowing: "  We  have  carefully  perused  this  little  volume,*and  are  of  opinion  that  it  is  by 
far  the  best  systematic  work  on  the  subject  in  the  English  language." — It  is  a  judicious 
compilation,  interwoven  with  much  original  and  acute  observation ;  and  it  gives  publicity 
to  instruments  which  promise  to  be  of  essential  benefit  to  operative  surgery." — Medico- 
Chirurgical  Review,  January,  1819. 

Perhaps  Dr.  Ducamp  imagined  that  the  slight  alteration  proposed  by  him  in  the  con- 
struction of  three  of  the  new  instruments,  might  be  a  shield  to  him  when  detected;  but  as 
the  chief  merit  was  in  the  analysis  of  the  subject  which  suggested  such  instruments,  and 
not  in  the  mere  mechanical  fulfilment  of  intentions,  even  a  considerable  improvement  in 
the  instruments  would  not  have  been  a  sufficient  excuse.  His  changes,  however,  were  either 
trifling  or  retrograde.  His  metallic  dilating  sound  is  less  perfect  than  metallic  sounds  con- 
trived by  J.  A.,  but  not  described,  because  the  fluid  dilating  sound  was  found  to  be  pre- 
ferable. His  porte-caustique  is  defective  in  not  distending  the  stricture  at  the.  moment  of 
applying  the  caustic  ;  and  his  mode  of  making  a  dilator  without  the  silken  tunic,  renders 
it  not  only  a  useless,  but  a  dangerous  instrument: — indeed,  such  as  obliged  him  to  use  the 
caustic  in  almost  every  case.  His  silence  with  respect  to  the  liquid  probe  favors  the  con- 
clusion that  he  did  not  understand  it,  although  Dr.  Amussat  of  Paris  has  since  used  it  with 
much  success : — and  the  same  remark  applies  to  the  double  catheter  (see  Arnott's  cases,) 
or  sonde  a  double  current,  as  it  has  been  called' by  those  who  have  since  used  it  in  Paris. 

The  following  are  extracts  from  the  report  made  by  the  commissioners  of  the  French 
Institute,  Doctors  Deschamps  and  Percy,  in  May,  1822,  on  the  subject  of  Ducamp's  work 
entitled  Traite  des  retentions  d'urine. 

"This  treatise  concerning  a  most  important  malady,  because  one  of  the  most  common 
and  painful  which  affects  humanity,  has  appeared  to  us  to  merit  more  than  ordinary 
attention. 

"When,  some  years  ago,  your  same  commissioners  had  to  express  their  opinion  of  an- 
other work  on  this  subjct,  they  commended  the  zeal  and  industry  of  its  estimable  author 
(Dr.  Petit;)  but  they  could  not  conceal  that  there  were  still  imperfections  in  his  modes  of 
treatment;  and  also  that  they  were  almost  entirely  either  borrowed  or  imitated  from  the 
English. 

"The  work  of  Dr.  Ducamp  now  leaves  us,  however,  nothing  more  to  desire,  and  we  have 
no  longer  reason,  as  regards  this  subject,  to  envy  our  neighbours.  Although  a  volume  of 
moderate  size,  it  is  incomparably  more  complete  and  full  of  matter  than  the  bulky  trea- 
tises lately  published  in  other  countries. 

"  *  *  Ducamp  leaves  all  these  authors  far  behind  him,  whether  as  to  the  soundness  of 
his  doctrines,  the  superiority  of  his  trials,  or  the  invention  of  instruments. 

"He  takes  a  print  or  model  of  the  stricture  by  an  instrument  of  his  invention,  called 
Sonde  Exploratrice.  (Arnott's  examining  sound,  page  471.) 

"  For  introducing  bougies  in  difficult  cases,  he  uses  an  elastic  gum  tube,  which  he  calls 
conductor.  (Described  above,  page  471.) 

"Mr.  D.  has  invented  for  measuring  the  length  of  strictures,  &c.,  an  instrument  which, 
when  introduced,  enlarges  beyond  the  stricture.  (The  dilating  sound,  page  471.) 

"  The  nitrate  of  silver,  or  common  caustic,  is  what  he  uses  for  destroying  strictures,  but 
he  employs  it  in  a  new  manner,  which  appears  to  us  to  give  it  new  powers,  and  to  deprive 
it  of  all  its  former  dangers.  *  *  He  carries  the  caustic  into  the  stricture  by  means  of  his 
porte  caustique.  (See  above,  page  473.  No.  7  of  Mr.  Arnott.) 

" To  enlarge  the  canal  at  the  morbid  part  of  its  true  caliber,  he  uses  an  instrument 

which  he  names  a  dilatateur.  (Dilator,  page  471.)  He  does  not  conceal  that  this  instru- 


474          FLUIDITY    IN    RELATION    TO    ANIMALS. 


Stone  in  the  bladder  is  another  disease  relievable  chiefly  by  mechanical 

means 

• 

The  urine  as  secreted  in  the  kidneys,  contains  dissolved  in  it,  a  variety  of 
substances  which,  under  certain  circumstances,  separate  and  assume  the  solid 
form, — as  sugar  separates  in  small  crystals  from  cooling  syrup,  or  salt  from 
cooling  brine  : — and  it  is  thus  that  those  minute  grains  are  produced  which 
we  call  urinary  gravel.  A  single  particle  of  gravel  remaining  by  an  acci- 
dent in  the  bladder,  soon  attracts  to  itself  more  matter  of  the  same  kind,  and 
becomes  the  nucleus  or  centre  of  an  increasing  mass,  which  is  what  we  call 
the  stone  in  the  bladder. 

In  a  second  Tract  by  the  author's  brother,  published  in  1820,*  the  follow 
ing  paragraph  appears : 

"  From  the  severe  suffering  of  the  patient  labouring  under  stone  in  the 
bladder,  and  the  remedy  being  an  operation  so  painful  and  dangerous,  that 
many  wear  out  their  lives  in,  certain  misery,  rather  than  submit  to  it,  it  has 
arisen  that  no  part  of  surgery  has  excited  more  attention,  either  in  the  medical 
profession  or  out  of  it.")"  No  very  important  change  in  the  treatment  of  this 
disease  has  now  been  made  for  upwards  of  a  century ;  and,  indeed,  it  has 
appeared  to  be  the  opinion  of  modern  surgeons,  that  the  manner  of  operating 
practised  in  Cheselden,  about  a  century  ago,  and  which  has  been  called  the 
*  glory  of  English  surgery'  was  so  nearly  perfect  as  to  leave  little  room  for 
improvement.  The  hopes  which  the  rapid  progress  of  chemistry,  and  the 
grand  discoveries  relating  to  stone  of  Scheele,  Wollaston,  Fourcroy,  and 
others  some  time  ago  gave  birth  to,  that  we  should  be  able  to  dissolve  stone 
by  lithontriptics,  and  thus  save  the  horrors  of  lithotomy,  had  again  died 
away,  and  the  researches  of  many  ingenious  men  who  have  been,  and  still 
are  employed  about  the  question,  have  for  their  end,  more  to  prevent  the 
formation  of  stone  by  remedies  and  regimen,  than  to  improve  the  manner  of 
removing  it  when  once  formed.  I  trust,  however,  notwithstanding  the  sup- 
posed exhausted  nature  of  the  subject,  that  the  following  essay  will  prove 
that  much  was  still  possible  in  the  improvement  of  this  department  of  the 
healing  art." 

The  publication  from  which  the  above  paragraph  is  taken,  and  the  "  Trea- 
tise" which  preceded  it,  in  both  of  which  new  instruments  and  new  pro- 
cesses were  described,  and  interesting  facts  were  detailed,  aroused  the  public 
attention  in  England  to  the  possibility  of  improving  the  treatment  of  stone  j 
and  about  the  same  time,  a  similar  spirit  awoke  with  more  decided  effects  in 
France.  The  results  have  now  become  of  great  importance  to  humanity. 
In  the  medical  publications  since  that  time,  cases  soon  began  to  be  re- 
corded in  lithotomy  superseded  by  new  means,  and  lately  such  cases  form 
the  majority.  We  shall  now  briefly  animadvert  to  the  principal  of  these 

ment  had  been  imagined  before  him,  but  he  has  the  merit  of  perfecting  it,  and  of  reducing 
to  practice  what  before  had  only  existed  as  a  project. 

" In  rendering  justice  to  the  able  men  who  have  preceded  Ducamp,  we  must  still 

say,  that  no  one  has  displayed  so  much  industry,  dexterity,  and  talent,  and  we  think  that 
he  has  high  claims  to  the  confidence  of  patients  and  the  gratitude  of  the  profession,  and 
that  his  work  merits  the  eulogium  of  the  Academy. 

(Signed  "DESCHAMPS, — PERCY,  Reporters. 
CUVIER, Secretary. 

*  Cases  illustrative  of  the  Treatment  of  Urethral  Obstructions  and  of  Stone.  By  James 
Arnott. — Longman  and  Co.,  1820. 

f  The  Catalogue  of  authors  who  have  written  upon  stone  occupies  in  Plocquet's  Litera- 
tura  Mtdica,  no  less  than  twenty-nine  very  closely  printed  quarto  pages. 


PELVIC    APPARATUS.  475 

means,  intending,  however,  only  to  interest  the  reader  in  a  manner  that  may 
lead  him  to  the  persual  of  the  original  works,  where  more  minute  information 
is  to  be  found.  They  shall  be  named  in  the  order  in  which  they  have  come 
into  use. 

The  dilator,  as  applied  to  the  treatment  of  stone,  has  already  been  spoken 
of  in  the  preceding  pages. 

The  double  catheter.  This  instrument,  with  its  applications  to  causes  of 
stone  and  other  affections  of  the  bladder,  is  described  in  Arnott's  Cases.  It 
has  two  channels,  by  one  of  which  a  fluid  may  pass  into  the  bladder,  while 
by  the  other  there  is  a  returning  current  mixed  with  urine.  It  is  equipped 
with  two  pliant  tubes,  of  which  one  leads,  from  a  supplying  reservoir,  and 
the  other  to  a  waste  vessel.  It  will  soothe  irritation  of  the  bladder,  whether 
arising  from  stone  or  not,  by  keeping  the  acrid  urine  in  a  dilated  state,  or  by 
applying  bland  and  medicated  liquids  directly  to  the  internal  surface  of  the 
bladder.  Not  being  larger  than  a  common  catheter,  it  may  be  worn  for  any 
period  as  the  common  catheter  now  is.  It  need  prevent  no  sedentary  occu- 
pation, and  may  be  used  during  sleep.  It  will  act  powerfully  to  dilate  a 
contracted  bladder,  if  the  reservoirs  be  placed  high,  and  the  fluid  be  caused  to 
distend  with  the  pressure  of  a  lofty  column.  It  also  affords  by  far  the  best 
means  of  admitting  to  the  bladder  any  solvent  of  stone.  Even  pure  water  is 
a  weak  solvent  of  most  animal  calculi,  as  is  proved  by  placing  them  in  a  run- 
ning stream ;  but  the  living  bladder  bears  with  impunity  a  diluted  acid  or 
alkali. 

The  syphon  catheter  (also  first  described  in  Arnott's  Cases')  is  merely  a 
catheter  of  a-length  that  will  allow  its  external  part  to  descend,  so  as  to  con- 
stitute the  long  leg  of  a  syphon.  (See  Pneumatics.')  Its  outer  extremity 
is  turned  up  a  little,  or  has  a  portion  of  soft  animal  gut  tied  upon  it  to  act 
as  a  valve,  for  preventing  the  entrance  of  air.  The  most  useful  application 
of  this  instrument  is  to  keep  the  bladder  empty  after  operations,  until  the 
healing  process  has  made  a  certain  advance.  The  diffusion  of  urine  among 
the  surrounding  parts  after  lithotomy,  particularly  after  the  high  operation, 
is  often  a  cause  of  death  ;  and  the  syphon  catheter,  by  providing  a  channel 
by  which  the  urine  must  immediately  pass  away  as  secreted,  obviates  the 
clanger.  This  instrument  is  sometimes  useful  in  very  irritable  bladders,  by 
preventing  the  repeated  distensions  of  the  bladder,  with  the  consequent 
excruciating  contraction.  It  has  also  relieved  in  the  deplorable  case  of  the 
bladder  torn  or  opened  by  sloughing  in  parturition,  as  it  can  keep  the  unhap- 
py patient  quite  dry. 

A  forceps,  calculated  to  pass  through  a  tube  into  the  bladder,  and  to  open 
there,  for  the  purpose  of  seizing  any  small  stone  or  other  solid  object  offered 
to  it,  was  described  long  ago  in  the  Armamentum  Chiruryicum  of  Scultetus, 
but  was  again  forgotten  until  John  Hunter's  investigations  led  him  to  a 
second  invention  of  it.  Such  an  instrument  had  for  a  considerable  time  passed 
under  the  appellation  of  Hunter's  urethra  or  bladder  forceps,  answering  for 
extracting  small  stones,  and  therefore,  if  used  in  time,  preventing  occasion- 
ally, the  necessity  of  lithotomy.  Soon  after  the  publication  of  Arnott's 
Essay,  it  was  modified  and  much  more  extensively  used  by  Sir  Astley  Cooper 
and  other  surgeons  in  England. 

But  a  new  and  intense  interest  has  now  been  excited  with  respect  to  the 
forceps,  as  a  means  for  removing  stone,  by  the  discovery — also  an  old  disco- 
very revived — that  a  straight  tube  may  be  passed  to  the  bladder,  as  a  conductor 
instead  of  the  bent  tubes  or  catheters  commonly  used.  A  door  is  thus,  as  it 
were,  opened  directly  into  the  bladder,  through  which  a  stone  might  even  be 


476  FLUIDITY    IN    RELATION    TO    ANIMALS. 

seen,  if  light  were  directed  upon  it,  and  through  which  it  easily  may  be  caught 
and  broken  to  pieces,  and  brought  away  without  doing  injury  to  the  living 
parts.  Dr.  Civiale,  of  Paris,  had  the  merit  first  of  contriving  good  instru- 
ments for  this  operation,  and  of  himself  operating  with  complete  success  in 
many  cases..  But  the  praise  of  carrying  the  operation  of  Lithotrity  (sfone- 
wearing-down,')  as  it  is  now  named,  to  its  present  state  of  perfection,  is  shared 
by  various  other  ingenious  surgeons,  as  Gruithuisen  (who  first  used  the 
straight  sound,)  Amussat,  Leroy,  Heurteloup,  (who  proposed  the  mode  of 
percussion,)  &c.  The  operator  introduces  a  strong  forceps,  which  seizes 
and  holds  fast  the  stone,  he  then  weakens  the  stone  by  boring  it  in  various 
directions  with  a  simple  drill,  which  passes  through  the  handle  of  the  forceps, 
and  is  turned  rapidly  by  a  drill-bow  acting  on  its  external  end,  or  with  a  drill 
of  which  the  point  can  be  bent  to  one  side,  so  as  to  excavate  to  any  desired 
extent ;  after  which  weakening,  the  stone  is  crushed,  either  by  the  forceps 
which  first  held  it,  or  by  another  instrument  called  brisecoque,  made  on  pur- 
pose ;  or  without  boring  at  all,  Heurteloup  and  others  at  once  break  the  stone 
to  pieces  by  blows  of  a  small  hammer  acting  on  a  sliding  limb  of  the  forceps. 

Dr.  Dartfin,  in  his  Zoonomia,  published  in  1790,  proposed  to  seize  stones 
by  forceps  passed  into  the  bladder,  and  then  to  break  them  down  or  destroy 
them  mechanically ;  but  the  supposed  necessity  of  working  through  a  long 
bent  tube  prevented  trials  from  being  made.  The  author  of  this  work  also 
showed  some  years  ago,  before  any  of  the  above-described  improvements 
were  made  (see  Cases,  page  93,)  that  it  was  possible  to  pass  a  bag  into  the 
living  bladder,  and  to  enclose  a  stone  there,  so  that  any  solvent  might  be 
injected  into  the  bag,  and  again  withdrawn  without  coming  into  contact  with 
the  bladder.  This  was  shown  rather  to  excite  attention  to  the  possibility  of 
operating  with  the  living  bladder  with  great  precision,  than  to  recommend 
that  precise  means  of  destroying  stone. 

To  all  the  ingenious  instruments  above  spoken  of  for  breaking  down  the 
stone,  there  is  still  this  objection,  that  it  is  broken  into  such  fragments,  that 
many  of  them  require  to  be  afterwards  treated  as  distinct  stones,  and  thus 
the  painful  operation  has  to  be  repeated  again  and  again,  and  whole  months 
may  pass  before  the  operation  be  completed. — The  author  deems  it  possible 
to  make  a  forceps  of  several  claws  or  ribs,  which  should  surround  the  stone 
so  loosely  as  to  leave  it  freedom  of  motion  within  the  claws,  like  a  loose 
kernal  in  a  shell,  and  so  that  on  making  the  forceps  itself  whirl  backwards 
and  forwards,  like  the  drill  in  Civiale's  apparatus,  the  stone  might  be  quickly 
rubbed  to  dust  by  the  friction  or  file  action  of  the  roughened  interior  of  the 
claws.  The  bladder  would  be  filled  during  the  operation,  with  water,  or  even 
air,  to  secure  plenty  of  room  for  the  turning  instrument : — or  a  slender  exter- 
nal forceps  might  be  used  as  a  guard,  to  prevent  contact  of  the  bladder  with 
the  moving  instrument.  Out  of  the  body,  a  stone  harder  than  urinary 
calculus,  placed  in  such  a  cage  with  rough  interior,  and  subjected  to  the 
action  described,  is  soon  reduced  to  dust.  There  are  various  ways  of  making 
a  forceps  or  cage  for  this  operation,  which  will  readily  suggest  themselves 
to  persons  knowing  what  has  already  been  achieved  in  this  department  of 
practice,  and  having  the  ingenuity  likely  to  engage  them  in  such  a  pursuit. 

The  high  operation  of  lithotomy  possesses  over  the  common  lateral  ope- 
ration such  advantages  as  the  following : — thinness  of  the  parts  cut  through 
— distance  of  the  knife  from  important  arteries — stones  of  very  large  size  may 
be  more  easily  extracted — the  prostate  gland  is  not  wounded.  But  the  high 
operation  has  not  become  general, — because — there  was  difficulty  in  avoid- 
ing the  peritoneum  while  making  the  opening  into  the  bladder — there  was 


PELVIC    APPARATUS.  477 

danger  of  effusion  of  urine  among  the  cut  parts,  after  the  operation — and 
where  the  bladder  was  contracted,  the  incision  had  to  be  very  deep.  Now 
these  objections  are  obviated  by,  1st,  the  double  catheter,  which  will  dilate 
the  contracted  bladder ;  2d,  by  the  syphon  catheter,  which  will  prevent  the 
effusion  of  urine;  and  3d,  by  the  jointed  sliding  sound,  (see  Cases,  page 
104,)  which  will  ensure  the  accurate  cutting  in  the  desired  place.  Had  we 
possessed  then,  for  the  removal  of  stone,  no  less  hazardous  means  than 
cutting,  the  high  operations  with  the  new  securities  might  have  been  best. 
When  a  catheter  has  to  be  retained  in  the  bladder  after  any  operation,  in 
cases  where,  if  it  slipped  out,  it  might  with  difficulty  be  replaced,  something 
should  be  passed  through  it  like  a  small  spring  forceps  to  expand  and 
become  an  internal  button  preventing  its  escape.  (See  Cases,  page  94.) 


UTERINE   PHENOMENA. 

Although  so  many  of  the  uterine  phenomena  are  mechanical,  there  are 
few  of  them  which  could  be  treated  of  with  advantage,  except  in  connection 
with  particulars,  of  which  the  consideration  does  not  belong  to  a  work  like 
this.  We  shall,  however,  cite  the  following  particulars  as  examples. 

The  protection  given  to  the  tender  foetus  by  the  liquor  amnii  in  which  it 
floats,  is  such,  that  a  blow  from  without  is  expended  on  the  surrounding 
water,  and  cannot  reach  the  foetus. 

The  head  of  the  foetus,  because  ossification  begins  in  it  first,  becomes  of 
greater  specific  gravity  than  the  other  parts  of  the  body,  and  therefore  gene- 
rally lies  at  the  bottom  of  its  liquid  bed.  It  is  thus  ready  to  appear  first  in 
parturition,  according  to  the  safest  course  of  delivery 

The  membranes  distended  by  the  liquor  amnii  descend  before  the  head, 
as  a  soft  but  powerful  wedge  preparing  the  way,  according  to  the  principle 
explained  in  a  previous  page. 

We  have  spoken,  at  page  168,  under  the  name  of  pneumatic  tractor,  of  a 
circular  piece  of  leather  or  similar  soft  substances,  kept  extended  by  included 
solid  rings  or  radii,  as  being  adapted  to  some  purposes  of  surgery.  Now  it 
seems  peculiarly  adapted  to  a  purpose  of  obstetric  surgery,  viz.,  as  a  substi- 
tute for  the  steel  forceps,  in  the  hands  of  men  who  are  deficient  in  manual 
dexterity,  whether  from  inexperience  or  natural  inaptitude.  The  forceps,  to 
be  well  and  safely  used,  requires  address,  which  even  the  naturally  dexterous 
man  cannot  possess  without  a  certain  degree  of  continued  practical  familiarity 
with  it,  and  except  in  large  towns,  a  man  must  be  very  unfortunate  in  his 
practice  who  often  requires  it :  hence  the  really  small  number  of  persons,  who 
use  it  well.  The  consideration  of  the  tractor  as  a  substitute  for  it  belongs 
properly  to  the  present  section :  but  as  the  true  mode  of  action  of  the  tractor 
is  not  very  readily  conceived,  by  persons  who  either  have  never  been  in- 
structed in  the  general  laws  of  physics,  or  who  have  ceased  to  be  familiar 
with  them,  such  persons  are  advised  to  read  this  paragraph  in  continuation 
of  that  at  page  168,  and  to  weigh  well  the  following  remarks.  A  tractor  of 
three  inches  in  diameter,  would  act  upon  any  body,  to  lift  or  draw  it,  with  a 
force  of  about  a  hundred  pounds — with  more  therefore,  than  is  ever  required 
or  allowable  in  obstetric  practice.  In  lifting  a  stone,  the  tractor  does  act 
as  if  it  were  glued  or  nailed  to  the  stone,  but  merely  bears  or  takes  off  the 
atmospheric  pressure  from  one  part,  and  allows  the  pressure  on  the  opposite 
side,  not  then  counterbalanced,  to  push  the  stone  in  the  direction  of  the  trac- 
tor ; — so  when  placed  upon  the  head,  it  would  not  pull  by  the  skin,  in  the 


478  FLUIDITY    IN    RELATION    TO    ANIMALS. 

manner  of  a  very  strong  adhesive  plaster  applied  there,  as  uninformed  per- 
sons would  be  apt  to  suppose,  but  by  taking  off  a  certain  atmospheric  pres- 
sure from  the  part  of  the  head  on  which  it  rested,  it  would  allow  the  pressure 
on  the  other  side  or  behind  to  urge  the  head  forward  on  its  way.  Of  course 
the  forwarding  pressure  in  such  a  case  would  not  operate  on  the  head  directly, 
but  through  the  intervening  parietes  and  contents  of  the  maternal  abdomen. 
It  would  be  much  better  to  have  a  gentle  and  diffused  action  of  the  tractor 
over  a  large  surface,  than  an  intense  action  on  a  small  surface,  and  therefore 
a  tractor  for  the  purpose  now  contemplated  should  not  be  very  small,  and 
should  have  a  little  air  underneath  it  in  a  slight  depression  or  cavity  at  its 
centre. — The  forceps  must  be  more  effective  than  the  tractor  for  rectifying 
malposition  of  the  head,  and  diminishing  its  transverse  diameter ;  but  the 
tractor  will  answer  both  these  purposes  in  a  degree  greater  than  many  would 
expect.*  The  author  proposes  to  publish  on  this  matter,  and  on  some  other 
strictly  professional  subjects  which  are  lightly  touched  upon  in  the  present 
general  work,  such  a  practical  detail,  as  for  the  dilator,  syphon  catheter,  &c., 
is  found  in  his  brother's  "  Treatise"  and  "  Cases." 

Conclusion. 

It  is.  almost,  superfluous  to  remark  here,  that,  for  the  practice  of  general 
and  obstetric  surgery,  learning  and  judgment  are  of  little  avail  unless  accom- 
panied by  manual  dexterity :  and  it  is  one  of  the  improvements  yet  to  be 
made  in  our  system  of  education  for  various  professions,  to  cultivate  more 
methodically  the  use  of  the  hands.  Children  and  young  people,  in  obtain- 
ing practical  familiarity  with  ingenious  toys,  tools  of  carpentry,  games  of 
address,  musical  instruments,  &c.,  are  often  fitting  themselves  for  the 
important  business  of  their  future  life. 

*  "We  have  been  already  compelled  on  one  or  two  occasions  to  differ  from  the  able  author 
of  this  work,  in  relation  to  the  practical  application  of  some  of  his  principles,  and  we  must 
be  again  permitted  to  record  our  dissent  from  his  opinion  that  the  pneumatic  tractor,  under 
certain  circumstances  is  peculiarly  adapted  as  a  substitute  for  the  obstetric  forceps.  Our 
author  cannot  be  a  practical  accoucheur  or  he  would  at  once  perceive  that  the  various  ma- 
noeuvres by  which  labour  is  assisted  with  the  forceps,  cannot  be  accomplished  with  the 
tractor.  That  address  and  knowledge  are  requisite  to  apply  the  forceps  properly,  is  no 
objection  to  their  use ;  it  only  shows  the  necessity  of  the  operator's  acquiring  this  dexterity 
and  knowledge  before  attempting  to  apply  the  instruments;  and  these  acquirments  are  not 
so  difficult  as  our  author  seems  to  think,  nor  do  we  believe  that  the  number  who  possess 
them  is  so  very  small.  It  is  not  contended  even  by  the  author  that  the  tractor  is  superior  to 
the  forceps  ;  he  only  recommends  it  as  being  less  dangerous  in  the  hands  of  the  unskilful. 

Now  it  might  be  supposed  from  this  that  the  tractor  is  readily  applied  and  cannot  effect  in- 
jury, both  of  which  are  erroneous.  Every  instrument  is  dangerous  in  the  hands  of  igno- 
rance. If  a  person  deficient  in  dexterity  could  succeed  in  applying  the  traction,  (of  which 
we  have  strong  doubts,  believing  it  would  require,  in  most  instances,  even  Dr.  Arnott's  skill 
and  knowledge)  it  is  quite  as  probable  that  he  would  produce  injury  as  benefit.  In  certain 
states  of  labour,  the  tractor  may  be  applied  to  the  neck  of  the  uterus  instead  of  the  head 
of  the  child,  or  to  both,  drawing  out  the  uterus  thus  as  well  as  the  child ;  it  may  be  applied 
before  the  uterus  is  sufficiently  dilated,  or  the  force  may  be  applied  in  the  wrong  direc- 
tion ;  indeed,  there  are  but  few  cases  in  which  force  could  be  applied  in  the  proper  direc- 
tion with  the  tractor,  <tc.  These  accidents  cannot  happen  to  the  well  instructed ;  but  in  the 
hands  of  such,  the  forceps  are  more  effectual  and  equally  safe.  The  tractor,  then,  requiring 
skill  for  its  proper  application,  and  being  a  less  efficient  instrument  than  the  forceps,  ought 
not,  independent  of  many  otherreasons,  to  be  recommended.  It  is  not  to  those  who  devise 
imperfect  substitutes  for  valuable  instruments,  or  temporary  palliatives  for  important  ope- 
rations, in  order  that  the  awkward  and  ignorant  may  imperfectly  perform  what  the  skilful 
or  instructed  should  only  attempt,  or  are  capable  of  accomplishing,  that  praise  is  to  be 
awarded.  It  is  the  just  meed  of  those  who  furnish  proper  instructions  for  the  use  of  instru- 
ments and  for  performing  operations,  and  present  the  means  of  gaining  information  and 
skill.— Am.  Ed. 


v  PELVIC    APPARATUS.  479 

While  the  author  directs  the  attention  of  the  profession  to  the  important 
physical  considerations  set  forth  in  the  preceding  pages,  he  deems  it  neces- 
sary most  pointedly  to  remark,  that  in  the  living  body  mechanical  principles 
are  generally  associated  in  their  operation  with  the  more  recondite  principles 
of  chemistry  and  of  life  j  and  that  the  man  who  allows  his  mind  to  dwell 
too  exclusively  on  any  one  of  the  three  classes,  must  be  a  very  bad  reasoner 
in  questions  either  of  health  or  disease.  It  is  within  a  very  recent  period, 
however,  that  just  views  on  this  subject  have  begun  to  prevail,  and  that  the 
titles  of  the  peculiarly  mechanical  physician,  or  chemical  physician,  or  phy- 
sician attending  only  to  the  influence  of  nerves  or  life,  are  likely  to  be  no 
longer  justly  applicable.  The  light  of  true  philosophy  is  at  last  breaking  in. 
upon  the  very  complex  and  difficult  subjects  of  medical  inquiry;  and  where 
formerly  keen  penetration  beheld  only  confusion,  even  common  minds  now 
begin  to  see  clear  divisions  and  beautiful  arrangement. 


THE 

ANALYTICAL  TABLE. 


INTRODUCTION. 

Progress  of  man  and  stationary  condition  of  inferior  animals. 
The  progress  more  rapid  at  present  than  ever. 
The  divisions  of  human  knowledge. 
Natural  philosophy  particularly  considered. 
SYNOPSIS. 

The  fundamental  truths  of  natural  philosophy  explained  under  the  terms 
atoms,  attraction,  repulsion,  and  inertia — the  divisions  of  this  work, 
page  19. 


PART  I.— The  FOUR  FUNDAMENTAL  TRUTHS  minutely  examined 
and  used  to  explain  the  general  constitution  of  material  substances,  and 
of  the  motions  going  on  among  them,  page  22. 

SECT.  I. — CONSTITUTION  or.  MATERIAL  MASSES,  22. 

ATOMS — minute — indestructible — occupying  space,  22. 
ATTRACTION  of  atoms  is  mutual,  25. 

Gravitation,  26 — Cohesion,  27 — Capillary  attraction,  28 — Che- 
mical attraction,  28 — Definite  proportions,  29. 
REPULSION — The  influence  of  heat  on  masses,  30. 
Solid — liquid — air,  31. 
Repulsion  of  surfaces,  32. 

Modification  of  Masses :  33 — Crystal,  33 — Porosity,  34 — Den- 
>  sity,  35, — Hardness,  36 — Elasticity,  37 — Brittleness,  38 — 

Malleability,  38— Ductility,  38— Pliancy,  39— Tenacity,  39. 
APPENDIX,  Properties  of  Matter,  41. 

SEQT.  II. — MOTIONS  AMONG  BODIES,  41. 

Motion  and  Rest; 
INERTIA  of  matter,  42. 

Motion  is  naturally  permanent,  46 — uniform,  47 — straight,  47'. 
Centripetal  and  centrifugal  forces,  49. 
Quantity  of  motion  and  force,  53 — momentum,  54. 
Direction  of  forces  and  composition  of  motion,  55. 
The  two  forces  of  nature  are  Attraction  and  Repulsion,  58.  • 
Accelerated  motion,  58. 
Retarded  motion,  60. 
Pendulum  and  balance  wheel,  60. 
Projectiles,  64. 

Tides,  winds,  currents,  &c.,  obey  attraction,  66. 
Explosion,  steam,  &c.,  obey  repulsion,  66. 
31 


482  ANALYTICAL    TABLE. 

All  great  velocities  are  results  of  continued  action,  and  are  de- 
stroyed by  continued  action,  67. 
Action  and  reaction  equal  and  contrary,  70. 
APPENDIX, — Definitions,   74 — Laws   of  motion,  75 — Impulsive 
force  and  rectilinear  motion,  75 — Constant  force  and  constantly 
accelerated  motion,  76 — Of  Gravity,  77 — Motion  produced  by 
joint  forces,  77 — Equilibriums,  78 — Joint^ction  of  an  impulsive 
and  constant  force,  78 — Laws  of  central  forces,  80 — Joint  effect 
of  active  and  inactive  forces,  80 — Pendulum,  80 — Impact  of 
bodies,  83. 


PART  II.— DOCTRINES  OF  SOLIDS  or  MECHANICS,  84. 

Centres  of  inertia  and  gravity,  85. 

In  animal  bodies,  90 — Sea  sickness,  91. 

Centres  of  percussion  and  oscillation,  93. 
Solids  moving  round  a  centre,  or  so  that  different  parts  may  have 

different  speed,  94, 
Simple  machines,  95. 
Lever,  95 — Wheel  and  Axle,  103 — Inclined  plane,  105 — 

Wedge,  106,— Screw,  107— Pulley,  108— Engine  of  oblique 

action,  109. 
Fly  wheels,  111. 
Complex  machines,  113. 

Friction,  114. 

Wheel  carriages,  115 — Rail- ways,  116. 
Strength  of  materials,  119. 
Influence  of  form — Arches,  &c.,  122. 


PART  III.— DOCTRINES  OF  FLUIDS  or  HYDRODYNAMICS,  126. 
SECT.  I. — HYDROSTATICS,  or  fluids  in  repose,  126. 

Pressure  in  a  fluid  extends  equally  through  the  whole,  127. 

Hydrostatic  press,  &c.,  129. 

Pressure  in  a>  fluid  increases  with  the  depth,  129. 

Compressibility  of  water,  &c.,  131. 
Not  influenced  by  shape  of  vessel,  132. 
Level  surface  of  fluids,  133. 

Spirit  level,  133 — Canals,  134 — Running  streams,  135 — 
gradual  change  of  the  earth's  surface  produced  by  running 
water,  136. 

Same  level  in  communicating  vessels,  139. 
City  water  works,  140. 
Springs  and  wells,  141. 
Support  of  bodies  floating  in  fluids,  143. 
Specific  gravities,  144. 
Floating  bodies,  148. 

Swimming  of  man  and  inferior  animals,  149. 
Ballast,  152. 
Fluids  of  different  density,  153. 


ANALYTICAL    TABLE.  483 

SECT.  II. — PNEUMATICS,  or  phenomena  of  air,  156. 

Lightness,  158. 
Elasticity,  158. 

Air-pumps,    159— Diving-bell,    162— Water-balloon,     163— 

Hiero's  fountain,  lb'4. 
Pressure  in  all  directions,  165. 
Pressure  as  the  depth,  165. 

Weight  of  the  atmosphere,  167. 
Atmospheric  pressure  on  solids,  167. 
Magdeburgh  hemispheres,  168. 
Pneumatic  tractor,  168. 
Atmospheric  pressure  on  liquids,  169. 

Pumps,    171 — Syphon,    172 — Intermitting   fountains,   173 — 

Bird-glass,  174 — Vent-plugs,  174. 
Atmospheric  pressure  on  animal  body,  175. 
Cupping,  &c.,  176. 
The  barometer,  176. 
Atmospheric  pressure  determines  the  liquid  or  aeriform  state  of 

aeriform  substances,  182. 
Boiling,  182. 

— at  different  heights,  183. 
— in  vacuo  and  distilling,  184. 
Elastic  force  of  steam,  186. 
Steam-engines,  187. 
Explosions,  190,  193. 
Atmospheric  pressure  affecting  combinations  of  bodies,  194. 

Effervescence — sparkling  liquids,  194. 
Atmospheric  pressure  affecting  the  density  and  temperature  of  the 

air,  195. 

Climate  depending  on  elevation,  196. 

Atmospheric  pressure  affecting  the  humidity  of  the  air,  196. 
Rain,  mist,  snow,  hail,  dew,  197 — Hygrometer,  198. 
Rain  and  clouds  among  mountains,  199. 
Fluid  support  of  floating  in  air,  200. 
Balloons,  200. 

Ascent  of  flame  and  smoke,  202. 
Chimneys,  203. 
Warming  and  ventilating  of  houses,  205. 

Apartments  for  consumptive  patients,  207. 
Winds,  208.— Trade  winds,  208. 
Land  and  sea  breezes,  209. 
Monsoons,  210. 
Pneumatic  trough,  210. 
Gasometer,  211. 
Pneumatic  chemistry,  212. 

SECT.  III. — HYDRAULICS,  or  fluids  in  motion,  213. 

Fluids  moving  in  channels  or  issuing  from  vessels,  213. 

Aqueducts,  216. 

Fountains  and  jets,  216. 
Waves,  216. 
Momentum  and  resistance  of  fluids,  220. 


484  ANALYTICAL    TABLE. 

Resistance  to  ships,  &c.,  increases  much  more  rapidly  than  the 
velocity,  220. 

Steam-boats,  221.  • 

Paddle-wheels,  221. 
Resistance  to  bodies  in  air,  222. 
Fluid  resistance  limits  many  velocities,  222. 
is  influenced  by  shape  of  solid,  223. 

Water-wheels,  224. 
Fluid  resistance  proportioned  to  surface  of  contact,  and  not  to 

quantity  of  matter,  224. 
Projectiles,  225— -levigating,  225 — Winnowing,  226 — Washing 

gold  dust,  226. 
Oblique  action  of  fluids,  226. 

Navigation— Sails,  226— Rudder,  227. 

Windmills,  229— Feathered  arrows,  229— Paper  kites,  230. 
Lifting  fluids,  231. 
Buckets — Pumps — Wheels — Water-screw,  231— Water-ram,  232. 

SECT.  IV. — ACOUSTICS,  or  doctrines  of  sound,  234. 

Nature  of  simple  sound,  235. 

Continued  sound  or  tone,  235 — Grave  and  sharp  sounds,  238. 
Musical  sounds,  238. 

Musical  scale,  239. 

Melody — Harmony — Accompaniment — Time,  241. 

Tuning-forks,  242. 

Musical  ear,  245. 
Spreading  of  sound — in  solid  and  fluid,  246 — Stethoscope,  247. 

Velocity  of  sound,  248 — Many  examples,  248. 
Reflection  of  sound,  248. 

Echo — whispering  galleries — Ear  trumpets — Speaking-trumpets, 
249. 

Animal  ear,  252. 


PART  IV.— Doctrines  of  imponderable  substance,  254. 
HEAT.  256. 


SECT.  1 — HEAT,  phenomena  which  it  produces,  257. 

'  Properties,  25$ — Means  of  measuring  quantity  in  bodies,  259. — 

Diffuses  itself  equally,  259. 
Cold,  260. 

Conducting  power  of  bodies,  260 — Natural  covering  of  animals, 
262— Clothing  262. 
Mode  in  which  heat  spreads,  266. 

By  the  motion  of  the  particles  in  fluids,  266. 
By  radiation,  269. 
Expansion  of  bodies  by  heat,  274. 

Capacity  of  bodies  for  heat,  275 — Influence -of  bulk  on  this, 
277— Of  density,  280. 


ANALYTICAL    TABLE.  485 

Each  substance  expands  in  a  degree  peculiar  to  itself,  281 — 
Expansion  of  solids,  282 — Expansion  of  fluids,  284 — Expan- 
sion of  gases,  284. 

Latent  heat,  290. 

Boiling,  295. 

Distillation,  295. 

Evaporation,  295. 

Table  of  temperature,  300— -Thermometer,  300. 

Absolute  quantity  of  heat,  304. 

Influence  of  heat  on  chemical  combinations,  305. 

Influence  of  heat  on  vegetables  and  animals,  307. 

Sources  of  heat,  309 — The  sun,  309— Electricity,  311— Com- 
bustion, 311 — Fuel,  317 — Condensation  and  friction,  322 — 
Functions  of  animal  life,  323. 

SEC.  II— LIGHT,  325— Sources  of,  327. 

Becomes  less  intense  as  it  spreads,  327 — Falling  on  bodies  ren- 
ders them  visible,  328. 
Shadows,  329,  331> 
Velocity  of  light,  329. 
Direction  of  light,  330. 
Transmission  of  bodies,  332. 
Refraction,  333. 
Lenses,  339. 

Camera  obscura,  solar  microscope,  magic  lantern,  341. 
Eye,  346. 
Vision,  3;48,  352. 
Distinct  vision,  350. 
Short-sightedness,  350 
Long-sightedness,  350. 
Visual  angle,  355. 
Apparent  size  of  objects,  357. 
Foreshortening,  360. 
Perspective,  362,  373. 
Intensity  of  light,  shade  and  colour,  369. 
Divergence  of  rays  of  light,  374. 
Convergence  of  axes  of  the  eyes,  375. 
Cosmorama,  377. 

Painting  representing  motion,  378. 
Art  of  painting,  379. 
Telescope.  381. 

Mirrors,  straight,  386 — Curved,  390. 
Comparison  of  light  and  sound,  392. 
Perfection  of  the  eye,  395. 

PART  V.— ANIMAL  AND  MEDICAL  PHYSICS. 
SECT.  I. — ANIMAL  MECHANICS,  399. 

Skull,  &c.,  399. 

Spine  and  its  distortions,  400. 

Limbs  and  mechanical  surgery,  401. 


486  ANALYTICAL    TABLE. 

Living  force,  409— Treadmill,  410. 
Surgical  instruments,  410. 

SECT.  II. — ANIMAL  HYDROSTATICS  AND  HYDRAULICS,  or  Fluidity  in 
relation  to  animals,  414. 

1.  Circulation  of  blood. 

In  arteries,  416. 

In  capillaries,  421. 

In  veins,  423. 
Force  of  the  heart,  430. 
Hydrostatic  or  floating  bed,  431. 
Velocity  of  blood,  435. 
The  pulse,  436. 
Circulation  in  the  head,  440. 
Effects  of  position  on  the  circulation,  441. 
Fainting  from  diminished  arterial  tension,  442. 
Diffused  pressure,  444. 
Mercurial  bath,  445. 
Transfusion  of  blood,  445. 

2.  Respiration  and  voice,  445. 

Action  of  chest,  446. 

Wounds  of  chest,  447. 

Hemoptysis,  448. 

Coughing,  448 — Sneezing — Hiccup,  &c.,  449. 

Suffocation,  449. 

Humane  Society's  apparatus,  450. 

Artificial  respiration,  450. 

Speech,  451. 

Modifications  of  voice,  452. 

Table  of  articulations,  455. 

Stuttering,  456.  . 

Ventriloquism,  461. 

3.  Digestion,  463. 

Mechanism  of  the  organs,  463. 
Effects  of  abdominal  pressure,  465. 
Vomiting,  466. 
Stomach  pump,  &c.,  466. 
Enema  funnel,  468. 

4.  Secretion  of  the  kidneys,  469. 

The  apparatus. 
Obstructions  in  urethra,  470. 
New  instruments  and  means  'for  treatment,  471. 
Stone  in  the  bladder,  474. 
-New  instruments  and  means,  475. 

5.  Uterine  phenomena. 

Protection  of  foetus  by  the  liquor  amnii,  477. 

Position  of  ditto,  477. 

Importance  of  physical  knowledge  and  manual  dexteirfcy,  478 


THE    END. 


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Churchill's  Midwifery, u  fl;-jaw.      «  9 

Dickson's  Elements  of  Medicine, "  10 

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18 
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yi 

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27 
27 
28 
29 
30 


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Watson's  Practice  of  Physic,  . 

Walshe  on  the  Lungs, "  30 

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Wins  low  on  Brain  and  Mind, "  32 

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ASHTON   (T.  J.), 

Surgeon  to  the  Blenheim  Dispensary,  &c. 

ON   THE   DISEASES,  INJURIES,  AND   MALFORMATIONS   OF   THE 

RECTUM  AND  ANUS;  with  remarks  on  Habitual  Constipation.  From  the  third  and  enlarged 
London  edition.  With  handsome  illustrations.  In  one  very  beautifully  printed  octavo  volume, 
of  about  300  pages.  (Now  Ready.)  $2  00. 

INTRODUCTION.  CHAPTER  I.  Irritation  and  Itching  of  the  Anus.  II.  Inflammation  and  Excoria- 
tion of  the  Anus.  IH.  Excrescences  of  the  Anal  Region.  IV.  Contraction  of  the  Anus.  V. 
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The  most  complete  one  we  possess  on  the  subject. 
Medico- Ckirurgical  Review. 


reach  of  American  practitioners.    We  are  satisfied 
after  a  careful  examination  of  the  volume,  and  a 
comparison  of  its  contents  with  those  of  its  leading; 
Its  merits  as  a  practical  instructor,  well  arranged, 
abundantly  furnished  with  illustrative  cases,  and 

vice  given  in  the  concluding  paragraph  above,  would 
be  to  provide  himself  with  a  copy  of  tne  book  from 


ciently  endorsed  by  the  verdict  of  his  countrymen 
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which  it  has  been  taken,  and  diligently  to  con  its 
instructive  pages.  They  may  secure  to  him  many 
a  triumph  and  fervent  blessing.—  Am.  Journal  Med. 
Sciences,  April,  1S58. 


ALLEN    (J.    M.),    M.  D., 

Professor  of  Anatomy  in  the  Pennsylvania  Medical  College,  &c. 

THE  PRACTICAL  ANATOMIST;  or,  The  Student's  Guide  in  the  Dissecting. 
ROOM.  With  266  illustrations.  In  one  handsome  royal  12mo.  volume,  of  over  600  pages,  lea- 
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However  valuable  may  be  the  "  Dissector's 
Guides"  which  we,  of  late,  have  had  occasion  to 
notice,  we  feel  confident  that  the  work  of  Dr.  Allen 
is  superior  to  any  of  them.  We  believe  with  the 
author,  that  none  is  so  fully  illustrated  as  this,  and 
the  arrangement  of  the  work  is  such  as  to  facilitate 
the  labors  of  the  student  in  acquiring  a  thorough 
practical  knowledge  of  Anatomy.  We  most  cordi- 


ally recommend  it  to  their  attention. — Western  Lan- 

We  believe  it  to  be  one  of  the  most  useful  works 
upon  the  subject  ever  written.  It  is  handsomely 
illustrated,  well  printed,  and  will  be  found  of  con- 
venient size  for  use  in  the  dissecting-roo«i. — Med. 
Examiner. 


ANATOMICAL    ATLAS, 

By  Professors  H.  II.  SMITH  and  W.  E.  HORNEB,,  of  the  University  of  Pennsyl- 
vania.   1  vol.  8vo.,  extra  cloth,  with  nearly  650  illustrations.    SSp"  See  SMITH,  p.  27. 


ABEL  (F.   A.),    F.  C.  S.    AND    C.    L.    BLOXAM. 

HANDBOOK  OF  CHEMISTRY,  Theoretical,  Practical,  and  Technical;  with  a 
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pages,  with  illustrations.  $3  25.  

ASHWELL   (SAMUEL),   M.  D., 

Obstetric  Physician  and  Lecturer  to  Guy's  Hospital,  London. 

A  PRACTICAL  TREATISE  ON  THE  DISEASES  PECULIAR  TO  WOMEN. 

Illustrated  by  Cases  derived  from  Hospital  and  Private  Practice.  Third  American,  from  the  TJmd 
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The  most  useful  practical  work  on  the  subject  in 
the  English  language.  —  Boston  Med.  and  Surg. 
Journal. 


The  most  able,  and  certainly  the  most  standard 
and  practical,  work  on  female  diseases  that  we  have 
yet  seen. — Medico-Chirwrgieal  Review. 


ARNOTT   (NEILL),  M.  D. 
ELEMENTS    OF    PHYSICS;    or  Natural  Philosophy,  General  and  Medical. 

Written  for  universal  use,  in  plain  or  non-technical  language.  A  new  edition,  by  ISAAC  HAYS, 
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tions. $2  50. 

BIRD  (GOLDING),  A.  M.,  M.  D.,  &c. 
URINARY     DEPOSITS:     THEIR     DIAGNOSIS,    PATHOLOGY,    AND 

THERAPEUTICAL  INDICATIONS.  Edited  by  EDMUND  LLOYD  BIRKETT,  M.  D.  A  new 
American,  from  the  fifth  and  enlarged  London  edition.  With  eighty  illustrations  on.  wood.  In  one 
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The  death  of  Dr.  Bird  has  rendered  it  necessary  to  entrust  the  revision  of  the  present  edition  to 
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the  text  as  the  progress  of  science  has  called  for.  Notwithstanding  the  utmost  care  to  keep  the 
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BUDD  (GEORGE),  M .  D.,  F.  R.  S., 

Professor  of  Medicine  in  King's  College,  London. 

ON  DISEASES  OF  THE  LIVER.  Third  American,  from  the  third  and 
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is  not  perceptibly  changed,  the  history  of  liver  dis- 
eases is  made  more  complete,  and  is  kept  upon  a  level 
with  the  progress  of  modern  science.  It  is  the  best 


Has  fairly  established  for  itself  a  place  among  the 
classical  medical  literature  of  England.— British, 
and  Foreign  Medico-Chir.  Review,  July,  1857. 


Dr.  Budd's  Treatise  on  Diseases  of  the  Liver  is  j  work  on  Diseases  of  the  Liver  in  any  language.— 


now  a  standard  work  in  Medical  literature,  and  dur- 
ing the  intervals  which  have  elapsed  between  the 


London  Med.  Times  and  Gazette,  June  27,  1857 
This  work,  now  the  standard  book  of  reference  on 


_____  _____  ,„_  .......  ________________________  __ 

successive  editions,  the  author  has  incorporated  into  the  diseases  of  which  it  treats,  has  been  carefully 
the  text  the  moststriking  novelties  which  have  cha-  i  revised,  and  many  new  illustrations  of  the  views  of 
racterized  the  recent  progress  of  hepatic  physiology  |  the  learned  author  added  in  the  present  edition.— 
and  pathology;  so  thatalthough  the  size  of  the  book  !  Dublin  Quarterly  Journal,  Aug.  1&57. 

BY  THE  SAME  AUTHOR. 

ON  THE   ORGANIC  DISEASES  AND  FUNCTIONAL  DISORDERS  OF 

THE  STOMACH.    In  one  neat  octavo  volume,  extra  cloth.    $1  50. 


BUCKNILL  (J.  C.),   M.  D., 

Medical  Superintendent  of  the  Devon  County  Lunatic  Asylum;  and 

DANIEL   H.  TUKE,   M .  D., 
Visiting  Medical  Officer  to  the  York  Retreat. 

A  MANUAL  OF  PSYCHOLOGICAL   MEDICINE;   containing  the  History, 

Nosology,  Description,  Statistics,  Diagnosis,  Pathology,  and  Treatment  of  INSANITY.    With 

a  Plate.     In  one  handsome  octavo  volume,  of  536  pages.     $3  00. 

The  increase  of  mental  disease  in  its  various  forms,  and  the  difficult  questions  to  which  it  is 
constantly  giving  rise,  reader  the  subject  one  of  daily  enhanced  interest,  requiring  on  the  part  of 
the  physician  a  constantly  greater  familiarity  with  this,  the  most  perplexing  branch  of  his  profes- 
sion. At  the  same  time  there  has  been  for  some  years  no  work  accessible  in  this  country,  present- 
ing the  results  of  recent  investigations  in  the  Diagnosis  and  Prognosis  of  Insanity,  and  the  greatly 
improved  methods  of  treatment  which  have  done  so  much  in  alleviating  the  condition  or  restoring 
the  health  of  the  insane.  To  fill  this  vacancy  the  publishers  present  this  volume,  assured  that 
the  distinguished  reputation  and  experience  of  the  authors  will  entitle  it  at  once  to  the  confidence 
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that  "their  aim  has  been  to  supply  a  text  book  which  may  serve  as  a  guide  in  the  acquisition  oi 
such  knowledge,  sufficiently  elementary  to  be  adapted  to  the  wants  of  the  student,  and  sufficiently 
modern  in  its  views  and  explicit  in  its  teaching  to  suffice  for  the  demands  of  the  practitioner." 


BENNETT   (J.    HUGHES),    M.  D.,    F.  R.  S.  E., 

Professor  of  Clinical  Medicine  in  the  University  of  Edinburgh,  &c. 

THE  PATHOLOGY  AND  TREATMENT  OF  PULMONARY  TUBERCU- 

LOSIS,  and  on  the  Local  Medication  of  Pharyngeal  and  Laryngeal  Diseases  frequently  mistaken 
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BENNETT   (HENRY),  M.  D. 
A  PRACTICAL  TREATISE   ON  INFLAMMATION  OF  THE  UTERUS, 

ITS  CERVIX  AND  APPENDAGES,  and  on  its  connection  with  Uterine  Disease.  To  which 
is  added,  a  Review  of  the  present  state  of  Uterine  Pathology.  Fifth  American,  from  the  third 
English  edition.  In  one  octavo  volume,  of  about  500  pages,  extra  cloth.  $2  00.  (Now  Ready.) 
The  ill  health  of  the  author  having  prevented  the  promised  revision  of  this  work,  the  present 

edition  is  a  reprint  of  the  last,  without  alteration.     As  the  volume  has  been  for  some  time  out  of 

print,  gentlemen  desiring  copies  can  now  procure  them. 


BOWMAN  (JOHN   E.),  M.D. 
PRACTICAL   HANDBOOK   OF   MEDICAL   CHEMISTRY.     Second  Ame- 

rican,  from  the  third  and  revised  English  Edition.    In  one  neat  volume,  royal  12mo.,  extra  cloth, 
with  numerous  illustrations,    pp.  288.    $1  25. 

BY  THE  SAME  AUTHOR. 

INTRODUCTION    TO    PRACTICAL    CHEMISTRY,    INCLUDING   ANA- 

LYSIS.    Second  American,  from  the  second  and  revised  London  edition.    With  numerous  illus- 
trations.   In  one  neat  vol.,  royal  12mo.,  extra  cloth,    pp.  350.    $1  25. 


BEALE  ON  THE  LAWS  OF  HEALTH  IN  RE- 
LATION TO  MIND  AND  BODY.  A  Series  of 
Letters  from  an  old  Practitioner  to  a  Patient.  In 
one  volume,  royal  12mo.,  extra  cloth,  pp.  296. 
80  cents. 

BUSHNAN'S  PHYSIOLOGY  OF  ANIMAL  AND 
VEGETABLE  LIFE;  a  Popular  Treatise  on  the 
Functions  and  Phenomena  of  Organic  Life.  In 
one  handsome  royal  12mo.  volume,  extra  cloth, 
with  over  100  illustrations,  pp.234.  80  cents. 


BUCKLER  ON  THE  ETIOLOGY,  PATHOLOGY, 
AND  TREATMENT  OF  FIBRO-BRONCHI- 
TIS  AND  RHEUMATIC  PNEUMONIA.  In 

one  8vo.  volume,  extra  cloth,    pp.  150.    SI  25. 

BLOOD  AND  URINE  (MANUALS  ON).  BY 
JOHN  WILLIAM  GRIFFITH,  G.  OWEN 
REESE,  AND  ALFRED  MARKWICK.  One 
thick  volume,  royal  12mo.,  extra  cloth,  with 
plates,  pp.  460.  $1  25. 

BRODIE'S  CLINICAL  LECTURES  ON  SUR- 
GERY. 1  vol.  8vo.  cloth.  350pp.  $125. 


AND    SCIENTIFIC    PUBLICATIONS. 


BARCLAY  (A.  W.),  M.  D., 

Assistant  Physician  to  St.  George's  Hospital,  &c. 

A  MANUAL  OF  MEDICAL  DIAGNOSIS;   being  an  Analysis  of  the  Signs 

and  Symptoms  of  Disease.    In  one  neat  octavo  volume,  extra  cloth,  of  424  pages.   $200.    (Lately 

issued.} 

Of  works  exclusively  devoted  to  this  important  |  The  task  of  composing  such  a  work  is  neither  an 
branch,  our  profession  has  at  command,  cornpara-  easy  nor  a  light  one ;  but  Dr.  Barclay  has  performed 
tively,  but  few,  and,  therefore,  in  the  publication  of  j  it  in  a  manner  which  meets  our  most  unqualified 


the  present  work,  Messrs.  Blanchard  &  Lea  have 
conferred  a  great  favor  upon  us.  Dr.  Barclay,  from 
having  occupied,  for  a  long  period,  the  position  of 
Medical  Registrar  at  St.  George's  Hospital,  pos- 
sessed advantages  for  correct  observation  and  reli- 
able conclusions,  as  to  the  significance  of  symptoms, 
which  have  fallen  to  the  lot  of  but  few,  either  in 
his  own  or  any  other  country.  He  has  carefully 
systematized  the  results  of  his  observation  of  over 
twelve  thousand  patients,  and  by  his  diligence  and 
judicious  classification,  the  profession  has  been 
presented  with  the  most  convenient  and  reliable 
work  on  the  subject  of  Diagnosis  that  it  has  been 
our  good  fortune  ever  to  examine;  we  can,  there- 
fore, say  of  Dr.  Barclay's  work,  that,  from  his  sys- 
tematic manner  of  arrangement,  his  work  is  one  of 
the  best  works  "  for  reference"  in  the  daily  emer- 
gencies of  the  practitioner,  with  which  we  are  ac- 
quainted ;  but,  at  the  same  time,  we  would  recom- 
mend our  readers,  especially  the  younger  ones,  to 
read  thoroughly  and  study  diligently  the  whole  work, 
and  the  "emergencies"  will  not  occur  so  often. — 
Southern  Med.  and  Surg.  Journ.,  March,  1858. 

To  give  this  information,  to  supply  this  admitted 
deficiency,  is  the  object  of  Dr.  Barclay's  Manual. 


approbation.  He  is  no  mere  theorist;  he  knows  his 
work  thoroughly,  and  in  attempting  to  perform  it, 
has  not  exceeded  his  powers. — British  Med.  Journal, 
Dec.  5,  1857. 

We  venture  to  predict  that  the  work  will  be  de- 
servedly popular,  and  soon  become,  like  Watson's 
Practice,  an  indispensable  necessity  to  the  practi- 
tioner.— N.  A.  Med.  Journal,  April,  1858. 

An  inestimable  work  of  reference  for  the  young 
practitioner  and  student. — Nashville  Med.  Journal, 
May,  1858. 

We  hope  the  volume  will  have  an  extensive  cir- 
culation, not  among  students  of  medicine  only,  but 
practitioners  also.  They  will  never  regret  a  faith- 
ful study  of  its  pages. — Cincinnati  Lancet,  Mar.  '58. 

An  important  acquisition  to  medical  literature. 
It  is  a  work  of  high  merits  both  from  the  vast  im- 
portance of  the  subject  upon  which  it  treats,  and 
also  from  the  real  ability  displayed  in  its  elabora- 
tion. In  conclusion,  let  us  bespeak  for  this  volume 
that  attention  of  every  student  of  our  art  which  it 
so  richly  deserves  —  that  place  in  every  medical 
library  which  it  can  so  well  adorn. — Peninsular 
Medical  Journal,  Sept.  1858. 


BARLOW  (GEORGE  H.),   M.  D, 

Physician  to  Guy's  Hospital,  London,  &c. 

A  MANUAL  OF  THE  PRACTICE  OF  MEDICINE.     With  Additions  by  D. 

F.  CONDIE,  M.  D.,  author  of  "  A  Practical  Treatise  on  Diseases  of  Children,"  &c.    In  one  hend- 
some  octavo  volume,  leather,  of  over  600  pages.    $2  75. 

will  be  found  hardly  less  useful  to  the  experienced 


We  recommend  Dr .  Barlow' s  Manual  in  the  warm- 
est manner  as  a  most  valuable  vade-mecum.  We 
have  had  frequent  occasion  to  consult  it,  and  have 
found  it  clear,  concise,  practical,  and  sound.  It  is 
eminently  a  practical  work,  containing  all  that  is 
essential,  and  avoiding  useless  theoretical  discus- 
sion. The  work  supplies  what  has  been  for  some 
time  wanting,  a  manual  of  practice  based  upon  mo- 
dern discoveries  in  pathology  and  rational  views  of 
treatment  of  disease.  It  is  especially  intended  for 
the  use  of  students  and  junior  practitioners,  but  it 


physician.  The  American  editor  has  added  to  the 
work  three  chapters— on  Cholera  Infantum,  Yellow 
Fever,  and  Cerebro-spinal  Meningitis.  These  addi- 
tions, the  two  first  of  which  are  indispensable  to  a 
work  on  practice  destined  for  the  profession  in  this 
country,  are  executed  with  great  judgment  and  fi- 
delity, by  Dr.  Condie,  who  has  also  succeeded  hap- 
pily in  imitating  the  conciseness  and  clearness  of 
style  which  are  such  agreeable  characteristics  of 
the  original  book. — Boston  Med.  and  Surg.  Journal. 


BARTLETT  (ELISHA),  M.  D. 
THE  HISTORY,  DIAGNOSIS,  AND  TREATMENT  OF  THE  FEVERS 

OF  THE  UNITED  STATES.    A  new  and  revised  edition.    By  ALONZO  CLARK,  M.  D.,  Prof. 

of  Pathology  and  Practical  Medicine  in  the  N.  Y.  College  of  Physicians  and  Surgeons,  &c.    In 

one  octavo  volume,  of  six  hundred  pages,  extra  cloth.    Price  $3  00. 

logy.  His  annotations  add  much  to  the  interest  of 
the  work,  and  have  brought  it  well  up  to  the  condi- 
tion of  the  science  as  it  exists  at  the  present  day 
in  regard  to  this  class  of  diseases.— Southern  Med. 
and  Surg.  Journal,  Mar.  1857. 

It  is  a  work  of  great  practical  value  and  interest, 
containing  much  that  is  new  relative  to  the  several 
diseases  of  which  it  treats,  and,  with  the  additions 
of  the  editor,  is  fully  up  to  the  times.  The  distinct- 
ive features  of  the  different  forms  of  fever  are  plainly 
and  forcibly  portrayed,  and  the  lines  of  demarcation 
carefully  and  accurately  drawn,  and  to  the  Ameri- 
can practitioner  is  a  more  valuable  and  safe  guide 
than  any  work  on  fever  extant. — Ohio  Med.  and 
Surg.  Journal,  May,  1857. 


It  is  the  best  work  on  fevers  which  has  emanated 
from  the  American  press,  and  the  present  editor  has 
carefully  availed  himself  of  all  information  exist- 
ing upon  the  subject  in  the  Old  and  New  World,  so 
that  the  doctrines  advanced  are  brought  down  to  the 
latest  date  in  the  progress  of  this  department  of 
Medical  Science.— London  Med.  Times  and  Gazette, 
May  2,  1857. 

This  excellent  monograph  on  febrile  disease,  has 
stood  deservedly  high  since  its  first  publication.  It 
will  be  seen  that  it  has  now  reached  its  fourth  edi- 
tion under  the  supervision  of  Prof.  A.  Clark,  a  gen- 
tleman who,  from  the  nature  of  his  studies  and  pur- 
suits, is  well  calculated  to  appreciate  and  discuss 
the  many  intricate  and  difficult  questions  in  patho- 


BROWN    (ISAAC    BAKER), 

Surgeon- Accoucheur  to  St.  Mary's  Hospital,  &c. 

ON  SOME  DISEASES  OF  WOMEN  ADMITTING  OF  SURGICAL  TREAT- 

MENT.    With  handsome  illustrations.    One  vol.  8vo.,  extra  cloth,  pp.  276.    f  1  60. 

Mr.  Brown  has  earned  for  himself  a  high  reputa-  [  and  merit  the  careful  attention  of  every  surgeon- 
tion  in  the  operative  treatment  of  sundry  diseases  accoucheur. — Association  Journal. 
and  injuries  to  which  females  are  peculiarly  subject. 
We  can  truly  say  of  his  work  that  it  is  an  important 
addition  to  obstetrical  literature.     The  operative 
suggestions  and  contrivances  which  Mr.  Brown  de- 
scribes, exhibit  much  practical  sagacity  and  skill, 


We  have  no  hesitation  in  recommending  this  book 
to  tlie  careful  attention  of  all  surgeons  who  make 
female  compiaints  a  partof  their  study  and  prsctioa. 
— Dublin  Quarterly  Journal. 


6 


BLANCHARD  &  LEA'S    MEDICAL 


CARPENTER  (WILLIAM    B.),   M.  D.,  F.  R.  S.,  &c., 

Examiner  in  Physiology  and  Comparative  Anatomy  in  the  University  of  London. 

PRINCIPLES  OF  HUMAN  PHYSIOLOGY;  with  their  chief  applications  to 

Psychology,  Pathology,  Therapeutics,  Hygiene,  and  Forensic  Medicine.  A  new  American,  from 
the  last  and  revised  London  edition.  With  nearly  three  hundred  illustrations.  Edited,  with  addi- 
tions, by  FRANCIS  GTJRNEY  SMITH,  M.  D.,  Professor  of  the  Institutes  of  Medicine  in  the  Pennsyl- 
vania Medical  College,  &c.  In  one  very  large  and  beautiful  octavo  volume,  of  about  nine  hundred 
large  pages,  handsomely  printed  and  strongly  bound  in  leather,  with  raised  bands.  $4  25. 

In  the  preparation  of  this  new  edition,  the  author  ha*  spared  no  labor  to  render  it,  as  heretofore, 
a  complete  and  lucid  exposition  of  the  most  advanced  condition  of  its  important  subject.  The 
amount  of  the  additions  required  to  effect  this  object  thoroughly,  joined  to  the  former  large  size  of 
the  volume,  presenting  objections  arising  from  the  unwieldy  bulk  of  the  work,  he  has  omitted  all 
those  portions  not  bearing  directly  upon  HUMAN  PHYSIOLOGY,  designing  to  incorporate  them  in 
his  forthcoming  Treatise  on  GENERAL  PHYSIOLOGY.  As  a  full  and  accurate  text-book  on  the  Phy- 
siology of  Man,  the  work  in  its  present  condition  therefore  presents  even  greater  claims  upon 
the  student  and  physician  than  those  which  have  heretofore  won  for  it  the  very  wide  and  distin- 
guished favor  which  it  has  so  long  enjoyed.  The  additions  of  Prof.  Smith  will  be  found  to  supply 
whatever  may  have  been  wanting  to  the  American  student,  while  the  introduction  of  many  new 
illustrations,  and  the  most  careful  mechanical  execution,  render  the  volume  one  of  the  most  at- 
tractive as  yet  issued. 


For  upwards  of  thirteen  years  Dr.  Carpenter's 
work  has  been  considered  by  the  profession  gene- 
rally, both  in  this  country  and  England,  as  the  most 
valuable  compendium  on  the  subject  of  physiology 
in  our  language.  This  distinction  it  owes  to  the  high 
attainments  and  unwearied  industry  of  its  accom- 
plished author.  The  present  edition  (which,  like  the 
last  American  one,  was  prepared  by  the  author  him- 
self)} is  the  result  of  such  extensive  revision,  that  it. 
may  almost  be  considered  a  new  work.  We  need 
hardly  say,  in  concluding  this  brief  notice,  that  while 
the  work  is  indispensable  to  every  student  of  medi- 
eine  in  this  country,  it  will  amply  repay  the  practi- 
tioner for  its  perusal  by  the  interest  and  value  of  its 
contents.— Boston  Med.  and  Surg.  Journal. 

This  is  a  standard  work — the  text-book  used  by  all 
medical  students  who  read  the  English  language. 
It  has  passed  through  several  editions  in  order  to 
keep  pace  with  the  rapidly  growing  science  of  Phy- 
siology. Nothing  need  be  said  in  its  praise,  for  its 
merits  are  universally  known ;  we  have  nothing  to 
say  of  its  defects,  for  they  only  appear  where  the 
science  of  which  it  treats  is  incomplete. — Western 
Lancet. 

The  most  complete  exposition  of  physiology  which 
any  language  can  at  present  give. — Brit .  and  For. 
Med.-Chirurg.  Review. 

The  greatest,  the  most  reliable,  and  the  best  book 
on  the  subject  which  we  know  of  in  the  English 
language. — Stethoscope . 


To  eulogize  this  great  work  would  be  superfluous. 

We  should  observe,  however,  that  in  this  edition 

i  the  author  has  remodelled  a  large  portion  of  the 

I  former,  and  the  editor  has  added  much  matter  of  in- 

j  terest,  especially  in  the  form  of  illustrations.    We 

I  may  confidently  recommend  it  as  the  most  complete 

I  work  on  Human    Physiology  in  our   language. — 

Southern  Med.  and  Surg.  Journal. 

The  most  complete  work  on  the  science  in  o-ur 
language. — Am.  Med.  Journal. 

The  most  complete  work  now  extant  in  our  lan- 
guage.— N.  O.  Med.  Register. 

The  best  text-book  in  the  language  on  this  ex- 
tensive subject.— London  Med.  Times. 

A  complete  cyclopaedia  of  this  branch  of  science. 
— N.  Y.  Med.  Times. 

The  profession  of  this  country,  and  perhaps  also 
of  Europe,  have  anxiously  and  for  some  time  awaited 
the  announcement  of  this  new  edition  of  Carpenter's 
Human  Physiology.  His  former  editions  have  for 
many  years  been  almost  the  only  text-book  on  Phy- 
siology in  all  our  medical  schools,  and  its  circula- 
tion among  the  profession  has  been  unsurpassed  by 
any  work  in  any  department  of  medical  science. 

It  is  quite  unnecessary  for  us  to  speak  of  this 
work  as  its  merits  would  justify.  The  mere  an- 
nouncement of  its  appearance  will  afford  the  highest 
pleasure  to  every  student  of  Physiology,  while  its 
perusal  will  be  of  infinite  service  in  advancing 
physiological  science. — Ohio  Med.  and  Surg.  Jour*. 


BY  THE  SAME  AUTHOR. 

PRINCIPLES  OF  COMPARATIVE  PHYSIOLOGY.    New  American,  from 

the  Fourth  and  Revised  London  edition.    In  one  large  and  handsome  octavo  volume,  with  over 
three  hundred  beautiful  illustrations,    pp.752.    Extra  cloth,  $4  80 ;  leather,  raised  bands,  $5  25. 

The  delay  which  has  existed  in  the  appearance  of  this  work  has  been  caused  by  the  very  thorough 
revision  and  remodelling  which  it  has  undergone  at  the  hands  of  the  author,  and  the  large  number 
of  new  illustrations  which  have  been  prepared  for  it.  It  will,  therefore,  be  found  almost  a  new 
work,  and  fully  up  to  the  day  in  every  department  of  the  subject,  rendering  it  a  reliable  text-book 
for  all  students  engaged  in  this  branch  of  science.  Every  effort  has  been  made  to  render  its  typo- 
graphical finish  and  mechanical  execution  worthy  of  its  exalted  reputation,  and  creditable  to  the 
mechanical  arts  of  this  country. 


This  book  should  not  only  be  read  but  thoroughly 
studied  by  every  member  of  the  profession.  None 
are  too  wise  or  old,  to  be  benefited  thereby.  But 
especially  to  the  younger  class  would  we  cordially 
commend  it  as  best  fitted  of  anytwork  in  the  English 
language  to  qualify  them  for  the  reception  and  com- 
prehension of  those  truths  which  are  daily  being  de- 
veloped in  physiology. — Medical  Counsellor. 

Without  pretending  to  it,  it  is  an  encyclopedia  of 
the  subject,  accurate  and  complete  in  all  respects — 
a  truthful  reflection  of  the  advanced  state  at  which 
the  science  has  now  arrived. — Dublin  Quarterly 
Journal  of  Medical  Science. 

A  truly  magnificent  work— in  itself  a  perfect  phy- 
siological study. — Ranking' s  Abstract. 

This  work  stands  without  its  fellow.  It  in  one 
few  men  in  Europe  could  have  undertaken ;  it  is  one 


no  man,  we  believe,  could  have  brought  to  so  suc- 
cessful an  issue  as  Dr.  Carpenter.  It  required  for 
its  production  a  physiologist  at  once  deeply  read  in 
the  labors  of  others,  capable  of  taking  a  general, 
critical,  and  unprejudiced  view  of  those  labors,  and 
of  combining  the  varied,  heterogeneous  materials  at 
his  disposal,  so  as  to  form  an  harmonious  whole. 
We  feel  that  this  abstract  can  give  the  reader  a  very 
imperfect  idea  of  the  fulness  of  this  work,  and  no 
idea  of  its  unity,  of  the  admirable  manner  in  which 
material  has  been  brought,  from  the  most  various 
sources,  to  conduce  to  its  completeness,  of  the  lucid- 
ity of  the  reasoning  it  contains,  or  of  the  clearness 
of  language  in  which  the  whole  is  clothed.  Not  the 
profession  only,  but  the  scientific  world  at  large, 
must  feel  deeply  indebted  to  Dr.  Carpenter  for  this 
great  work.  It  must,  indeed,  add  largely  even  to 
his  high  reputation.— Medical  Times. 


AND    SCIENTIFIC    PUBLICATIONS 


CARPENTER  (WILLIAM  B.),   M.  D.,  F.  R.  S., 

Examiner  in  Physiology  and  Comparative  Anatomy  in  the  University  of  London. 

DHE  MICROSCOPE  AND  ITS  REVELATIONS.      With  an  Appendix  con- 

taining  the  Applications  of  the  Microscope  to  Clinical  Medicine,  &c.  By  F.  G.  SMITH,  M.  D. 
Illustrated  by  four  hundred  and  thirty-four  beautiful  engravings  on  wood.  In  one  large  and  verp 
handsome  octavo  volume,  of  724  pages,  extra  cloth,  $4  00  ;  leather,  $4  50. 

Dr.  Carpenter's  position  as  a  microscopist  and  physiologist,  and  his  great  experience  as  a  teacher, 
jminently  qualify  him  to  produce  what  has  long  been  wanted — a  good  text-book  on  the  practical 
jse  of  the  microscope.  In  the  present  volume  his  object  has  been,  as  stated  in  his  Preface,  "  to 
jombine,  within  a  moderate  compass,  that  information  with  regard  to  the  use  of  his  '  tools,'  which 
.s  most  essential  to  the  working  microscopist,  with  such  an  account  of  the  objects  best  fitted  for 
lis  study,  as  might  qualify  him  to  comprehend  what  he  observes,  and  might  thus  prepare  him  to 
jeneh't  science,  whilst  expanding  and  refreshing  his  own  mind  "  That  he  has  succeeded  in  accom- 
jlishing  this,  no  one  acquainted  with  his  previous  labors  can  doubt. 

The  great  importance  of  the  microscope  as  a  means  of  diagnosis,  and  the  number  of  microsco- 
jists  who  are  also  physicians,  have  induced  the  American  publishers,  with  the  author's  approval,  to 
idd  an  Appendix,  carefully  prepared  by  Professor  Smith,  on  the  applications  of  the  instrument  to 
clinical  medicine,  together  with  an  account  of  American  Microscopes,  their  modifications  and 
iccessories.  This  portion  of  the  work  is  illustrated  with  nearly  one  hundred  wood-cuts,  and,  it  is 
loped,  will  adapt  the  volume  more  particularly  to  the  use  of  the  American  student. 

Every  care  has  been  taken  in  the  mechanical  execution  of  the  work,  which  is  confidently  pre- 
sented as  in  no  respect  inferior  to  the  choicest  productions  of  the  London  press. 

The  mode  in  which  the  author  has  executed  his  intentions  may  be  gathered  from  the  following 
sondensed  synopsis  of  the 

CONTENTS. 

[INTRODUCTION — History  of  the  Microscope.  CHAP.  I.  Optical  Principles  of  the  Microscope. 
CHAP.  II.  Construction  of  the  Microscope.  CHAP.  III.  Accessory  Apparatus.  CHAP.  IV. 
Management  of  the  Microscope  CHAP.  V.  Preparation,  Mounting,  and  Collection  of  Objects. 
CHAP.  VI.  Microscopic  Forms  of  Vegetable  Life — Protophytes.  CHAP.  VII.  Higher  Cryptoga- 
mia.  CHAP.  VIII.  Phanerogamic  Plants.  CHAP.  IX.  Microscopic  Forms  of  Animal  Life — Pro- 
tozoa— Animalcules.  CHAP.  X.  Foraminifera,  Polycystina,  and  Sponges.  CHAP.  XL  Zoophytes. 
CHAP.  XII.  Echinodermata.  CHAP.  XIII.  Polyzoa  and  Compound  Tunicata.  CHAP.  XIV. 
Molluscous  Animals  Generally.  CHAP.  XV.  Annulosa.  CHAP.  XVI.  Crustacea.  CHAP.  XVII. 
Insects  and  Arachnida.  CHAP.  XVIII.  Vertebrated  Animals.  CHAP.  XIX.  Applications  of  the 
Microscope  to  Geology.  CHAP.  XX.  Inorganic  or  Mineral  Kingdom — Polarization.  APPENDIX. 
Microscope  as  a  means  of  Diagnosis — Injections — Microscopes  of  American  Manufacture. 

Those  who  are  acquainted  with  Dr.  Carpenter's 
previous  writings  on  Animal  and  Vegetable  Physio- 
fogy,  will  fully  understand  how  vast  a  store  of  know- 
ledge he  is  able  to  bring  to  bear  upon  so  comprehen- 
sive a  subject  as  the  revelations  of  the  microscope  ; 
and  even  those  who  have  no  previous  acquaintance 
with  the  construction  or  uses  of  this  instrument, 
will  find  abundance  of  information  conveyed  in  clear 
and  simple  language.— Med.  Times  and  Gazette. 

Although  originally  not  intended  as  a  strictly 


medical  work,  the  additions  by  Prof.  Smith  give  it 
a  positive  claim  upon  the  profession,  for  which  we 
doubt  not  he  will  receive  their  sincere  thanks.  In- 
deed, we  know  not  where  the  student  of  medicine 
will  find  such  a  complete  and  satisfactory  collection 
of  microscopic  facts  bearing  upon  physiology  and 
practical  medicine  as  is  contained  in  Prof.  Smith's 
appendix;  and  this  of  itself,  it  seems  to  us,  is  fully 
worth  the  cost  of  the  volume.— Louisville  Medical 
Review,  Nov.  1856. 


BY   THE  SAME   AUTHOR. 

ELEMENTS  (OR  MANUAL)  OF  PHYSIOLOGY,  INCLUDING  PHYSIO- 
LOGICAL ANATOMY.  Second  American,  from  a  new  and  revised  London  edition.  With 
one  hundred  and  ninety  illustrations.  In  one  very  handsome  octavo  volume,  leather,  pp.  566. 
$3  00. 

In  publishing  the  first  edition  of  this  work,  its  title  was  altered  from  that  of  the  London  volume, 
by  the  substitution  of  the  word  "  Elements"  for  that  of  "  Manual,"  and  with  the  author's  sanction 
tae  title  of  "  Elements"  is  still  retained  as  being  more  expressive  of  the  scope  of  the  treatise. 


To  say  that  it  is  the  best  manual  of  Physiology 
now  before  the  public,  would  not  do  sufficient  justice 
to  the  author.— Buffalo  Medical  Journal. 

In  his  former  works  it  would  seem  that  he  had 
exhausted  the  subject  of  Physiology.  In  the  present, 
he  gives  the  essence,  as  it  were,  of  the  whole. — N.  Y. 
Journal  of  Medicine . 


Those  who  have  occasion  for  an  elementary  trea- 
tise on  Physiology,  cannot  do  better  than  to  possess 
themselves  of  the  manual  of  Dr.  Carpenter. — Medical 
Examiner. 

The  best  and  most  complete  expose"  of  modern 
Physiology,  in  one  volume,  extant  in  the  English 
language. — St.  Louis  Medical  Journal. 


BY  THE  SAME  AUTHOR.     (Preparing.) 

PRINCIPLES  OF   GENERAL   PHYSIOLOGY,   INCLUDING   ORGANIC 

CHEMISTRY  AND  HISTOLOGY.    With  a  General  Sketch  of  the  Vegetable  and  Animal 
Kingdom.    In  one  large  and  very  handsome  octavo  volume,  with  several  hundred  illustrations. 
The  subject  of  general  physiology  having  been  omitted  in  the  last  editions  ot  the  author's  "  Com- 
parative Physiology"  and  "Human  Physiology,"  he  has  undertaken  to  prepare  a  volume  which 
shall  present  it  more  thoroughly  and  fully  than  has  yet  been  attempted,  and  which  may  be  regarded 
as  an  introduction  to  his  other  works. 

BY  THE  SAME   AUTHOR. 

A  PRIZE  ESSAY  ON  THE  USE  OF  ALCOHOLIC  LIQUORS  IN  HEALTH 

AND  DISEASE.    New  edition,  with  a  Preface  by  D.  F.  CONDIE,  M.  D.,  and  explanations  of 
Hp.ip.ntifif.  words.     Tn  ons  neat  12rno.  vnlnme.  extra  eloth.    tm.  178.     50  cents. 


8 


BLANCHAKD  &  LEA'S  MEDICAL 


CONDIE  (D.  F.),  M.  D.,  &c. 
A  PRACTICAL  TREATISE  ON  THE  DISEASES  OF  CHILDREN.    Fifth 

edition,  revised  and  augmented.    In  one  large  volume,  8vo.,  leather,  of  over  750  pages.  $3  25. 

(Just  Issued,  1859.) 

In  presenting  a  new  and  revised  edition  of  this  favorite  work,  the  publishers  have  only  to  state 
that  the  author  has  endeavored  to  render  it  in  every  respect  "a  complete  and  faithful  exposition  of 
the  pathology  and  therapeutics  of  the  maladies  incident  to  the  earlier  stages  of  existence — a  full 
and  exact  account  of  the  diseases  of  infancy  and  childhood.."  To  accomplish  this  he  has  subjected 
the  whole  work  to  a  careful  and  thorough  revision,  rewriting  a  considerable  portion,  and  adding 
several  new  chapters.  In  this  manner  it  is  hoped  that  any  deficiencies  which  may  have  previously 
existed  have  been  supplied,  that  the  recent  labors  of  practitioners  and  observers  have  been  tho- 
roughly incorporated,  and  that  in  every  point  the  work  will  be  found  to  maintain  the  high  reputation 
it  has  enjoyed  as  a  complete  and  thoroughly  practical  book  of  reference  in  infantile  affections. 

A  few  notices  of  previous  editions  are  subjoined. 


Dr.  Condie's  scholarship,  acumen,  industry,  and 
practical  sense  are  manifested  in  this,  as  in  all  his 
numerous  contributions  to  science. — Dr.  Holmes's 
Report  to  the  American  Medical  Association. 

Taken  as  a  whole,  in  our  judgment,  Dr.  Condie's 
Treatise  is  the  one  from  the  perusal  of  which  the 
practitioner  in  this  country  will  rise  with  the  great- 
est satisfaction. — Western  Journal  of  Medicine  and 
Surgery. 

One  of  the  best  works  upon  the  Diseases  of  Chil- 
dren in  the  English  language. — Western  Lancet. 

We  feel  assured  from  actual  experience  that  no 
physician's  library  can  be  complete  without  a  copy 
of  this  work.— N.  Y.  Journal  of  Medicine. 

A  veritable  psediatric  encyclopaedia,  and  an  honor 
to  American  medical  literature. — Ohio  Medical  and 
SurgicalJournal. 

We  feel  persuaded  that  the  American  medical  pro- 
fession will  soon  regard  it  not  only  as  a  very  good, 
but  as  the  VERY  BEST  "  Practical  Treatise  on  the 
Diseases  of  Children." — American  Medical  Journal. 

In  the  department  of  infantile  therapeutics,  the 
work  of  Dr.  Condie  is  considered  one  of  the  best 
which  has  been  published  in  the  English  language. 
— The  Stethoscope. 


We  pronounced  the  first  edition  to  be  the  best 
work  on  the  diseases  of  children  in  the  English 
language,  and,  notwithstanding  all  that  has  been 


we  still  regard  it  in  that  light.  —  Medical 


anguage, 
published, 
Examiner. 

The  value  of  works  by  native  authors  on  the  dis- 
eases which  the  physician  is  called  upon  to  combat, 
will  be  appreciated  by  all  ;  and  the  work  of  Dr.  Con- 
die  has  gained  for  itself  the  character  of  a  safe  guide 
for  students,  and  a  useful  work  for  consultation  by 
those  engaged  in  practice.—  N.  Y.  Med.  Times. 

This  is  the  fourth  edition  of  this  deservedly  popu- 
lar treatise.  During  the  interval  since  the  last  edi- 
tion, it  has  been  subjected  to  a  thorough  revision 
by  the  author;  and  all  new  observations  in  the 
pathology  and  therapeutics  of  children  have  been 
included  in  the  present  volume.  As  we  said  before, 
we  do  not  know  of  a  better  book  on  diseases  of  chil- 
dren, and  to  a  large  part  of  its  recommend.itions  we 
yield  an  unhesitating  concurrence.  —  Buffalo  Med. 
Journal. 

Perhaps  the  most  full  and  complete  work  now  be- 
fore the  profession  of  the  United  States  ;  indeed,  we 
may  say  in  the  English  language.  It  is  vastly  supe- 
rior to  most  of  its  predecessors.  —  Transylvania  Med. 
Journal  . 


CHRISTISON  (ROBERT),  M.  D.,  V.  P<  R.  S.  E.,  &c. 
A  DISPENSATORY;  or.  Commentary  on  the  Pharmacopeias  of  Great  Britain 

and  the  United  States ;  comprising  the  Natural  History,  Description,  Chemistry,  Pharmacy,  Ac- 
tions, Uses,  and  Doses  of  the  Articles  of  the  Materia  Medica.  Second  edition,  revised  and  im- 
proved, with  a  Supplement  containing  the  most  important  New  Remedies.  With  copious  Addi- 
tions, and  two  hundred  and  thirteen  large  wood -engravings.  By  R.  EGLESFELD  GRIFFITH,  M.  D. 
In  one  very  large  and  handsome  octavo  volume,  leather,  raised  bands,  of  over  1000  pages.  $3  50. 

COOPER  (BRANSBY  B.),  F.  R.  S. 
LECTURES  ON  THE  PRINCIPLES  AND  PRACTICE  OF  SURGERY. 

in  one  very  large  octavo  volume,  extra  cloth,  of  750  pages.    $300. 


COOPER  ON  DISLOCATIONS  AND  FRAC- 
TURES OF  THE  JOINTS.— Edited  by  BRANSBY 
B.  COOPER,  F.  R.  S.,  &c.  With  additional  Ob- 
servations by  Prof.  J.  C.  WARREN.  A  new  Ame- 
rican edition.  In  one  handsome  octavo  volume, 
extra  cloth,  of  about  500  pages,  with  numerous 
illustrations  on  wood.  $3  25. 

COOPER  ON  THE  ANATOMY  AND  DISEASES 
OF  THE  BREAST,  with  twenty-five  Miscellane- 
ous and  Surgical  Papers.  One  large  volume,  im- 
perial 8vo.,  extra  cloth,  with  252  figures,  on  36 
plates.  $2  50. 

COOPER  ON  THE  STRUCTURE  AND  DIS- 
EASES OF  THE  TESTIS,  AND  ON  THE 
THYMUS  GLAND.  One  vol.  imperial  8vo.,  ex- 
tra cloth,  with  177  figures  on  29  plates.  $2  00. 


COPLAND  ON  THE  CAUSES,  NATURE,  AND 
TREATMENT  OF  PALSY  AND  APOPLEXY. 
In  one  volume,  royal  12mo.,  extra  cloth,  pp.  328. 
80  cents. 

CLYMER  ON  FEVERS;  THEIR  DIAGNOSIS, 
PATHOLOGY,  AND  TREATMENT  In  one 
octavo  volume,  leather,  of  600  pages,  f  1  50. 

COLOMBAT  DE  L'ISERE  ON  THE  DISEASES 

OF  FEMALES,  and  on  the  special  Hygiene  of 
their  Sex.  Translated,  with  many  Notes  and  Ad- 
ditions, by  C.  D.  MEIGS,  M.  D.  Second  edition, 
revised  and  improved.  In  one  large  volume,  oc- 
tavo, leather,  with  numerous  wood-cuts,  pp.  720. 
$350. 


CARSON  (JOSEPH),  M.  D., 

Professor  of  Materia  Medica  and  Pharmacy  in  the  University  of  Pennsylvania. 

SYNOPSIS  OF  THE  COURSE  OF  LECTURES  ON  MATERIA  MEDICA 

AND  PHARMACY,  delivered  in  the  University  of  Pennsylvania,    Second  and  revised  edi- 
tion.   In  one  very  neat  octavo  volume,  extra  cloth,  of  208  pages.    $1  50. 

CURLING    (T.    B.),    F.R.S., 

Surgeon  to  the  London  Hospital,  President  of  the  Hunterian  Society,  &c. 

A  PRACTICAL  TREATISE  ON  DISEASES  OF  THE  TESTIS,  SPERMA- 
TIC CORD,  AND  SCROTUM.  Second  American,  from  the  second  and  enlarged  English  edi- 
tion. In  one  handsome  octavo  volume,  extra  cloth,  with  numerous  illustrations,  pp.  420.  $2  00. 


AND    SCIENTIFIC   PUBLICATIONS 


CHURCHILL  (FLEET WOOD),  M.  D.,  M.  R.  I.  A. 
ON  THE  THEORY  AND  PRACTICE  OF  MIDWIFERY.     A  new  American 

from  the  fourth  revised  and  enlarged  London  edition.  With  Notes  and  Additions,  by  D.  FRANCIS 
CONDIE,  M.  D.,  author  of  a  "Practical  Treatise  on  the  Diseases  of  Children,"  &c.  With  194 
illustrations.  Tn  one  very  handsome  octavo  volume,  leather,  of  nearly  700  large  pages.  $3  50. 
(Now  Ready,  October,  I860.) 

This  work  has  been  so  long  an  established  favorite,  both  as  a  text-book  for  the  learner  and  as  a 
reliable  aid  in  consultation  for  the  practitioner,  that  in  presenting  a  new  edition  it  is  only  necessary 
to  call  attention  to  the  very  extended  improvements  which  it  has  received.  Having  had  the  benefit 
of  two  revisions  by  the  author  since  the  last  American  reprint,  it  has  been  materially  enlarged,  and 
Dr.  Churchill's  well-known  conscientious  industry  is  a  guarantee  that  every  portion  has  been  tho- 
roughly brought  up  with  the  latest  results  of  European  investigation  in  all  departments  of  the  sci- 
ence and  art  of  obstetrics.  The  recent  date  of  the  last  Dublin  edition  has  not  left  much  of  novelty 
for  the  American  editor  to  introduce,  but  he  has  endeavored  to  insert  whatever  has  since  appeared, 
together  with  such  matters  as  his  experience  has  shown  him  would  be  desirable  for  the  American 
student,  including  a  large  number  of  illustrations.  With  the  sanction  of  the  author  he  has  added 
in  the  form  of  an  appendix,  some  chapters  from  a  little  "Mmiual  for  Mid  wives  and  Nurses,"  re- 
cently issued  by  Dr.  Churchill,  believing  that  the  details  there  presented  can  hardly  fail  to  prove  of 
advantage  to  the  junior  practitioner.  The  result  of  all  these  additions  is  that  the  work  now  con- 
tains fully  one-half  more  matter  than  the  last  American  edition,  with  nearly  one-half  more  illus- 
trations, so  that  notwithstanding  the  use  of  a  smaller  type,  the  volume  contains  almost  two  hundred 
pages  more  than  before. 

No  effort  has  been  spared  to  secure  an  improvement  in  the  mechanical  execution  of  the  work 
equal  to  that  which  the  text  has  received,  and  the  volume  is  confidently  presented  as  one  of  the 
handsomest  that  has  thus  far  been  laid  before  the  American  profession;  while  the  very  low  price 
at  which  it  is  offered  should  secure  for  it  a  place  in  every  lecture-room  and  on  every  office  table. 

The  most  popular  work  on  midwifery  ever  issued 


A  better  book  in  which  to  learn  these  important 
points  we  have  not  met  than  Dr.  Churchill's.  Every 
page  of  it  is  full  of  instruction;  the  opinion  of  all 
writers  of  authority  is  given  on  questions  of  diffi- 
culty, as  well  as  the  directions  and  advice  of  the 
learned  author  himself,  to  which  he  adds  the  result 
of  statistical  inquiry,  putting  statistics  in  their  pro- 
per place  and  giving  them  their  due  weight,  and  no 
more.  We  have  never  read  a  book  more  free  from 
professional  jealousy  than  Dr.  Churchill's.  It  ap- 
pears to  be  written  with  the  true  design  of  a  book  on 
medicine,  viz :  to  give  all  that  is  known  on  the  sub- 
ject of  which  he  treats,  both  theoretically  and  prac- 
tically, and  to  advance  such  opinions  of  his  own  as 
he  believes  will  benefit  medical  science,  and  insure 
the  safety  of  the  patient.  We  have  said  enough  to 
convey  to  the  profession  that  this  book  of  Dr.  Chur- 
chill's is  admirably  suited  for  a  book  of  reference 
for  the  practitioner,  as  well  as  a  text-book  for  the 
student,  and  we  hope  it  may  be  extensively  pur- 
chased amongst  our  readers.  To  "them  we  most 
strongly  recommend  it.  —  Dublin  Medical  Press , 
June  20,  1860. 

To  bestow  praise  on  a  book  that  has  received  such 
marked  approbation  would  be  superfluous.  We  need 
only  say,  therefore,  that  if  the  first  edition  was 
thought  worthy  of  a  favorable  reception  by  the 
medical  public,  we  can  confidently  affirm  that  this 
will  be  found  much  more  so.  The  lecturer,  the 
practitioner,  and  the  student,  may  all  have  recourse 
to  its  pages,  and  derive  from  their  perusal  much  in- 
terest and  instruction  in  everything  relating  to  theo- 
retical and  practical  midwifery. — Dublin  Quarterly 
Journal  of  Medical  Science. 

A  work  of  very  great  merit,  and  such  as  we  can 
confidently  recommend  to  the  study  of  every  obste- 
tric practitioner.— London  Medical  Gazette. 

This  is  certainly  the  most  perfect  system  extant. 
It  is  the  best  adapted  for  the  purposes  of  .a  teixt- 
book,  and  that  which  he  whose  necessities  confine 
him  to  one  book,  should  select  in  preference  to  all 


from  the  American  press. — Charleston  Med.  Journal. 

Were  we  reduced  to  the  necessity  of  having  but 
sne  work  on  midwifery,  and  permitted  to  choose, 
we  would  unhesitatingly  take  Churchill.— Western 
Med.  and  Surg.  Journal. 

It  is  impossible  to  conceive  a  more  useful  anil 
jlegant  manual  than  Dr.  Churchill's  Practice  of 
Midwifery.— Provincial  Medical  Journal. 

Certainly,  in  our  opinion,  the  very  best  work  on 
he  subject  which  exists.— IV.  Y.  Annalist. 

No  work  holds  a  higher  position,  or  is  more  de- 
serving of  being  placed  in  the  hands  of  the  tyro, 
the  advanced  student,  or  the  practitioner. — Medical 
Examiner. 

Previous  editions,  under  the  editorial  supervision 
of  Prof  R.  M.  Huston,  have  been  received  with 
marked  favor,  and  they  deserved  it;  but  this,  re- 
printed from  a  very  late  Dublin  edition,  carefully 
revised  and  brought  up  by  the  author  to  the  present 
time,  does  present  an  unusually  accurate  and  able 
exposition  of  every  important  particular  embraced 
in  the  department  of  midwifery.  #  *  The  clearness^ 
directness,  and  precision  of  its  teachings,  together 
with  the  great  amount  of  statistical  research  which 
its  text  exhibits,  have  served  to  place  it  already  in 
the  foremost  rank  of  works  in  this  department  of  re- 
medial science.— N.  O.  Med.  and  Surg.  Journal. 

In  our  opinion,  it  forms  one  of  the  best  if  not  the 
very  best  text-book  and  epitome  of  obstetric  science 
which  we  at  present  possess  in  the  English  lan- 
guage.— Monthly  Journal  of  Medical  Science. 

The  clearness  and  precision  of  style  in  which  it  is 
written,  and  the  greatamount  of  statistical  research 
which  it  contains,  have  served  to  place  it  in  the  first 
rank  of  works  in  this  departmentof  medical  science. 
— N.  Y.  Journal  of  Medicine. 

Few  treatises  will  be  found  better  adapted  as  a 
text-book  for  the  student,  or  as  a  manual  for  the 
frequent  consultation  of  the  young  practitioner. — 
American  Medical  Journal. 


others. — Southern  Medical  and  Surgical  Journal. 

BY  THE  SAME  AUTHOR.    (Lately  Published.) 

ON  THE  DISEASES  OF  INFANTS  AND  CHILDREN.     Second  American 

Edition,  revised  and  enlarged  by  the  author.    Edited,  with  Notes,  by  W.  V.  KEATING,  M.  D.    In 
one  large  and  handsome  volume,  extra  cloth,  of  over  700  pages.    $3  00,  or  in  leather,  $3  25. 
In  preparing  this  work  a  second  time  for  the  American  profession,  the  author  has  spared  no 
labor  in  giving  it  a  very  thorough  revision,  introducing  several  new  chapters,  and  rewriting  others, 
while  every  portion  of  the  volume  has  been  subjected  to  a  severe  scrutiny.     The  efforts  of  the 
American  editor  have  been  directed  to  supplying  such  information  relative  to  matters  peculiar 
to  this  country  as  might  have  escaped  the  attention  of  the  author,  and  the  whole  may,  there- 
fore, be  safely  pronounced  one  of  the  most  complete  works  on  the  subject  accessible  to  the  Ame- 
rican Profession.     By  an  alteration  in  the  size  of  the  page,  these  very  extensive  additions  have 
been  accommodated  without  unduly  increasing  the  size  of  the  work. 

BY   THE   SAME   AUTHOR. 

ESSAYS  ON  THE  PUERPERAL  FEVER,  AND  OTHER  DISEASES  PE- 
CULIAR TO  WOMEN.  Selected  from  the  writings  of  British  Authors  previous  to  the  close  of 
the  Eighteenth  Century.  In  one  neat  octavo  volume,  extra  cloth,  of  about  450  pages.  $2  50. 


10  BLANCHARD    &    LEA'S    MEDICAL 

CHURCHILL  (FLEETWOOD),    M.  D.,  M .  R.  I.  A.,    &c 
ON  THE  DISEASES  OF  WOMEN;  including  those  of  Pregnancy  and  Child- 
bed.    A  new  American  edition,  revised  by  the  Author.    With  Notes  and  Additions,  by  D.  FRAN- 
CIS CONDIE,  M.  D.,  author  ot  "  A  Practical  Treatise  on  the  Diseases  of  Children."    With  nume- 
rous illustrations.    In  one  large  and  handsome  octavo  volume,  leather,  of  768  pages.   $3  00. 
This  edition  of  Dr.  Churchill's  very  popular  treatise  may  almost  be  termed  a  new  work,  so 
thoroughly  has  he  revised  it  in  every  portion.     It  will  be  found  greatly  enlarged,  and  completely 
brought  up  to  the  most  recent  condition  of  the  subject,  while  the  very  handsome  series  of  illustra- 
tions introduced,  representing  such  pathological  conditions  as  can  be  accurately  portrayed,  present 
a  novel  feature,  and  afford  valuable  assistance  to  the  young  practitioner.     Such  additions  as  ap- 
peared desirable  for  the  American  student  have  been  made  by  the  editor,  Dr.  Condie,  while  a 
marked  improvement  in  the  mechanical  execution  keeps  pace  with  the  advance  in  all  other  respects 
which  the  volume  has  undergone,  while  the  price  has  been  kept  at  the  former  very  moderate  rate. 
It  comprises,  unquestionably,  one  of  the  most  ex-  I  extent  that  Dr.  Churchill  does.    His,  indeed,  is  the 
act  and  comprehensive  expositions  of  the  present  |  only  thorough  treatise  we  know  of  on  the  subject; 
state  of  medical  knowledge  in  respect  to  the  diseases  j  and  it  may  be  commended  to  practitioners  and  stu- 
dents as  a  masterpiece  in  its  particular  department. 


of  women  that  has  yet  been  published. — Am.  Journ. 
Med.  Sciences,  July,  1857. 

This  work  is  the  most  reliable  which  we  possess 
on  this  subject;  and  is  deservedly  popular  with  the 
profession.— Charleston  Med.  Journal,  July,  1857. 

We  know  of  no  author  who  deserves  that  appro- 
bation, on  "the  diseases  of  females,"  to  the  same 


— Tht  Western  Journal  of  Medicine  and  Surgery. 

As  a  comprehensive  manual  for  students,  or  a 
work  of  reference  for  practitioners,  it  surpasses  any 
other  that  has  ever  issued  on  the  same  subject  from 
the  British  press.— Dublin  Quart.  Journal. 


DICKSON   (S.    H.),    M.  D., 

Professor  of  Practice  of  Medicine  in  the  Jefferson  Medical  College,  Philadelphia. 

ELEMENTS  OF  MEDICINE;  a  Compendious  View  of  Pathology  and  Thera- 
peutics, or  the  History  and  Treatment  of  Diseases.  Second  edition,  revised.  In  one  large  and 
handsome  octavo  volume;  of  750  pages,  leather.  $3  75.  (Just  Issued.) 

The  steady  demand  which  has  so  soon  exhausted  the  first  edition  of  this  work,  sufficiently  shows 
that  the  author  was  not  mistaken  in  supposing  that  a  volume  of  this  character  was  needed — an 
elementary  manual  of  practice,  which  should  present  the  leading  principles  of  medicine  with  the 
practical  results,  in  a  condensed  and  perspicuous  manner.  Disencumbered  of  unnecessary  detail 
and  fruitless  speculations,  it  embodies  what  is  most  requisite  for  the  student  to  learn,  and  at  the 
same  time  what  the  active  practitioner  wants  when  obliged,  in  the  daily  calls  of  his  profession,  to 
refresh  his  memory  on  special  points.  The  clear  and  attractive  style  of  the  author  renders  the 
whole  ea^y  of  comprehension,  while  his  long  experience  gives  to  his  teachings  an  authority  every- 
where acknowledged.  Few  physicians,  indeed,  have  had  wider  opportunities  for  observation  and 
experience,  and  few,  perhaps,  have  used  them  to  better  purpose.  As  the  result  of  a  long  life  de- 
voted to  study  and  practice,  the  present  edition,  revised  and  brought  up  to  the  date  of  publication, 
will  doubtless  maintain  the  reputation  already  acquired  as  a  condensed  and  convenient  American 
text-book  on  the  Practice  of  Medicine. 


DRUITT   (ROBERT),   M.R.  C.S.,   &c. 
THE  PRINCIPLES  AND  PRACTICE   OF  MODERN  SURGERY.     A  new 

and  revised  American  from  the  eighth  enlarged  and  improved  London  edition.  Illustrated  with 
four  hundred  and  thirty-two  wood-engravings.  In  one  very  handsomely  printed  octavo  volume, 
leather,  of  nearly  700  large  pages.  $3  50.  (Now  Ready,  October,  1860.) 

A  work  which  like  DRUITT'S  SURGERY  has  for  so  many  years  maintained  the  position  of  a  lead- 
ing favorite  with  all  classes  of  the  profession,  needs  no  special  recommendation  to  attract  attention 
to  a  revised  edition.  It  is  only  necessary  to  state  that  the  author  has  spared  no  paiijs  to  keep  the 
work  up  to  its  well  earned  reputation  of  presenting  in  a  small  and  convenient  compass  the  latest 
condition  of  every  department  of  surgery,  considered  both  as  a  science  and  as  an  art;  and  that  the 
services  of  a  competent  American  editor  have  been  employed  to  introduce  whatever  novelties  may 
have  escaped  the  author's  attention,  or  may  prove  of  service  to  the  American  practitioner.  As 
several  editions  have  appeared  in  London  since  the  issue  of  the  last  American  reprint,  the  volume 
has  had  the  benefit  of  repeated  revisions  by  the  author,  resulting  in  a  very  thorough  alteration  and 
improvement.  The  extent  of  these  additions  may  be  estimated  from  the  fact  that  it  now  contains 
about  one-third  more  matter  than  the  previous  American  edition,  and  that  notwithstanding  the 
adoption  of  a  smaller  type,  the  pages  have  been  increased  by  about  one  hundred,  while  nearly  two 
hundred  and  fifty  wood-cuts  have  been  added  to  the  former  list  of  illustrations. 

A  marked  improvement  will  also  be  perceived  in  the  mechanical  and  artisJical  execution  of  the 
work,  which,  printed  in  the  best  style,  on  new  type,  and  fine  paper,  leaves  little  to  be  desired  as 
regards  external  finish ;  while  at  the  very  low  price  affixed  it  will  be  found  one  of  the  cheapest 
volumes  accessible  to  the  profession. 

This  popular  volume,  now  a  most  comprehensive  j  nothing  of  real  practical  importance  has  been  omit- 
work  on  surgery,  has  undergone  many  corrections,  ted  ;  it  presents  a  faithful  epitome  of  everything  re- 
improvements,  and  additions,  and  the  principles  and  j  lating  t )  surgery  up  to  the  present  hour.  It  is  de- 
the  practice  of  the  art  have  been  brought  down  to  j  servedly  a  popular  manual,  both  with  the  student 
the  latest  record  and  observation.  Of  the  operations  and  practitioner.— London  Lancet,  Nov.  19,  1859. 

SK^ 

trations  so  accurate  and  numerous,  that  the  student  Jial'v  "  ?verTthls  ™°8t  >  comP/ehensiv« 

can  have  no  difliculty,  with  instrument  in  hand,  and  hand-book.    It  must  prove  a  vast  assistance,  not 

book  by  his  side,  over  the  dead  body,  in  obtaining  only  !° .the  stud.ent  of  surgery,  but  also  to  the  busy 

a  propfr  knowledge  and  sufficient  Jc    in  this  much  practi  loner  whc  may  not  have    ^f^  ?±V»°!! 


neglected  department  of  medical  education. — Briti&h 
and  Foreign  Mtdico-Chirurg.  Review,  Jan.  1960. 

In  the  present  edition  the  author  has  entirely  re- 
written many  of  the  chapters,  and  has  incorporated 
the  various  improvements  and  additions  in  modern 
surgery.  On  carefully  going  over  it,  we  find  that 


himself  to   the  study  of  more  lengthy  volumes.— 
London  Med.  Times  and  Gazette,  Oct  22,  1859. 

In  a  word,   this  eighth  edition  of  Dr.  Druitt' 
Manual 
or  practiti 
Journal  of  Med.  Sciences,  Nov. 


of  Surgery  is  all  that  the  surgical  student 
titioner  could  desire.  —  Dublin  Quarterly 


AND    SCIENTIFIC    PUBLICATIONS. 


11 


DALTON,  JR.  (J.   C.),   M.  D. 

Professor  of  Physiology  in  the  College  of  Physicians,  New  York. 

A  TREATISE  ON  HUMAN  PHYSIOLOGY,  designed  for  the  use  of  Students 

and  Practitioners  of  Medicine.  With  two  hundred  and  fifty-four  illustrations  on  wood.  In  one 
very  beautiful  octavo  volume,  of  over  600  pages,  extra  cloth,  $4  00 ;  leather,  raised  bands,  $4  25. 
(Just  Issued.) 


This  system  of  Physiology,  both  from  the  ex- 
cellence of  the  arrangement  studiously  observed 
throughout  every  page,  and  the  clear,  lucid,  and  in- 
structive manner  in  which  each  subject  is  treated, 
promises  to  form  one  of  the  most  generally  received 
class-books  in  the  English  language.  It  is,  in  fact, 
a  most  admirable  epitome  of  ail  the  really  important 
discoveries  that  have  always  been  received  as  incon- 
testable truths,  as  well  as  of  those  which  have  been 
recently  added  to  our  stock  of  knowledge  on  this  sub- 
ject. We  will,  however,  proceed  to  give  a  few  ex- 
tracts from  the  book  itself,  as  a  specimen  of  its  style 
and  composition,  and  this,  we  conceive,  will  be  quite 
sufficient  to  awaken  a  general  interest  in  a  work 
which  isimmeasurabl}  superior  in  its  details  to  the 
majority  of  those  of  the  same  class  t^  which  it  he- 
longs.  In  its  purity  of  style  and  elegance  of  com- 
position it  may  safely  take  its  place  with  the  very 
best  of  our  English  classics;  while  in  accuracy  of 
description  it  is  impossible  that  it  could  be  surpass 
ed.  In  every  line  is  beautifully  shadowed  forth  the 
emanations  of  the  polished  scholar,  whose  reflec- 
tions are  clothed  in  a  garb  as  interesting  as  they  are 
impressive;  with  the  one  predominant  feeling  ap- 
pearing to  pervade  the  whole — an  anxious  desire  to 
please  and  at  the  same  time  to  instruct. — Dublin 
Quarterly  Journ.  of  Med.  Sciences,  Nov.  1859. 

The  work  before  us,  however,  in  our  humble  judg- 
ment, is  precisely  what  it  purports  to  be,  and  will 
answer  admirably  the  purpose  for  which  it  is  in- 
tended. It  is  par  excellence,*  text-book;  and  the 
best  text-book  in  this  department  that  we  have  ever 
seen.  We  have  carefully  read  the  book,  and  speak 
of  its  merits  from  a  more  than  cursory  perusal. 
Looking  back  upon  the  work  we  have  just  finished, 
we  must  say  a  word  concerning  the  excellence  of  its 
illustrations.  No  department  is  so  dependent  upon 
good  illustrations,  and  those  which  keep  pace  with 
our  knowledge  of  the  subject,  as  that  of  physiology. 
The  wood- cuts  in  the  work  before  us  are  the  best 
we  have  ever  seen,  and,  being  original,  serve  to 
illustrate  precisely  what  is  desired — Buffalo  Med. 
Journal,  March,  1859. 

A  book  of  genuine  merit  like  this  deserves  hearty 
praise  before  subjecting  it  tu  any  minute  criticism. 
We  are  not  prepared  to  find  any  fault  with  its  design 
until  we  have  had  more  time  to  appreciate  its  merits 
as  a  manual  for  daily  consultation,  and  to  weigh 
its  statements  and  conclusions  more  deliberately. 
Its  excellences  we  are  sure  of;  its  defects  we  have 
yet  to  discover.  It  is  a  work  highly  honorable  to 


its  author  ;  to  his  talents,  his  industry,  his  training  ; 
to  the  institution  with  whi^h  he  is  connected,  and 
to  American  science.—  Boston  Med.  and  Surgical 
Journal,  Feb.  24,  1859. 

A  NEW  book  and  a  first  rate  one  ;  an  original  book, 
and  one  which  cannot  be  too  highly  appreciated, 
and  which  we  are  proud  to  see  emanating  from  our 
country's  press.  It  is  by  an  author  who,  though 
young,  is  considerably  famous  for  physiological  re- 
search, and  who  in  this  work  has  erected  for  him- 
self an  enduring  monument,  a  token  at  once  of  his 
labor  and  his  success.—  Nashville  Medical  Journal, 
March,  1859. 

Throughout  the  entire  work,  the  definitions  are 
clear  and  precise,  the  arrangement  admirable,  the 
argument  briefly  and  well  stated,  and  the  style 
nervous,  simple,  and  concise.  Section  third,  treat- 
ing of  Reproduction  is  a  monograph  of  unap- 
proached  excellence,  upon  this  subject,  in  the  Eng- 
lish tongue.  For  precision,  elegance  and  force  of 
style,  exhaustive  method  and  extent  of  treatment, 
fulness  of  illustration  and  weight  of  personal  re- 
search, we  know  of  no  American  contribution  to 
medical  science  which  surpasses  it,  and  the  day  is 
far  distant  when  its  claims  to  the  respectful  atten- 
tion of  even  the  best  informed  scholars  will  not  be 
cheerfully  conceded  by  all  acquainted  with  its  range 
anu  depth.—  Charleston  Med.  Journal,  May,  1859. 

A  new  elementary  work  on  Human  Physiology 
lifting  up  its  voice  in  the  presence  of  late  and  sturdy 
editions  of  Kirke's,  Carpenter's,  Todd  and  Bow- 


i have  something  superior  in  the  matter  or  the 
jr  of  its  utterance  in  order  to  win  for  itself 


man's,  to  say  nothing  of  Durglison's  and  Draper's, 
should  '-  — 
manner 

deserved  attention  and  a  name.  That  matter  and 
that  manner,  after  a  candid  perusal,  we  think  dis- 
tinguish this  work,  and  we  are  proud  to  welcome  it 
not  merely  for  its  nativity's  sake,  but  for  its  own 
intrinsic  excellence.  Its  language  we  find  to  be 
plain,  direct,  unambitious,  and  falling  with  a  just 
conciseness  on  hypothetical  or  unsettled  questions, 
and  yet  with  sufficient  fulness  on  those  living  topics 
already  understood,  or  the  path  to  whose  solution 
is  definitely  marked  out.  It  does  not  speak  exhaust- 
i/ely  upon  every  subject  that  it  notices,  but  it  does 
speak  suggestively,  experimentally,  and  to  their 
main  utilities.  Into  the  subject  of  Reproduction 
our  author  plunges  with  a  kind  of  loving  spirit. 
Throughout  this  interesting  and  obscure  department 
he  is  a  clear  and  admirable  teacher,  sometimes  a 
brilliant  leader.—  Am.  Med.  Monthly,  May,  1859. 


DUNGLISON,   FORBES,   TWEEDIE,   AND   CONOLLY. 
THE  CYCLOPAEDIA  OF  PRACTICAL  MEDICINE:  comprising  Treatises  on 

the  Nature  and  Treatment  of  Diseases,  Materia  Medica,  and  Therapeutics,  Diseases  of  Women 
and  Children,  Medical  Jurisprudence,  &c.  &c.     In  four  large  super-royal  octavo  volumes,  of 
3254  double-columned  pages,  strongly  and  handsomely  bound,  with  raised  bands.    $12  00. 
*^*  This  work  contains  no  less  than  four  hundred  and  eighteen  distinct  treatises,  contributed  by 

sixty-eight  distinguished  physicians,  rendering  it  a  complete  library  of  reference  for  the  country 

practitioner. 
The  most  complete  work  on  Practical  Medicine  I  titioner.    This  estimate  of  it  has  not  been  formed 


extant;    or,  at  least,  in    our   language.— Buffalo 
Medical  and  Surgical  Journal. 

For  reference,  it  is  above  all  price  to  every  prac- 
titioner.— Western  Lancet. 

One  of  the  most  valuable  medical  publications  of 
the  day— as  a  work  of  reference  it  is  invaluable.— 
Western  Journal  of  Medicine  and  Surgery. 

It  has  been  to  us,  both  as  learner  and  teacher,  a 
work  for  ready  and  frequent  reference,  one  in  which 
modern  English  medicine  is  exhibited  in  the  most 
advantageous  light.— Medical  Examiner. 

We  rejoice  that  this  work  is  to  be  placed  within 
the  reach  of  the  profession  in  this  country,  it  bein«- 
unquestionably  one  of  very  great  value  to  the  prac- 


from  a  hasty  examination,  but  after  an  intimate  ac- 
quaintance derived  from  frequent  consultation  of  it 
during  the  past  nine  or  ten  years.  The  editors  are 
practitioners  of  established  reputation,  and  the  list 
of  contributors  embraces  many  of  the  most  eminent 
professors  and  teachers  of  London,  Edinburgh,  Dub- 
lin, and  Glasgow.  It  is,  indeed,  the  great  merit  of 
this  work  that  the  principal  articles  have  been  fur- 
nished by  practitioners  who  have  not  only  devoted 
especial  attention  to  the  diseases  about  which  they 
have  written,  but  have  also  enjoyed  opportunities 
for  an  extensive  practical  acquaintance  with  them, 
and  whose  reputation  carries  the  assurance  of  their 
competency  justly  to  appreciate  the  opinions  of 
others,  while  it  stamps  their  own  doctrines  with 
high  and  just  authority.— American  Medical  Journ. 


DEWEES'S  COMPREHENSIVE  SYSTEM  OF 
MIDWIFERY.  Illustrated  by  occasional  cases 
and  many  engravings.  Twelfth  edition,  with  the 
author's  last  improvements  and  corrections  In 
one  octavo  volume,  extra  cloth,  of  600 pages.  $320. 

DEWEES'S  TREATISE  ON   THE  PHYSICAL 


AND  MEDICAL  TREATMENT  OF  CHILD- 
REN. The  last  edition.  In  one  volume,  octavo, 
extra  cloth,  548  pages.  $2  80 

DEWEES'S  TREATISE  ON  THE  DISEASES 
OF  FEMALES.  Tenth  edition.  In  one  volume, 
octavo  extra  cloth,  532  pages,  with  plates.  $3  QO 


12 


BLANCHARD   &    LEA'S    MEDICAL 


DUNGLISON    (ROBLEY),    M.D., 

Professor  of  Institutes  of  Medicine  in  the  Jefferson  Medical  College,  Philadelphia. 

NEW  AND  ENLARGED  EDITION. 
MEDICAL  LEXICON;   a  Dictionary  of  Medical  Science,  containing  a  concise 

Explanation  of  the  various  Subjects  and  Terms  of  Anatomy,  Physiology,  Pathology,  Hygiene, 
Therapeutics;  Pharmacology,  Pharmacy,  Surgery,  Obstetrics,  Medical  Jurisprudence,  Dentistry, 
Sec.  Notices  of  Climate  and  of  Mineral  Waters;  Formulae  for  Officinal,  Empirical,  and  Dietetic 
Preparations,  &c.  With  French  and  other  Synonymes.  Revised  and  very  greatly  enlarged. 
In  one  very  large  and  handsome  octavo  volume,  of  992  double-columned  pages,  ia  small  type; 
strongly  bound  in  leather,  with  raised  bands.  Price  $4  00. 

Especial  care  has  been  devoted  in  the  preparation  of  this  edition  to  render  it  in  every  respect 
worthy  a  continuance  of  the  very  remarkable  favor  which  it  has  hitherto  enjoyed.  The  rapid 
sale  of  FIFTEEN  large  editions,  and  the  constantly  increasing  demand,  show  that  it  is  regarded  by 
the  profession  as  the  standard  authority.  Stimulated  by  this  fact,  the  author  has  endeavored  in  the 
present  revision  to  introduce  whatever  might  be  necessary  "  to  make  it  a  satisfactory  and  desira- 
ble— if  not  indispensable — lexicon,  in  which  the  student  may  search  without  disappointment  for 
every  term  that  has  been  legitimated  in  the  nomenclature  of  the  science."  To  accomplish  this, 
large  additions  have  been  found  requisite,  and  the  extent  of  the  author's  labors  may  be  estimated 
from  the  fact  that  about  Six  THOUSAND  subjects  and  terms  have  been  introduced  throughout,  ren- 
dering the  whole  number  of  definitions  about  SIXTY  THOUSAND,  to  accommodate  which,  the  num- 
ber of  pages  has  been  increased  by  nearly  a  hundred,  notwithstanding  an  enlargement  in  the  size 
of  the  page.  The  medical  press,  both  in  this  country  and  in  England,  has  pronounced  the  work  in- 
dispensable to  all  medical  students  and  practitioners,  and  the  present  improved  edition  will  not  lose 
that  enviable  reputation. 

The  publishers  have  endeavored  to  render  the  mechanical  execution  worthy  of  a  volume  of  such 
universal  use  in  daily  reference.  The  greatest  care  hns  been  exercised  to  obtain  the  typographical 
accuracy  so  necessary  in  a  work  of  the  kind.  By  the  small  but  exceedingly  clear  type  employed, 
an  immense  amount  of  matter  is  condensed  in  its  thousand  ample  pages,  while  the  binding  will  be 
found  strong  and  durable.  With  all  these  improvements  and  enlargements,  the  price  has  been  kept 
at  the  former  very  moderate  rate,  placing  it  within  the  reach  of  all. 


This  work,  the  appearance  of  the  fifteenth  edition 
of  which,  it  has  become  our  duty  and  pleasure  to 
announce,  is  perhaps  the  most  stupendous  monument 
of  labor  and  erudition  in  medical  literature.  One 
would  hardly  suppose  after  constant  use  of  the  pre- 
ceding editions,  where  we  have  never  failed  to  find 
a  sufficiently  full  explanation  of  ever)  medical  term, 
that  in  this  edition  "about  six  thousand  subjects 
and  terms  have  been  added,"  with  a  careful  revision 
and  correction  of  the  entire  work.  It  is  only  neces- 
sary to  announce  the  advent  of  this  edition  to  make 
it  occupy  the  place  of  the  preceding  one  on  the  table 
of  every  medical  man,  as  it  is  without  doubt  the  best 
and  most  comprehensive  work  of  the  kind  which  has 
ever  appeared. — Buffalo  Med.  Journ.,  Jan.  1858. 

The  work  is  a  monument  of  patient  research, 
skilful  judgment,  and  vast  physical  labor,  that  will 
perpetuate  the  name  of  the  author  more  effectually 
than  any  possible  device  of  stone  or  metal.  Dr. 
Dunglison  deserves  the  thanks  not  only  of  the  Ame- 
rican profession,  but  of  the  whole  medical  world. — 
North  Am.  Medico-Chir.  Review,  Jan.  1858. 

A  Medical  Dictionary  better  adapted  for  the  wants 
of  the  profession  than  any  other  with  which  we  are 
acquainted,  and  of  a  character  which  places  it  far 
above  comparison  and  competition.— Am.  Journ. 
Med.  Sciences,  Jan.  1858. 

We  need  only  say,  that  the  addition  of  6,000  new 
terms,  with  their  accompanying  definitions,  may  be 
said  to  constitute  a  new  work,  by  itself.    We  have 
examined  the  Dictionary  attentively,  and  are  most 
happy  to  pronounce  it  unrivalled  of  its  kind.    The 
erudition  displayed,  and  the  extraordinary  industry 
which  must  have  been  demanded,  in  its  preparation 
and  perfection,  redound  to  the  lasting  credit  of  its 
author,  and  have  furnished  us  with  a  volume  indis-  \ 
jiensable  at  the  present  day,  to  all  who  would  find  i 
themselves  au  niveau  with  the  highest  standards  of  j 
medical  information. — Boston  Medical  and  Surgical 
Journal,  Dec.  31,  1857. 

Good  lexicons  and  encyclopedic  works  generally, 
are  the  most  labor-saving  contrivances  which  lite- 
rary men  enjoy ;  and  the  labor  which  is  required  to 
produce  them  in  the  perfect  manner  of  this  example 
is  something  appalling  to  contemplate.  The  author 


tells  us  in  his  preface  that  he  has  added  about  six 
thousand  terms  and  subjects  to  this  edition,  which, 
before,  was  considered  universally  as  the  best  work 
of  the  kind  in  any  language. — Silliman's  Journal, 
March,  1858. 

He  has  razed  his  gigantic  structure  to  the  founda- 
tions, and  remodelled  and  reconstructed  the  entire 
pile.  No  less  than  six  thousand  additional  subjects 
and  terms  are  illustrated  and  analyzed  in  this  new 
edition,  swelling  the  grand  aggregate  to  beyond 
sixty  thousand  !  Thus  is  placed  before  the  profes- 
sion a  complete  and  thorough  exponent  of  medical 
terminology,  without  rival  or  possibility  of  rivalry. 
—Nashville  Journ.  of  Med.  and  Surg.,  Jan.  1858. 

It  is  universally  acknowledged,  we  believe,  that 
this  work  is  incomparably  the  best  and  most  com- 
plete Medical  Lexicon  in  the  English,  language. 
The  amount  of  labor  which  the  distinguished  author 
has  bestowed  upon  it  is  truly  wonderful,  and  the 
learning  and  research  displayed  in  its  preparation 
are  equally  remarkable.  Comment  and  commenda- 
tion are  unnecessary,  as  no  one  at  the  present  day 
thinks  of  purchasing  any  other  Medical  Dictionary 
than  this. — St.  Louis  Med.  and  Surg.  Journ..  Jan. 
1858. 

It  is  the  foundation  stone  of  a  good  medical  libra- 
ry, and  should  always  be  included  in  the  first  list  of 
books  purchased  by  the  medical  student. — Am.  Med. 
Monthly,  Jan.  1858. 

A  very  perfect  work  of  the  kind,  undoubtedly  the 
most  perfect  in  the  English  language. — Med.  and 
Surg.  Reporter,  Jan.  1858. 

It  is  now  emphatically  the  Medical  Dictionary  of 
the  English  language,  and  for  it  there  is  no  substi- 
tute.— N.  H.  Med.  Journ.,  Jan.  1858. 

It  is  scarcely  necessary  to  remark  that  any  medi- 
cal library  wanting  a  copy  of  Dunglison's  Lexicon 
must  be  imperfect. — Cin.  Lancet,  Jan.  1858. 

We  have  ever  considered  it  the  best  authority  pub- 
lished, and  the  present  edition  we  may  safely  say  has 
no  equal  in  the  world. — Peninsular  Med.  Journal, 
Jan.  1858. 

The  most  complete  authority  on  the  subject  to  be 
foundin  any  language. —  Va.  Med.  Journal,  Feb.  '58. 


BY  THE  SAME  AUTHOR. 

THE  PRACTICE  OF  MEDICINE.     A  Treatise  OH  Special  Pathology  and  The- 
rapeutics.    Third  Edition.    In  two  large  octavo  volumes,  leather,  of  1,500  pages.    $635. 


AHD    SCIENTIFIC   PUBLICATIONS. 


DUNGLISON   (ROBLEY,   M.  D., 


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GENERAL   THERAPEUTICS   AND    MATKRT4  MEDICA: 
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BT  XKZ  SAXK  ACXHB.     {A  MM*  E&titm.) 

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ELLIS  (BENJAMIN),  M.D. 

THE  MEDICAL  FORMULARY:  beis** 
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BLANCHARD  &  LEA'S  MEDICAL 


ERICHSEN    (JOHN), 

Professor  of  Surgery  in  University  College,  London,  &c. 

THE  SCIENCE  AND  ART  OF  SURGERY;  BEING  A  TREATISE  ON  SURGICAL 

INJURIES,  DISEASES,  AND  OPERATIONS.    New  and  improved  American,  from  the  second  enlarged 

and  carefully  revised  London  edition.     Illustrated  with  over  four  hundred  engravings  on  wood. 

In  one  large  and  handsome  octavo  volume,  of  one  thousand  closely  printed  pages,  leather, 

raised  bands.     -$4  50.     (Just  Issued.) 

The  very  distinguished  favor  with  which  this  work  has  been  received  on  both  sides  of  the  Atlan- 
tic has  stimulated  the  author  to  render  it  even  more  worthy  of  the  position  which  it  has  so  rapidly 
attained  as  a  standard  authority.  Every  portion  has  been  carefully  revised,  numerous  additions 
have  been  made,  and  the  most  watchful  care  has  been  exercised  to  render  it  a  complete  exponent 
of  the  most  advanced  condition  of  surgical  science.  In  this  manner  the  work  has  been  enlarged  by 
about  a  hundred  pages,  while  the  series  of  engravings  has  been  increased  by  more  than  a  hundred, 
rendering  it  one  of  the  most  thoroughly  illustrated  volumes  before  the  profession.  The  additions  oJ 
the  author  having  rendered  unnecessary  most  of  the  notes  of  the  former  American  editor,  but  little 
has  been  added  in  this  country;  some  few  notes  and  occasional  illustrations  have,  however,  been 
introduced  to  elucidate  American  modes  of  practice. 

It  is,  in  our  humble  judgment,  decidedly  the  best    step  of  the  operation,  and  not  deserting  him  until  the 


book  of  the  kind  in  the  English  language.  Strange 
that  just  such  books  are  notoftener  produced  by  pub- 
lic teachers  of  surgery  in  this  country  and  Great 
Britain  Indeed,  it  is  a  matter  of  great  astonishment, 
but  no  less  true  than  astonishing,  that  of  the  many 
works  on  surgery  republished  in  this  country  within 
the  last  fifteen  or  twenty  years  a?  text-books  for 
medical  students,  this  is  the  only  one  that  even  ap- 
proximates to  the  fulfilment  of  the  peculiar  wants  of 
young  men  just  entering  upon  the  study  of  this  branch 
of  the  profession. —  Western  Jour,  of  Med.  and  Surgery. 

Its  value  is  greatly  enhanced  by  a  very  copious 
well-arranged  index.  We  regard  this  as  one  of  the 
most  valuable  contributions  to  modern  surgery.  To 
one  entering  his  novitiate  of  practice,  we  regard  it 
the  mo«t  serviceable  guide  which  he  can  consult.  He 
will  find  a  fulness  of  detail  leadinghim  through  every 


final  issue  of  the  case  is  decided. — Sethoscope. 

Embracing,  as  will  be  perceived,  the  whole  surgi- 
cal domain,  and  each  division  of  itself  almost  com- 
plete and  perfect,  each  chapterfull  and  explicit,  each 
subject  faithfully  exhibited,  we  can  only  express  ouj 
estimate  of  it  in  the  aggregate.  We  consider  it  an 
excellent  contribution  to  surgery,  as  probably  the 
best  single  volume  now  extant  on  the  subject,  and 
with  great  pleasure  we  add  it  to  onr  text-books.— 
Nashville  Journal  of  Medicine  and  Surgery. 

Prof.  Erichsen's  work,  for  its  size,  has  not  beeK 
surpassed;  his  nine  hundred  and  eight  pages,  pro- 
fusely illustrated,  are  rich  in  physiological,  patholo- 
gical, and  operative  suggestions,  doctrines,  details, 
and  processes ;  and  will  prove  a  reliable  resource 
for  information,  both  to  physician  and  sursreon,  in  the 
hour  of  peril. — N.  0.  Med.  and  Surg.  Journal. 


FLINT  (AUSTIN),  M .  D., 

Professor  of  the  Theory  and  Practice  of  Medicine  in  the  University  of  Louisville,  Ac. 

PHYSICAL  EXPLORATION  AND  DIAGNOSIS  OP  DISEASES  AFFECT- 
ING THE  RESPIRATORY  ORGANS.  In  one  large  and  handsome  octavo  volume,  extra 
cloth,  636  pages.  $3  00. 
We  regard  it,  in  point  both  of  arrangement  and  of 

the  marked  ability  of  its  treatment  of  the  subjects, 

as  destined  to  take  the  first  rank  in  works  of  this 


class.  So  far  as  our  information  extends,  it  has  at 
present  no  equal.  To  the  practitioner,  as  well  as 
the  student,  it  will  be  invaluable  in  clearing  up  the 
diagnosis  of  doubtful  cases,  and  in  shedding  light 
upon  difficult  phenomena. — Buffalo  Med.  Journal. 


A  work  of  original  observation  of  the  highest  merit. 
We  recommend  the  treatise  to  every  one  who  wishes 


to  become  a  correct  auscultator.  Based  to  a  very 
large  extent  upon  cases  numerically  examined,  it 
carries  the  evidence  of  careful  study  and  discrimina- 
tion upon  every  page.  It  does  credit  to  the  author, 
and,  through  him,  to  the  profession  in  this  country. 
It  is,  what  we  cannot  call  every  book  upon  auscul- 
tation, a  readable  book. — Am.  Jour.  Med.  Sciences, 


BY  THE  SAME  AUTHOR.     (Now  Ready.) 

A  PRACTICAL  TREATISE  ON  THE  DIAGNOSIS,  PATHOLOGY,  AND 


TREATMENT  OF  DISEASES  OF  THE  HEART. 

500  pages,  extra  cloth.     $2  75. 

We  do  no*  know  that  Dr.  Flint  has  written  any- 
thing which  is  not  first  rate  ;  but  this,  his  latest  con- 
tribution to  medical  literature,  in  our  opinion,  sur- 


passes all  the  others.  The  work  is  most  comprehen- 
sive in  its  scope,  and  most  sound  in  the  views  it  enun- 
ciates. The  descriptions  are  clear  and  methodical ; 
the  statements  are  substantiated  by  facts,  ai.d  are 
made  with  such  simplicity  and  sincerity,  that  with- 
out them  they  would  carry  conviction.  The  style 
is  admirably  clear,  direct,  and  free  from  dryness 
With  Dr.  Walshe's  excellent  treatise  before  us,  we 
have  no  hesitation  in  saying  that  Dr.  Flint's  book  is 
the  best  work  on  the  heart  in  the  English  language. 
—Boston  Med.  and  Surg.  Journal,  Dec.  15,  1S59. 

We  have  thus  endeavored  to  present  our  readers 
with  a  fair  analysis  of  tiiis  remarkable  work.  Pre- 
ferring to  employ  the  very  words  of  thedisr.inguished 
author,  wherever  it  was  possible,  we  have  essayed 
to  condense  into  the  briefest  spacea  general  view  of 
his  observations  and  suggestions,  and  to  direct  the 
attention  of  our  brethren  to  the  abounding  stores  of 
valuable  matter  here  collected  and  arranged  for  their 
use  and  instruction.  No  medical  library  will  here 
after  be  considered  complete  without  this  volume; 
and  we  trust  it  will  promptly  find  its  way  into  the 
hands  of  every  Amoican  student  and  physician. — 
N.  Am.  Med.  Chir.  Review,  Jan  1860. 

This  last  work  of  Prof.  Flint  will  add  much  to 
his  previous  well-earned  celebrity,  as  a  writer  of 
great  force  and  beauty,  and,  with  his  previous  work, 
places  him  ut  the  head  of  American  writers  upon 


In  one  neat  octavo  volume,  of  about 


diseases  of  the  chest.  We  have  adopted  his  work 
upon  the  heart  as  a  t^xt-book,  believing  it  to  be 
more  valuable  for  that  purpose  than  any  work  of  the 


kind  that  has  yet  appeared. — Nashville  Med.Journ.. 
Dec.  1859. 

With  more  than  pleasure  do  we  hail  the  advent  of 
this  work,  for  it  fills  a  wide  gap  on  the  list  nf  text- 
books for  our  schools,  and  is,  Jor  the  practitioner, 
the  most  valuable  practical  work  of  its  kind.— JV.  O. 
Med.  Newt,  Nov.  1859. 

In  regard  to  the  merits  of  the  work,  we  have  no 
hesitation  in  pronouncing  it  full,  accurate,  and  ju- 
dicious. Considering  the  present  state  of  science, 
such  a  work  was  mucb  needed.  It  should  be  in  the 
hands  of  every  practitioner. — Chicago  Med.  Journal, 
April,  1860. 

Bat  these  are  very  trivial  spots,  and  in  nowise 
prevent  us  from  declaring  our  most  hearty  approval 
of  the  author's  ability,  industry,  and  conscientious- 
ness.— Dublin  Quarterly  Journal  of  Med.  Sciences, 
Feb.  1860. 

He  has  labored  on  with  the  same  industry  and  care, 
and  his  place  among  the  first  authors  of  our  country 
is  becoming  fully  established.  To  this  end,  the  work 
whose  title  is  given  above,  contributes  in  no  small 
degree.  Our  spa^e  will  not  admit  of  tn  extended 
analysis,  and  we  will  close  this  orief  no«ice  by 
commending  it  without  reserve  to  every  class  of 
readers  in  the  profession — Peninsular  Med.  Journ., 
Feb.  1860. 


AND    SCIENTIFIC    PUBLICATIONS. 


ir, 


FOWNES  (GEORGE),  PH.  D.,  &c. 
A  MANUAL  OF  ELEMENTARY  CHEMISTRY;  Theoretical  and  Practical. 

From  the  seventh  revised  and  corrected  London  edition.     With  one  hundred  and  ninety-seven 

illustrations.     Edited  by  ROBERT  BRIDGES,  M.  D.    In  one  large  royal  12mo.  volume,  of  600 

pages.     In  leather,  $1  65 ;  extra  cloth,  $1  50.     (Just  Issued.) 

The  death  of  the  author  having  placed  the  editorial  care  of  this  work  in  the  practised  hands  of 
Drs.  Bence  Jones  and  A.  W.  Hoffman,  everything  has  been  done  in  its  revision  which  experience 
eould  suggest  to  keep  it  on  a  level  with  the  rapid  advance  of  chemical  science.  The  additions 
requisite  to  this  purpose  have  necessitated  an  enlargement  of  the  page,  notwithstanding  which  the 
work  has  been  increased  by  about  fifty  pages.  At  the  same  time  every  care  has  been  used  to 
maintain  its  distinctive  character  as  a  condensed  manual  for  the  student,  divested  of  all  unnecessary 
detail  or  mere  theoretical  speculation.  The  additions  have,  of  course,  been  mainly  in  the  depart- 
ment of  Organic  Chemistry,  which  has  made  such  rapid  progress  within  the  last  few  years,  but 
yet  equal  attention  has  been  bestowed  on  the  other  branches  of  the  subject — Chemical  Physics  and 
Inorganic  Chemistry — to  present  all  investigations  and  discoveries  of  importance,  and  to  keep  up 
•the  reputation  of  the  volume  as  a  complete  manual  of  the  whole  science,  admirably  adapted  for  the 
learner.  By  the  use  of  a  small  but  exceedingly  clear  type  the  matter  of  a  large  octavo  is  compressed 
within  the  convenient  and  portable  limits  of  a'moderate  sized  duodecimo,  and  at  the  very  low  price 
affixed,  it  is  offered  as  one  of  the  cheapest  volumes  before  the  profession. 


Dr.  Fownes'  excellent  work  has  been  universally 
recognized  every  where  in  his  own  and  this  country, 
as  the  best  elementary  treatise  on  chemistry  in  the 
English  tongue,  and  is  very  generally  adopted,  we 
believe,  as  the  standard  text- book  in  all<  ur  colleges, 
both  literary  and  scientific. — Charleston  Med.  Journ. 
and  Review,  Sept.  1859. 

A  standard  manual,  which  has  long  enjoyed  the 
reputation  of  embodying  much  knowledge  in  a  small 
space.  The  author  has  achieved  the  difficult  task  of 
condensation  with  masterly  tact.  His  book  is  con- 
cise without  being  dry,  and  brief  without  being  too 
dogmatical  or  general. — Virginia  Med.  and  Surgical 
Journal. 


The  work  of  Dr.  Fownes  has  long  been  before 
the  public,  and  its  merits  have  been  fully  appreci- 
ated as  the  best  text-book  on  chemistry  now  in 
existence.  We  do  not,  of  course,  place  it  in  a  rank 
superior  to  the  works  of  Brande,  Graham,  Turner, 


Gregory 

for  students,  it  is  preferable  to  any  of  them 


or  Gmelin,  but  we  say  that,   as  a  work 

Lon- 


don Journal  of  Medicine. 

A  work  well  adapted  to  the  wants  of  the  student 
It  is  an  excellent  exposition  of  the  chief  doctrines 
and  facts  of  modern  chemistry  .  The  size  of  the  work, 
and  still  more  the  condensed  yet  perspicuous  style 
in  which  it  is  written,  absolve  it  from  the  charges 
very  properly  urged  against  most  manuals  termed 
popular.  —  Edinburgh  Journal  of  Medical  Science. 


FISKE    FUND    PRIZE    ESSAYS  —THE    EF- 
FECTS OF  CLIMATE   ON   TUBERCULOUS 


EDWARD  WARREN,  M.  D  ,  of  Edenton,  N.  C.  To- 
gether in  one  neat  8vo.  volume,  extra  cloth.  $1  00. 


DISEASE.    By  EDWIN  LEE,  M.R.C.S  ,  London,  |  FRICK  ON  RENAL  AFFECTIONS;  their  Diag 


and  THE  INFLUENCE  OF  PREGNANCY  ON 
THE  DEVELOPMENT  OF  TUBERCLES     By 


nosis  and  Pathology.     With  illustrations.     One 
volume,  royal  I2mo.,  extra  cloth.    75  cents 


FERGUSSON  (WILLIAM),  F.  R.  S., 

Professor  of  Surgery  in  King's  College,  London,  &c. 

A  SYSTEM  OF  PRACTICAL  SURGERY.     Fourth  American,  from  the  third 

and  enlarged  London  edition.  In  one  large  and  beautifully  printed  octavo  volume,  of  about  700 
pages,  with  393  handsome  illustrations,  leather.  $3  00. 

GRAHAM  (THOMAS),  F.  R.  S. 
THE  ELEMENTS   OF   INORGANIC   CHEMISTRY,  including  the  Applica- 

tions of  the  Science  in  the  Arts.  New  and  much  enlarged  edition,  by  HENRY  WATTS  and  ROBERT 
BRIDGES,  M.  D.  Complete  in  one  large  and  handsome  octavo  volume,  of  over  800  very  large 
pages,  with  two  hundred  and  thirty-two  wood-cuts,  extra  cloth.  $4  00. 

j<*%.  Part  II.,  completing  the  work  from  p.  431  to  end,  with  Index,  Title  Matter,  &c.,  may  be 
had  separate,  cloth  backs  and  paper  sides.    Price  $2  50. 


From  Prof.  E.  N.  Horsford,  Harvard  College. 

It  has,  in  its  earlier  and  less  perfect  editions,  been 
familiar  to  me,  and  the  excellence  of  its  plan  and 
the  clearness  and  completeness  of  its  discussions, 
have  long  been  my  admiration. 

No  reader  of  English  works  on  this  science  can 


afford  to  be  without  this  edition  of  Prof.  Graham's 
Elements. — Silliman's  Journal,  March,  1858. 

From  Prof.  Wolcott  Gibbs,  N.  Y.  Free  Academy. 

The  work  is  an  admirable  one  in  all  respects,  and 
its  republication  here  cannot  fail  to  exert  a  positive 
influence  upon  the  progress  of  science  in  this  country. 


GRIFFITH  (ROBERT   E.),   M.  D.,  &c. 

A  UNIVERSAL  FORMULARY,  containing  the  methods  of  Preparing  and  Ad- 
ministering  Officinal  and  other  Medicines.  The  whole  adapted  to  Physicians  and  Pharmaceu- 
lists.  SECOND  EDITION,  thoroughly  revised,  with  numerous  additions",  by  ROBERT  P.  THOMAS, 
M.  D.,  Professor  of  Materia  Medica  in  the  Philadelphia  College  of  Pharmacy.  In  one  large  and 
handsome  octavo  volume,  extra  cloth,  of  650  pages,  double  columns.  $3  00;  or  in  sheep,  $3  25. 


It  was  a  work  requiring  much  perseverance,  and 
when  published  was  looked  upon  as  by  far  the  best 
work  of  its  kind  that  had  issued  from  the  American 
press.  Prof  Thomas  has  certainly  "improved,"  as 
well  as  added  to  this  Formulary,  and  lias  rendered  il 
additionally  deserving  of  the  confidence  of  pharma- 
ceutists and  physicians. — Am.  Journal  of  Pharmacy. 

We  are  happy  to  announce  a  new  and  improved 
edition  of  this,  one  of  the  most  valuable  and  useful 
works  that  have  emanated  from  an  American  pen. 
Et  would  do  credit  to  any  country,  and  will  be  found 
of  daily  usefulness  to  practitioners  of  medicine;  it  is 
better  adapted  to  their  purposes  than  the  dispensato 
ries. — Southern  Med.  and  Surg.  Journal. 

It  is  one  of  the  most  useful  books  a  country  practi- 
tioner can  possibly  have.— Medica  I  Chronicle. 


This  is  a  work  of  six  hundred  and  fifty  one  pages, 
imbracing  all  on  the  subject  of  preparing  and  admi- 


medicines  that  can  be  desired  by  the  physi- 
pharmaceutist.  —  Western  Lancet. 


nisterin 

an  an 

The  amountof  useful,  every-day  matter.  for  a  prac- 
ticing physician,  is  really  immense.—  Boston  Med. 
and  Surg.  Journal. 

This  edition  has  been  greatly  improved  by  the  re- 
vision and  ample  additions  of  Dr  Thomas,  and  is 
now,  we  believe,  one  of  the  mosi  complete  works 
of  its  kind  iu  any  language.  The  addition?  amount 
to  about  seventy  pages,  and  no  effort  has  been  spared 
to  include  in  them  all  the  recent  improvements.  A 
work  of  this  kind  appears  to  us  indispensable  to  the 
physician,  and  there  is  none  we  can  more  cordially 
recommend.  —  N.  Y.  Journal  of  Medicine. 


Itj 


BLANCHARD    &    LEA'S   MEDICAL 


GROSS  (SAMUEL  D.),   M.  D., 

Professor  of  Surgery  in  the  Jefferson  Medical  College  of  Philadelphia,  &c. 
Just  Issued. 

A  SYSTEM  OF  SURGERY :  Pathological,  Diagnostic,  Therapeutic,  and  Opera- 
tive. Illustrated  by  NINE  HUNDRED  AND  THIRTY-SIX  ENGRAVINGS.  In  two  large  and  beautifully 
printed  octavo  volumes,  of  nearly  twenty-four  hundred  pages;  strongly  bound  in  leather,  with 
raised  bands.  Price  $12. 

FROM  THE  AUTHOR'S  PREFACE. 

"  The  object  of  this  work  is  to  furnish  a  systematic  and  comprehensive  treatise  on  the  science  and 
practice  of  surgery,  considered  in  the  broadest  sense;  one  that  shall  serve  the  practitioner  as  a 
faithful  and  available  guide  in  his  daily  routine  of  duty.  It  has  been  too  much  the  custom  of  mod- 
ern writers  on  this  department  of  the  healing  art  to  omit  certain  topics  altogether,  and  to  speak  of 
others  at  undue  length,  evidently  assuming  that  their  readers  could  readily  supply  the  deficiencies 
from  other  sources,  or  that  what  has  been  thus  slighted  is  of  no  particular  practical  value.  My  aim 
has  been  to  embrace  the  whole  domain  of  surgery,  and  to  allot  to  every  subject  its  legitimate  claim 
to  notice  in  the  great  family  of  external  diseases  and  accidents.  How  far  this  object  has  been  accom- 
plished, it  is  not  for  me  to  determine.  It  may  safely  be  affirmed,  however,  that  there  is  no  topic, 
properly  appertaining  to  surgery,  that  will  not  be  found  to  be  discussed,  to  a  greater  or  less  extent, 
in  these  volumes.  If  a  larger  space  than  is  customary  has  been  devoted  to  the  consideration  of 
inflammation  and  its  results,  or  the  great  principles  of  surgery,  it  is  because  of  the  conviction, 
grounded  upon  long  and  close  observation,  that  there  are  no  subjects  so  little  understood  by  the 
general  practitioner.  Special  attention  has  also  been  bestowed  upon  the  discrimination  of  diseases; 
and  an  elaborate  chapter  has  been  introduced  on  general  diagnosis." 

That  these  intentions  have  been  carried  out  in  the  fullest  and  most  elaborate  manner  is  sufficiently 
shown  by  the  great  extent  of  the  work,  and  the  length  of  time  during  which  the  author  has  been 
concentrating  on  the  task  his  studies  and  his  experience,  guided  by  the  knowledge  which  twenty 
years  of  lecturing  on  surgical  topics  have  given  him  of  the  wants  of  the  profession. 


Of  Dr.  Gross's  treatise  on  Surgery  we  can  say 
no  more  than  that  it  is  the  most  elaborate  and  com- 

Elete  work  on  this  branch  of  the  healing  art  which 
as  ever  been  published  in  any  country.  A  sys- 
tematic work,  it  admits  of  no  analytical  review; 
but,  did  our  space  permit,  we  should  gladly  give 
some  extracts  from  it,  to  enable  our  readers  to  judge 
of  the  c'assical  style  of  the  author,  and  the  exhaust- 
ing way  in  which  each  subject  is  treated. — Dublin 
Quarterly  Journal  of  Med.  Science,  Nov.  1859. 

The  work  is  so  superior  to  its  predecessors  in 
matter  and  extent,  as  well  as  in  illustrations  and 
style  of  publication,  that  we  can  honestly  recom- 
mend it  as  the  best  work  of  the  kind  to  be  taken 
home  by  the  young  practitioner.— Am.  Med.  Journ., 
Jan. I860. 

The  treatise  of  Prof.  Gross  is  not,  therefore,  a 
mere  text-book  for  undergraduates,  but  a  systema- 
tic record  of  more  than  thirty  years'  experience,, 
reading,  and  reflection  by  a  man  of  observation, 
sound  judgment,  and  i  are  practical  tact,  and  as  such 
deserves  to  take  rank  with  the  renowned  produc- 
tions of  a  similar  character,  by  Vidal  and  Boyer,  of 
France,  or  those  of  Chelius,  Blasius,  and  Langen- 
beck,  of  Germany.  Henee,  we  do  not  hesitate  to 
express  the  opinion  that  it  will  speedily  take  the 
same  elevated  position  in  regard  to  surgery  that  has 
been  given  by  common  consent  to  the  masterly  work 
of  Percira  in  Materia  Medica,  or  to  Todd  and  Bow- 
man in  Physiology.— N.  O.  Med.  and  Surg.  Journal, 
Jan. 1860. 


At  present,  however,  our  object  is  not  to  review 
the  work  (this  we  purpose  doing  hereafter),  but 
simply  to  announce  its  appeal ance,  that  in  the 
meantime  our  readers  may  procure  and  examine  it 
for  themselves.  But  even  this  much  we  cannot  do 
without  expressing  the  opinion  that,  in  putting  forth 
these  two  volumes,  Dr.  Gross  has  reared  for  him- 
self a  lasting  monument  to  his  skill  as  a  surgeon, 
and  to  his  industry  and  learning  as  an  author. — St. 
Louis  Med.  and  Surg.  Journal,  Nov.  1859. 

With  pleasure  we  record  the  completion  of  this 
long-anticipeted  work.  The  reputation  which  the 
author  has  for  many  years  sustained,  both  as  a  sur- 
geon and  as  a  writer,  had  prepared  us  to  expect  a 
treatise  of  great  excellence  and  originality;  but  we 
confess  we  were  by  no  means  prepared  tor  the  work 
which  is  before  us — the  most  complete  treatise  upon 
surgery  ever  published,  either  in  this  or  any  othtr 
country,  and  we  might,  perhaps,  safely  say,  the 
most  original.  There  is  no  subject  belonging  pro- 
perly to  surgery  which  has  not  received  from  the 
author  a  due  share  of  attention.  Dr.  Grots  has  sup- 
plied a  want  in  surgical  literature  which  has  long 
been  felt  by  practitioners;  he  has  furnished  us  with 
a  complete  practical  treatise  upon  surgery  in  all  its 
departments  As  Anaericins,  we  are  proud  of  the 
achievement;  as  surgeons,  we  are  most  sincerely 
thankful  to  him  for  his  extraord  nary  labors  in  our 
behalf.— N.  Y.  Monthly  Review  and  Buffalo  Med. 
Journal,  Oct.  1850. 


BY  THE  SAME  AUTHOR. 

ELEMENTS  OF  PATHOLOGICAL  ANATOMY.    Third  edition,  thoroughly 

revised  and  greatly  improved.    In  one  large  and  very  handsome  octavo  volume,  with  about  three 
hundred  and  fifty  beautiful  illustrations,  of  which  a  large  number  are  from  original  drawings. 
Price  in  extra  cloth,  $4  75;  leather,  raised  bands,  $5  25.     (Lately  Published.) 
The  very  rapid  advances  in  the  Science  of  Pathological  Anatomy  during  the  last  few  years  have 
rendered  essential  a  thorough  modification  of  this  work,  with  a  view  of  making  it  a  correct  expo- 
nent of  the  present  state  of  the  subject.     The  very  careful  manner  in  which  this  task  has  been 
executed,  and  the  amount  of  alteration  which  it  has  undergone,  have  enabled  the  author  to  say  that 
"  with  the  many  changes  and  improvements  now  introduced,  the  work  may  be  regarded  almost  as 
a  new  treatise,"  while  the  efforts  of  the  author  have  been  seconded  as  regards  the  mechanical 
execution  of  the  volume,  rendering  it  one  of  the  handsomest  productions  of  the  American  press. 


We  most  sincerely  congratulate  the  author  on  the 
successful  manner  in  which  he  has  accomplished  his 
proposed  object.  His  book  is  most  admirably  cal- 
culated to  fill  up  a  blank  which  has  long  been  felt  to 
exist  in  this  department  of  medical  literature,  and 
us  such  must  become  very  widely  circulated  amongst 
all  classes  of  the  profession.  —  Dublin  Quarterly 
Journ.  of  Med.  Science,  Nov.  1857. 


We  have  been  favorably  impressed  with  the  gene- 
ral manner  in  which  Dr.  Gross  has  executed  his  task 
of  affording  a  comprehensive  digest  of  the  present 
state  of  the  literature  of  Pathological  Anatomy,  and 
have  much  pleasure  in  recommending  his  work  to 
our  readers,  as  we  believe  one  well  deserving  of 


diligent 
Chiron., 


;rusal  and  careful  study, 
ept.  1857. 


Montreal  Med. 


BY  THE  SAME  AUTHOR. 

A  PRACTICAL  TREATISE  ON  FOREIGN  BODIES  IN  THE  AIR-PAS- 

SAGES.    In  one  handsome  octavo  volume,  extra  cloth,  with  illustrations,    pp.  468.    $2  75. 


AND    SCIENTIFIC  PUBLICATIONS. 


17 


GROSS  (SAMUEL    D.),    M .  D., 

Professor  of  Surgery  in  the  Jefferson  Medical  College  of  Philadelphia,  &c. 

A   PRACTICAL    TREATISE   ON   THE    DISEASES,    INJURIES,  AND 

MALFORMATIONS  OF  THE  URINARY  BLADDER,  THE  PROSTATE  GLAND,  AND 
THE  URETHRA.  Second  Edition,  revised  and  much  enlarged,  with  one  hundred  and  eighty- 
four  illustrations.  In  one  large  and  very  handsome  octavo  volume,  of  over  nine  hundred  pages. 
In  leather,  raised  bands,  $5  25 ;  extra  cloth,  $4  75. 


Philosophical  in  its  design,  methodical  in  its  ar- 
rangement, ample  and  sound  in  its  practical  details, 
it  may  in  truth  be  said  to  leave  scarcely  anything  to 
be  desired  on  so  important  a  subject. — Boston  Med. 
and  Surg.  Journal. 

Whoever  will  peruse  the  vast  amount  of  valuable 
practical  information  it  contains,  will,  we  think, 


agree  with  us,  that  there  is  no  work  in  the  English 
language  which  can  make  any  just  pretensions  to 
be  its  equal. — N.  Y.  Journal  of  Medicine. 

A  volume  replete  with  truths  and  principles  of  the 
almost  value  in  the  investigation  of  these  diseases.  - 
American  Medical  Journal . 

70  1 1.-.  ,•>;>. yJoV  GY/ii'JO  6fli  vbaeil  V"f,  thrill?!  *>'X, 


GRAY  (HENRY),   F.  R.  S., 

Lecturer  on  Anatomy  at  St.  George's  Hospital,  London,  &«. 

ANATOMY,  DESCRIPTIVE  AND   SURGICAL.      The  Drawings  by  H.  V. 

CARTER,  M.  D.,  late  Demonstrator  on  Anatomy  at  St.  George's  Hospital ;  the  Dissections  jointly 
by  the  AUTHOR  and  Dr.  CARTER.  In  one  magnificent  imperial  oclavo  volume,  of  nearly  800 
pages,  with  363  large  and  elaborate  engravings  on  wood.  Price  in  extra  cloth,  $6  25;  leather 
raised  bands,  $7  00.  (Just  Issued.) 

The  author  has  endeavored  in  this  work  to  cover  a  more  extended  range  of  subjects  than  is 
customary  in  the  ordinary  text-books,  by  giving  not  only  the  details  necessary  for  the  student,  but 
also  the  application  of  those  details  in  the  practice  of  medicine  and  surgery,  thus  rendering  it  both 
a  grade  for  the  learner,  and  an  admirable  work  of  reference  lor  the  active  practitioner.  The 
engravings  form  a  special  feature  in  the  work,  many  of  them  being  the  size  of  nature,  nearly  all 
original,  and  having  the  names  of  the  various  parts  printed  on  the  body  of  the  cut,  in  place  of  figures 
of  reference  with  descriptions  at  the  foot.  They  thus  form  a  complete  and  splendid  series,  which 
will  greatly  assist  the  student  ia  obtaining  a  clear  idea  of  Anatomy,  and  will  also  serve  to  refresh 
the  memory  of  those  who  may  find  in  the  exigencies  of  practice  the  necessity  of  recalling  the  details 
of  the  dissecting  room  ;  while  combining,  as  it  does,  a  complete  Atlas  of  Anatomy,  with  a  thorough 
treatise  on  systematic,  descriptive,  and  applied  Anatomy,  the  work  will  be  found  of  essential  use 
to  all  physicians  who  receive  students  in  their  offices,  relieving  both  preceptor  and  pupil  of  much 
labor  in  laying  the  groundwork  of  a  thorough  medical  education. 

to  exist  in  this  country.    Mr.  Gray  writes  through- 
out with  both  branches  of  his  subject  in  view.    His 


The  work  before  us  is  one  entitled  to  the  highest 
praise,  and  we  accordingly  welcome  it  as  a  valu- 
able addition  to  medical  literature.  Intermediate 
in  fulness  of  detail  between  the  treatises  of  S.iar 
pey  and  of  Wilson,  its  characteristic  merit  lies  in 
the  number  and  excellence  of  the  engravings  it 
contains.  Most  of  these  are  original,  of  much 
larger  than  ordinary  size,  and  admirably  executed. 
The  various  parts  are  also  lettered  after  the  plan 
adopted  in  Holden's  Osteology.  It  would  be  diffi- 
cult to  over-estimate  the  advantages  offered  by  this 
mode  of  pictorial  illusiration.  Bones,  ligaments, 
muscles,  bloodvessels,  and  nerves  are  each  in  turn 
figured,  and  marked  with  their  appropriate  names; 
thus  enabling  the  student  to  comprehend,  at  a  glance, 
what  would  otherwise  often  be  ignored,  or  at  any 
rate,  acquired  only  by  prolonged  and  irksome  ap- 
plication. In  conclusion,  we  heartily  commend  the 
work  of  Mr.  Gray  to  the  attention  of  the  medical 
profession,  feeling  certain  that  it  should  be  regarded 


description  of  each  particular  part  is  followed  by  a 
notice  of  its  relations  to  tiie  parts  with  which  it  is 
connected,  and  this,  too,  sufficiently  ample  for  all 
the  purposes  of  the  operative  surgeon.  After  de- 
scribing the  bones  and  muscles,  he  gives  a  concise 
statement  of  the  fractures  to  which  the  bones  of 
the  extremities  are  most  liable,  together  with  the 
amount  and  direction  of  the  displacement  to  which 
the  fragments  are  subjected  by  muscular  action. 
The  section  on  arteries  is  remarkably  full  and  ac- 
curate. Not  only  is  the  surgical  anatomy  given  to 
every  important  vessel,  with  directions  for  its  liga- 
tion,  but  at  the  end  of  the  description  of  each  arte- 
rial trunk  we  have  a  useful  summary  of  the  irregu- 
larities which  may  occur  in  its  origin,  course,  and 
termination.— N.  A.  Med.  Chir.  Review,  Mar.  1359. 

Mr.  Gray's  book,  in  excellency  of  arrangement 


as  one  of  the  most  valuable  contributions  eve?  made    and  completeness  of  execution,  exceeds  any  work 

on  anatomy  hitherto  published  in  the  English  lan- 


to  educational  literature. — N.  Y.  Monthly  Review. 
Dec.  1859. 

In  this  view,  we  regard  the  work  of  Mr.  Gray  as 
far  belter  adapted  to  the  wants  of  the  profession, 
and  especially  of  the  student,  than  any  treatise  on 
anatomy  yet  published  in  this  country.  1 1  is  destined, 
we  believe,  to  supersede  all  others,  both  as  a  manual 
of  dissections,  and  a  standard  of  reference  to  the 
etudent  of  general  or  relative  anatomy.  —  N.  Y. 
Journal  of  Medicine,  Nov.  1859. 

This  is  by  all  comparison  the  most  excellent  work 
on  Anatomy  extant.  It  is  just  the  thing  that  has 
been  long  desired  by  the  profession.  With  such  a 
guide  as  this,  the  student  of  anatomy,  the  practi- 
tioner of  medicine,  and  the  surgical  devotee  have 
all  a  newer,  clearer,  and  more  radiant  light  thrown 
upon  the  intricacies  and  mysteries  of  this  wonder- 
ful science,  and  are  thus  enabled  to  accomplish  re- 
sults which  hitherto  seemed  possible  only  to  the 
specialist.  The  plates,  which  are  copied  from  re- 
cent dissections,  are  so  well  executed,  that  the  most 
superficial  observer  cannot  fail  to  perceive  the  posi- 
tions, relations,  and  distinctive  features  of  the  vari- 
ous parts,  and  to  take  in  more  of  anatomy  at  a  glance, 
than  by  many  long  hours  of  diligent  study  over  the 
most  erudite  treatise,  or,  perhaps,  at  the  dissecting 
table  itself.— Med.  Journ.  of  N.  Carolina,  Oct.  1859. 

For  this  truly  admirable  work  the  profession  is 
indebted  to  the  distinguished  author  of  "  Gray  on 
the  Spleen."  The  vacancy  it  fills  has  been  long  felt 


guage,  affording  a  complete  view  of  the  structure  of 
the  human  body,  with  especial  reference  to  practical 
surgery.  Thus  the  volume  constitutes  a  perfect  book 
of  reference  for  the  practitioner,  demanding  a  place 
in  even  the  most  limited  library  of  the  physician  or 
surgeon,  and  a  work  of  necessity  for  the  student  to 
fix  in  his  mind  what  he  has  learned  by  the  dissecting 
knife  from  the  book  of  nature. — The  Dublin  Quar- 
terly Journal  of  Med.  Sciences,  Nov.  1858. 

In  our  judgment,  the  mode  of  illustration  adopted 
in  the  present  volume  cannot  but  present  many  ad- 
vantages to  the  student  of  anatomy.  To  the  zealous 
disciple  of  Vesalius,  earnestly  desirous  of  real  im- 
provement, the  book  will  certainly  be  of  immense 
value;  but,  at  the  same  time,  we  must  also  confess 
that  to  those  simply  desirous  of  "cramming"  it 
will  be  an  undoubted  godsend.  The  peculiar  value 
of  Mr.  Gray's  mode  of  illustration  is  nowhere  more 
markedly  evident  than  in  the  chapter  on  osteology, 
and  especially  in  those  portions  which  treat  of  the 
bones  of  the  head  and  of  thsir  development.  The 
study  of  these  parts  is  thus  made  one  of  comparative 
ease,  if  not  of  positive  pleasure ;  and  those  bugbears 
of  the  student,  the  temporal  and  sphenoid  bones,  are 
shorn  of  half  their  terrors.  It  is,  in  our  estimation, 
an  admirable  and  complete  text-book  for  the  student, 
and  a  useful  work  of  reference  for  the  practitioner; 
its  pictorial  character  forming  a  novel  element,  to 
which  we  have  already  sufficiently  alluded. — Am. 
Journ.  Med.  Sci.,  July,  1859. 


IS 


BLANCHARD    &    LEA'S    MEDICAL 


GIBSON'S  INSTITUTES  AND  PRACTICE  OF 
SURGERY.  Eighth  edition,  improved  and  al- 
tered. With  thirty- four  plates.  In  two  handsome 
octavo  volumes,  containing  about  1,000  pages, 
leather,  raised  bandi.  $6  50. 

GARDNER'S  MEDICAL  CHEMISTRY,  for  the 
use  of  Students  and  the  Profession.  In  one  royal 
l'2mo.  vol.,  cloth,  pp.  396,  with  wood-cuts.  81. 

GLUGE'S  ATLAS  OF  PATHOLOGICAL  HIS- 
TOLOGY. Translated,  with  Notes  and  Addi- 


tions, by  JOSEPH  LEIDY,  M.  D.  In  one  volume, 
very  bvrge  imperial  quarto,  extra  cloth,  witi  320 
copper-  plate  figures,  plain  and  colored,  $5  00. 

HUGHES'  INTRODUCTION  TO  THE  PRAC- 
TICE OF  AUSCULTATION  AND  OTHER 
MODES  OF  PHYSICAL  DIAGNOSIS  IN  DIS- 
EASES OF  THE  LUNGS  AND  HEART.  Se- 
cond edition  1  vol.  royal  12mo.,  ex.  cloth,  pp. 
304.  $1  00. 


HAMILTON  (FRANK    H.),   M.  D., 

Professor  of  Surgery  in  the  University  of  Buffalo,  &c. 

A  PRACTICAL  TREATISE  ON  FRACTURES  AND  DISLOCATIONS.     In 

one  large  and  handsome  octavo  volume,  of  over  750  pages,  with  289  illustrations.   $4  25.    (Now 

Ready,  January,  1860.) 

This  is  a  valuable  contribution  to  the  surgery  of  [  illustrated,  which  will  be  a  desideratum  for  those 
most  important  affections,  and  is  the  more  welcome,  i  practitioners  who  cannot  conveniently  see  the  mo- 
inasmuch  as  at  the  present  time  we  do  not  possess  j  dels  applied.— New  York  Med.  Press,  Feb.  4,  1860. 
a  single  complete  treatise  on  Fractures  and  Dislo-  j  We  re?ard  thig  work  ag  an  h(mor  m)t  on,  to  its 
cations  in  the  English  language.  It  has  remained  for  |  author,  but  to  the  profession  of  our  country.  Were 
our  American  brother  to  produce  a  complete  treatise  j  we  to  r'eviewit  thoroughly,  we  could  not  convey  to 
upon  the  subject,  and  bring  together  in  a  convenient  j  the  mind  of  lhe  reade°r  more  forcibly  our  ho^est 
form  those  alterations  and  improvements  that  have  ;  Opmion  f  Xpressed  in  the  few  words— we  think  it  the 
been  made  from  time  to  time  in  the  treatment  of  these  j  best  book  &  its  kmd  extant.  Every  man  interested 
affections.  One  great  and  valuable  feature  in  the  1  in  Burgery  will  8Oon  nave  this  work  on  his  desk, 
work  before  us  is  the  fact  that  it  comprises  all  the  He  wlfo  d{5ea  not  will  be  the  io8er._New  OrUans 
improvements  introduced  into  the  practice  of  both 
English  and  American  surgery,  and  though  far  from 
omitting  mention  of  our  continental  neighbors,  the 


.author  by  no  means  encourages  the  notion — but  too 


Medical  News,  March.  1860. 

Now  that  it  is  before  us,  we  feel  bound  to  say  that 
much  as  was  expected  from  it,  and  onerous  as  was 


prevalent  in  some  quarters- that  nothing  is  good  !  the  undertaking,  it  has  surpassed  expectation,  and 
unless  imported  from  France  or  Germany.  Tne  j  achieved  more  than  was  pledged  in  its  behalf;  for 
latter  half  of  the  work  is  devoted  to  the  considera-  lts  tltle  does  not  express  in  full  the  richness  of  its 
tion  of  the  various  dislocations  and  their  appropri-  contents.  On  the  whole  we  are  prouder  of  this 
ate  treatment,  and  its  merit  is  fully  equal  to  that  of  !  w<>rk  than  of  any  which  has  for  years  emanated 
the  preceding  portion.-TAe  London  Lancet, May  5,  from  thKe  American  medical  press ;  its  sale  will  cer- 
- —  1  tamly  be  very  large  in  this  country,  and  we  antici- 

pate its  eliciting  much  attention  in  Europe. — Nash- 
ville Medical  Record,  Mar.  1860. 


1860. 


It  is  emphatically  the  book  upon  the  subjects  of 


Every  surgeon,  young  and  old,  should  possess 


which  it  treats,  and  we  cannot  doubt  that  it  will 

continue  so  to  be  for  an  indefinite  period  of  time.    ,          ..-   , ..-      ,-  .- 

When  we  say,  however,  that  we  believe  it  will  at  j  himself  of  it,  and  give  it  a  careful  perusal,  in  doing 

once  take  its  place  as  the  best  book  for  consultation    wh,lcch  he  Wl11  be,  r£hly  ?*%&*— St"  L°Ul$  Med' 

by  the  practitioner;  and  that  it  will  form  the  most  j  and  SurS-  Journal,  March,  I860. 

complete,  available,  and  reliable  guide  in  emergen-  I      Dr.  Hamilton  is  fortunate  in  having  succeeded  in 

ciesof  every  nature  connected  with  its  subjects;  and  i  filling  the  void,  so  long  felt,  with  what  cannot  fail 

also  that  the  student  of  surgery  may  make  it  his  text-  i  to  be  at  once  accepted  as  a  model  monograph  in  some 

book  wi Hi  entire  confidence,  and  with  pleasure  also,  !  respects,  and  a  work  of  classic*!  authority.     We 

from  its  agreeable  and  easy  style — wefhink  our  own  j  sincerely  congratulate  the  profession  of  the  United 


opinion  may  be  gathered  as  to  its  value. — Boston 
Medical  and  Surgical  Journal,  March  1,  1860. 

The  work  is  concise,  judicious,  and  accurate,  and 
adapted  to  the  wants  of  the  student,  practitioner, 
and  investigator,  honorable  to  the  author  and  to  the 
profession.— Chicago  Med.  Journal,  March,  1860. 


States  on  the  appearance  of  such  a  publication  from 
one  of  their  number.  We  have  reason  to  be  proud 
of  it  as  an  original  work,  both  in  a  literary  and  sci- 
entific point  of  view,  and  to  esteem  it  as  a  valuable 
guide  in  a  most  difficult  and  important  branch  of 
study  and  practice.  On  every  account,  therefore, 


,  we  hope  that  it  may  soon  be  widely  known  abroad 
We  venture  to  say  that  this  is  not  alone  the  only  ;  as  an  evideaee  of  genuine  progress  on  this  side  of 
complete  treatise  on  the  subject  in  the  language,  j  the  Atlantic,  and  further,  that  it  may  be  still  more 
but  the  best  and  most  practical  we  have  ever  read,  j  widely  known  at  home  as  an  authoritative  teacher 
The  arrangement  is  simple  and  systematic,  the  die-  i  from  which  every  one  may  profitably  learn,  and  as 
tion  clear  and  graphic,  and  the  illustrations  nume-  j  affording  an  example  of  honest,  well-directed,  and 
rous  and  remarkable  for  accuracy  of  delineation.  UL  tiring  industry  in  authorship  which  every  surgeon 
The  various  mechanical  appliances  are  faithfully  may  emulate.-  Am.  Med.  Journal,  April,  1860. 

HOBLYN  (RICHARD  D.),  M.  D. 
A  DICTIONARY  OF  THE  TERMS   USED  IN  MEDICINE  AND  THE 

COLLATERAL  SCIENCES.    A  new  American  edition.    Revised,  with  numerous  Additions, 
by  ISAAC  HAYS,  M.  D.,  editor  of  the  "  American  Journal  of  the  Medical  Sciences."    In  one  large 
royal  12mo.  volume,  leather,  of  over  500  double  columned  pages.    $1  50. 
To  both  practitioner  and  student,  we  recommend   use  ;  embracing  every  department  of  medical  science 


this  dictionary  as  being  convenient  in  size,  accurate 
in  definition,  and  sufficiently  full  and  complete  for 
ordinary  consultation.— Charleston  Med.  Journ. 

We  know  of  no  dictionary  better  arranged  and 
adapted.  It  is  not  encumbered  with  the  obsolete  terms 
of  a  bygone  age,  but  it  contains  all  that  are  now  in 


down  to  the  very  latest  date.— Western  Lancet. 

Hoblyn's  Dictionary  has  long  been  a  favorite  with 
us.  It  is  the  best  book  of  definitions  we  have,  and 
ought  always  to  be  upon  the  student's  table. — 
Southern  Med.  and  Surg.  Journal. 


HOLLAND'S  MEDICAL  NOTES  AND  RE- 
FLECTIONS. From  the  third  London  edition. 
In  one  handsome  octavo  volume,  extra  cloth.  S3. 

HORNER'S   SPECIAL  ANATOMY  AND  HIS- 


TOLOGY. Eighth  edition.  Extensively  revised 
and  modified.  In  two  large  octavo  volumes,  ex- 
tra cloth,  of  more  than  1000  pages,  with  over  300 
illustrations.  $6  00. 


HABERSHON  (S.  O.),  M.  D., 

Assistant  Physician  to  and  Lecturer  on  Materia  Medica  and  Therapeutics  at  Guy's  Hospital,  &c. 

.PATHOLOGICAL   AND    PRACTICAL  OBSERVATIONS  ON  DISEASES 

OF  THE  ALIMENTARY  CANAL,  CESOPHAGUS,  STOMACH,  CAECUM,  AND  INTES- 
TINES. With  illustrations  on  wood.  In  one  handsome  octavo  volume  of  312  page*,  extra 
cloth  $1  75.  (Now  Ready.} 


AND    SCIENTIFIC    PUBLICATIONS. 


10 


HODGE  (HUGH    L.),    M.  D., 

Professor  of  Midwifery  and  the  Diseases  of  Women  and  Children  in  the  University  of  Pennsylvania,  &c. 

ON  DISEASES  PECULIAR  TO  WOMEN,  including  Displacements  of  the 
Uterus.  With  original  illustrations.  In  one  beautifully  printed  octavo  volume,  of  nearly  500 
pages.  (Now  Ready.) 

The  profession  will  look  with  much  interest  on  a  volume  embodying  the  long  and  extensive  ex- 
perience of  Professor  Hodge  on  an  important  branch  of  practice  in  which  his  opportunities  for 
investigation  have  been  so  extensive.  A  short  summary  of  the  contents  will  show  the  scope  of 
the  work,  and  the  manner  in  which  the  subject  is  presented.  It  will  be  seen  that,  with  the  excep- 
tion of  Displacements  of  the  Uterus,  he  divides  the  Diseases  peculiar,  to  Women  into  iwo  great 
constitutional  class-es — those  arising  from  irritation,  and  those  arising  from  sedation. 

CONTENTS. 

PART  I.  DISEASES  OF  IRRITATION. — CHAPTER  I.  Nervous  Irritation,  and  its  Consequences  — II. 
Irritable  Uterus. — III.  Local  Symptoms  of  Irritable  Uterus:  Menorrhagia  and  Hgemorrhagia; 
Leucorrhoea ;  Dysmenorrhoea  — IV.  Local  Symptoms  of  Irritable  Uterus  ;  Complications. — V. 
General  Symptoms  of  Irritable  Uterus  :  Cerebro-spinal  Irritations. — VI.  General  Symptoms  of 
Irritable  Uterus. — VII.  Progress  and  Results  of  Irritable  Uterus. — VIII.  Causes  and  Pathology 
of  Irritable  Diseases  — IX.  Treatment  of  Irritable  Uterus;  Removal  or  Palliation  of  the  Cause. 
— X.  Treatment  of  Irritable  Uterus:  To  Diminish  or  Destroy  the  Morbid  Irritability — X[. 
Treatment  of  the  Complications  of  Irritable  Uterus. — XII,  Treatment  of  the  Complications  of 
Irritable  Uterus. 

PART  II.  DISPLACEMENTS  OF  THE  UTERUS. — CHAPTER  I.  Natural  Position  and  Supports  of  the 
Uterus. — II.  Varieties  of  Displacements  of  the  Uterus,  and  their  Causes. — III.  Symptoms  of 
Displacements  of  the  Uterus. — IV.  Treatment  of  Displacements  of  the  Uterus. — V.  Treatment 
of  Displacements;  Internal  Supports. — VI.  Treatment  of  Displacements ;  Lever  Pessaries. — 
VII.  Treatment  of  the  Varieties  of  Displacements. — VIII.  Treatment  of  Complications  of  Dis- 
placements of  the  Uterus. — IX.  Treatment  of  Enlargements  and  Displacements  of  the  Ovaries,  &c. 

PART  III.  DISEASES  OF  SEDATION. — CHAPTER  I.  Sedation  and  its  Consequences:  Organic  and 
Nervous  Sedation;   Passive  Congestion;   Reaction;  Treatment — II.  Sedation  of  the  Uterus; 
Amenorrhoea:  Sedation  of  the  Uterus  from  Moral  Causes;  Sedation  of  the  Uterus  from  Physical 
Causes. — III.  Diagnosis  and  Treatment  of  Sedation,  of  the  Uterus. 
The  illustrations,  which  are  all  original,  are  drawn  to  a  uniform  scale  of  one-half  the  natural  size. 


JONES  (T.   WHARTON),   F.  R.  S., 

Professor  of  Ophthalmic  Medicine  and  Surgery  in  University  College,  London,  &c. 

THE   PRINCIPLES  AND  PRACTICE  OF   OPHTHALMIC    MEDICINE 

AND  SURGERY.  With  one  hundred  and  ten  illustrations.  Second  American  from  the  second 
and  revised  London  edition,  with  additions  by  EDWARD  HARTSHORNE,  M.  D.,  Surgeon  to  Wills' 
Hospital,  &c.  In  one  large,  handsome  royal  12mo.  volume,  extra  cloth,  of  500  pages.  $1  50. 


JONES  (C.  HAN  DPI  ELD),  F.  R.  S.,  &.  EDWARD  H.  SIEVEKING,  M.D., 

Assistant  Physicians  and  Lecturers  in  St.  Mary's  Hospital,  London. 

A  MANUAL  OF  PATHOLOGICAL  ANATOMY.    First  American  Edition, 

Revised.    With  three  hundred  and  ninety-seven  handsome  wood  engravings.    In  one  large  and 
beautiful  octavo  volume  of  nearly  750  pages,  leather.    $3  75. 

As  a  concise  text-book,  containing,  in  a  condensed  obliged  to  glean  from  a  great  number  of  monographs, 

form,  a  complete  outline  of  what  is  known  in  the  and  the  field  was  so  extensive  that  but  few  cultivated 

domain  of  Pathological  Anatomy,  it  is  perhaps  the  it  with  any  degree  of  success.    As  a  simple  work 

best  work  in  the  English  language.    Its  great  merit  of  reference,  therefore,  it  is  of  great  value  to  the 

consists  in  its  completeness  and  brevity,  and  in  this  student  of  pathological  anatomy,  and  should  be  in 

respect  it  supplies  a  great,  desideratum  in  our  lite-  every  physician's  library. — Western  Lancet. 
rature.    Heretofore  the  student  of  pathology  was 


KIRKES  (WILLIAM   SENHOUSE),   M.D., 

Demonstrator  of  Morbid  Anatomy  at  St.  Bartholomew's  Hospital,  &c. 

A  MANUAL  OF  PHYSIOLOGY.  A  new  American,  from  the  third  and 
improved  London  edition.  With  two  hundred  illustrations.  In  one  large  and -handsome  royal 
12mo.  volume,  leather,  pp.  586.  $2  00.  (Lately  Published.) 


This  is  a  new  and  very  much  improved  edition  of 
Dr.  Kirkes'  well-known  Handbook  of  Physiology. 
It  combines  conciseness  with  completeness,  and  is, 
therefore,  admirably  adapted  for  consultation  by  the 
busy  practitioner.— Dublin  Quarterly  Journal. 

Its  excellence  is  in  its  compactness,  its  clearness, 
and  its  carefully  cited  authorities.  It  is  the  most 
convenient  of  text-books.  These  gentlemen,  Messrs. 
Kirkes  and  Paget,  have  really  an  immense  talent  for 
silence,  which  is  not  so  common  or  so  cheap  as  prat- 
ing people  fancy.  They  have  the  gift  of  telling  us 
what  we  want  to  know,  without  thinking  it  neces- 
sary to  tell  us  all  they  know.— 5oszo»  Med  and 
Surg.  Journal. 


One  of  the  very  best  handbooks  of  Physiology  we 
possess— presenting  just  such  an  outline  of  the  sci- 
ence as  the  student  requires  during  his  attendance 
upon  a  course  of  lectures,  or  for  reference  whilst 
preparing  for  examination.—  Am.  Medical  Journal. 

For  the  student  beginning  this  study,  and  the 
practitioner  who  has  but  leisure  to  refresh  his 
memory,  this  book  is  invaluable,  as  it  contains  all 
that  it  is  important  to  know,  without  special  details, 
which  are  read  with  interest  only  by  those  who 
would  make  a  specialty,  or  desire  to  possess  a  criti- 
cal knowledge  of  the  subject. — Charleston  Med. 
Journal. 


20  BLANCHARD  &  LEA'S  MEDICAL 


KNAPP'S  TECHNOLOGY ;  or,  Chemistry  applied 
to  the  Arts  and  to  Manufactures.  Edited  by  Dr. 
RONALDS,  Dr.  RICHARDSON,  and  Prof.  XV.  R. 


LAYCOCK'S  LECTURES  ON  THE  PRINCI- 
PLES AND  METHODS  OF  MEDICAL  OB- 
SERVATION AND  RESEARCH.  For  the  Use 


JOHNSON.  In  two  handsome  Svo.vols.,  with  about  I      of  Advanced  Students  and  Junior  Practitioners. 
500  wood- engravings.    $600.  In  one  royal  12mo.  volume,  extra  cloth.  Price  $1. 


LALLEMAND  AND  WILSON. 
A    PRACTICAL    TREATISE    ON    THE    CAUSES,    SYMPTOMS,    AND 

TREATMENT  OF  SPERMATORRHOEA.     By  M.  LALLEMAND.     Translated  and  edited  bv 

HENRY  J  McDouGALL.     Third  American  edition.    To  which  is  added ON  DISEASES 

OF  THE  VESICUL^;  SEMINALES;  AND  THEIR  ASSOCIATED  ORGANS.  With  special  refer- 
ence to  the  Morbid  Secretions  of  the  Prostatic  and  Urethra!  Mucous  Membrane.  By  MARRIS 
WILSON,  M.  D.  In  one  neat  octavo  volume,  of  about  400  pp.,  extra  cloth.  $2  00.  (Just  Issued.} 

LA   ROCHE  (R.),    M.  D.,  &c. 

YELLOW  FEVER,  considered  in  its  Historical,  Pathological,  Etiological,  and 
Therapeutical  Relations.  Including  a  Sketch  of  the  Disease  as  it  has  occurred  in  Philadelphia 
from  1699  to  1854,  with  an  examination  of  the  connections  between  it  and  the  fevers  known  under 
the  same  name  in  other  parts  of  temperate  as  well  as  in  tropical  regions.  In  two  large  and 
handsome  octavo  volumes  of  nearly  1500  pages,  extra  cloth.  $7  00. 


From  Professor  S.  H.  Dickson,  Charleston,  S.  C., 
September  18,  1855. 

A  monument  of  intelligent  and  well  applied  re- 
search, almost  without  example.  It  is,  indeed,  in 
itself,  a  large  library,  and  is  destined  to  constitute 
the  special  resort  as  a  book  of  reference,  in  the 
subject  of  which  it  treats,  to  all  future  time. 

We  have  not  time  at  present,  engaged  as  we  are, 
by  da,y  and  by  night,  in  the  work  of  combating  this 
very  disease,  now  prevailing  in  our  city,  to  do  more 
than  give  this  cursory  notice  of  what  we  consider 
as  undoubtedly  the  most  able  and  erudite  medical 
publication  our  country  has  yet  produced.  But  in 


nant  and  unmanageable  disease  of  modern  times, 
has  for  several  years  been  prevailing  in  our  country 
to  a  greater  extent  than  ever  before;  that  it  is  no 
longer  confined  to  either  large  or  small  cities,  but 
penetrates  country  villages,  plantations,  and  farm- 
houses ;  that  it  is  treated  with  scarcely  better  suc- 
cess now  than  thirty  or  forty  years  ago ;  that  there 
is  vast  mischief  done  by  ignorant  pretenders  to  know- 
ledge in  regard  to  the  disease,  and  in  view  of  the  pro- 
bability that  a  majority  of  southern  physicians  will 
be  called  upon  to  treat  the  disease,  we  trust  that  this 
able  and  comprehensive  treatise  will  he  very  gene- 
rally read  in  the  south. — Memphis  Med.  Recorder. 


view  of  the  startling  fact,  that  this,  the  most  malig- 

BY  THE  SAME  AUTHOR. 

PNEUMONIA ;  its  Supposed  Connection,  Pathological  and  Etiological,  with  Au- 
tumnal Fevers,  including  an  Inquiry  into  the  Existence  and  Morbid  Agency  of  Malaria.  In  one 
handsome  octavo  volume,  extra  cloth,  of  500  pages.  $3  00. 

LUDLOW  (J.  L.),   M.  D. 
A  MANUAL   OP    EXAMINATIONS   upon   Anatomy,   Physiology,   Surgery, 

Practice  of  Medicine,  Obstetrics,  Materia  Medica,  Chemistry,  Pharmacy,  and  Therapeutics.  To 
which  is  added  a  Medical  Formulary.  Third  edition,  thoroughly  revised  and  greatly  extended 
and  enlarged.  With  370  illustrations.  In  one  handsome  royal  12mo.  volume,  leather,  of  816 
large  pages.  $2  50. 

The  great  popularity  of  this  volume,  and  t  he  numerous  demands  for  it  during  the  two  years  in  which 
it  has  been  out  of  print,  have  induced  the  author  in  its  revision  to  spare  no  pains  to  render  it  a 
correct  and  accurate  digest  of  the  most  recent  condition  of  all  the  branches  of  medical  science.  In 
many  respects  it  may,  therefore,  be  regarded  rather  as  a  new  book  than  a  new  edition,  an  entire 
section  on  Physiology  having  been  added,  as  also  one  on  Organic  Chemistry,  and  many  portions 
having  been  rewritten.  A  very  complete  series  of  illustrations  has  been  introduced,  and  every 
care  has  been  taken  in  the  mechanical  execution  to  render  it  a  convenient  and  satisfactory  book  for 
study  or  reference.  The  arrangement  of  the  volume  in  the  form  of  question  and  answer  renders  it 
especially  suited  for  the  office  examination  of  students  and  for  those  preparing  for  graduation. 

We  know  of  no  better  companion  for  the  student  I  crammed  into  his  head  by  the  various  professors  to 
during  the  hours  spent  in  the  lecture  room,  or  to  re-  whom  he  is  compelled  to  listen.— Western  Lancet, 
fresh,  at  a  glance,  his  memory  of  the  various  topics  |  May,  1857. 

LEHMANN    (C.  G.) 

PHYSIOLOGICAL  CHEMISTRY.  Translated  from  the  second  edition  by 
GEORGE  E.  DAY,  M.  D.,  F.  R.  S.,  &c.,  edited  by  R.  E.  ROGERS,  M.  D.,  Professor  of  Chemistry 
in  the  Medical  Department  of  the  University  of  Pennsylvania,  with  illustrations  selected  from 
Funke's  Atlas  of  Physiological  Chemistry,  and  an  Appendix  of  plates.  Complete  in  two  large 
and  handsome  octavo  volumes,  extra  cloth,  containing  1200  pages,  with  nearly  two  hundred  illus- 
trations. $6  00. 

The  most  important  contribution  as  yet  made  to 


The  work  of  Lehmann  stands  unrivalled  as  the 
most  comprehensive  book  of  reference  and  informa- 
tion extant  on  every  branch  of  the  subject  on  which 


Physiological  Chemistry. — Am.  Journal  Med.  Sci- 
ences, Jan. 1856. 


it  treats. — Edinburgh  Journal  of  Medical  Science. 

BY  THE  SAME  AUTHOR.     (Lately  Published.} 

MANUAL  OF  CHEMICAL   PHYSIOLOGY.      Translated  from  the  German, 

with  Notes  and  Additions,  by  J.  CHESTON  MORRIS,  M.  D.,  with  an  Introductory  Essay  on  Vital 
Force,  by  Professor  SAMUEL  JACKSON,  M.  D.,  of  the  University  of  Pennsylvania.  With  illus- 
trations on  wood.  In  one  very  handsome  octavo  volume,  extra  cloth,  of  336  pages.  $2  25. 

From  Prof.  Jackson's  Introductory  Essay. 

In  adopting  the  handbook  of  Dr.  Lehmann  as  a  manual  of  Organic  Chemistry  for  the  use  of  the 
students  of  the  University,  and  in  recommending  his  original  work  of  PHYSIOLOGICAL  CHEMISTRY 
for  their  more  mature  studies,  the  high  value  of  his  researches,  and  the  great  weight  of  his  autho- 
rity in  that  important  department  of  medical  science  are  fully  recognized. 


AND    SCIENTIFIC    PUBLICATIONS. 


21 


LAWRENCE  (W.),   F.  R.  S.,  &c. 
A  TREATISE    ON    DISEASES    OF    THE    EYE.     A    new  edition,  edited, 

with  numerous  additions,  and  243  illustrations,  by  ISAAC  HAYS,  M.  D.,  Surgeon  to  Will's  Hospi- 
tal, &c.  In  one  very  large  and  handsome  octavo  volume,  of  950  pages,  strongly  bound  in  leather 
with  raised  bands.  $5  00. 

MEIGS  (CHARLES  D.),  M.  D., 

Professor  of  Obstetrics,  &c.  in  the  Jefferson  Medical  College,  Philadelphia. 

OBSTETRICS :   THE   SCIENCE   AND   THE   ART.     Third  edition,  revised 

and  improved.   With  one  hundred  and  twenty-nine  illustrations.  In  one  beautifully  printed  octavo 

volume,  leather,  of  seven  hundred  and  fifty-two  large  pages.     $3  75. 

The  rapid  demand  for  another  edition  of  this  work  is  a  sufficient  expression  of  the  favorable 
verdict  of  the  profession.  In  thus  preparing  it  a  third  time  for  the  presS,  the  author  has  endeavored 
to  render  it  in  every  respect  worthy  of  the  favor  which  it  has  received.  To  accomplish  this  he 
has  thoroughly  revised  it  in  every  part.  Some  portions  have  been  rewritten,  others  added,  new 
illustrations  have  been  in  many  instances  substituted  for  such  as  were  not  deemed  satisfactory, 
while,  by  an  alteration  in  the  typographical  arrangement,  the  size  of  the  work  has  not  been  increased, 
and  the  price  remains  unaltered.  In  its  present  improved  form,  it  is,  therefore,  hoped  that  the  work 
will  continue  to  meet  the  wants  of  the  American  profession  as  a  sound,  practical,  and  extended 
SYSTEM  OF  MIDWIFERY. 


Though  the  work  has  received  only  five  pages  of 
enlargement,  its  chapters  throughout  wear  the  im- 
press of  careful  revision.  Expunging  and  rewriting, 
remodelling  its  sentences,  with  occasional  new  ma- 
terial, all  evince  a  lively  desire  that  it  shall  deserve 
to  be  regarded  as  improved  in  manner  as  well  as 
matter.  In  the  matter,  every  stroke  of  the  pen  has 


increased  the  value  of  the  book,  both  in  expungings 
and  additions  — Western  Lancet,  Jan.  1857. 


The  best  American  work  on  Midwifery  that  is 
accessible  to  the  student  and  practitioner— N.  W. 
Med.  and  Surg.  Journal,  Jan.  1857. 

This  is  a  standard  work  by  a  great  American  Ob- 
stetrician. It  is  the  third  and  last  edition,  and,  in 
the  language  of  the  preface,  the  author  has  "brought 
the  subject  up  to  the  latest  dates  of  real  improve- 


ment in  our  art  and  Science." — Nashville  Journ.  of 
Med.  and  Surg.,  May,  1857. 

BY   THE   SAME   AUTHOR.      (Just  Issued.} 

WOMAN:  HER  DISEASES  AND  THEIR  REMEDIES.     A  Series  of  Lec- 
tures to  his  Class.    Fourth  and  Improved  edition.    In  one  large  and  beautifully  printed  octavo 
volume,  leather,  of  over  700  pages.     $3  60. 
In  other  respects,  in  our  estimation,  too  much  can- 
not be  said  in  praise  of  this  work.    It  abounds  with 
beautiful  passages,  and  for  conciseness,  for  origin- 


ality, and  for  all  that  is  commendable  in  a  work  on 
the  diseases  of  females,  it  is  not  excelled,  and  pro- 
bably not  equalled  in  the  English  language.  On  the 
whole,  we  know  of  no  worit  on  the  diseases  of  wo- 
men which  we  can  so  cordially  commend  to  the 
student  and  practitioner  as  the  one  before  us. — Ohio 
Med.  and  Surg.  Journal. 

The  body  of  the  book  is  worthy  of  attentive  con- 
sideration, and  is  evidently  the  production  of  a 
clever,  thoughtful,  and  sagacious  physician.  Dr. 
Meigs's  letters  on  the  diseases  of  the  external  or- 
gans, contain  many  interesting  and  rare  cases,  and 
many  instructive  observations.  We  take  our  leave 
of  Dr.  Meigs,  with  a  high  opinion  of  his  talents  and 
originality.— The  British  and  Foreign  Medico-Chi- 
rurgical  Review. 

Every  chapter  is  replete  with  practical  instruc- 
tion, and  bears  the  impress  of  being  the  composition 
of  an  acute  and  experienced  mind.  There  is  a  terse- 
ness, and  at  the  same  time  an  accuracy  in  his  de- 
scription of  symptoms,  and  in  the  rules  for  diagnosis, 
which  cannot  fail  to  recommend  the  volume  to  the 
attention  of  the  reader.— Ranking^  Abstract. 

It  contains  a  vast  amount  of  practical  knowledge, 
by  one  who  has  accurately  observed  and  retained 


the  experience  of  many  years. 
Journal. 


-Dublin  Quarterly 


Full  of  important  matter,  conveyed  in  a  ready  and 
agreeable  manner. — St. Louis  Med.  and  Surg.  Jour. 


There  is  an  off-hand  fervor,  a  glow,  and  a  warm- 
heartedness infecting  the  effort  of  Dr.  Meigs,  which 
is  entirely  captivating,  and  which  absolutely  hur- 
ries the  reader  through  from  beginning  to  end.  Be- 
sides, the  book  teems  with  solid  instruction,  and 
it  shows  the  very  highest  evidence  of  ability,  viz., 
the  clearness  with  which  the  information  is  pre- 
sented. We  know  of  no  better  test  of  one's  under- 
standing a  subject  than  the  evidence  of  the  power 
of  lucidly  explaining  it.  The  most  elementary,  as 
well  as  the  obscurest  subjects,  under  the  pencil  of 
Prof.  Meigs,  are  isolated  and  made  to  stand  out  in 
such  bold  relief,  as  to  produce  distinct  impressions 
upon  the  mind  and  memory  of  the  reader.  —  Tht 
Charleston  Med.  Journal. 

Professor  Meigs  has  enlarged  and  amended  this 
great  work,  for  such  it  unquestionably  is,  having 
passed  the  ordeal  of  criticism  at  home  and  abroad, 
but  been  improved  thereby  ;  for  in  this  new  edition 
the  author  has  introduced  real  improvements,  and 
increased  the  value  and  utility  of  the  book  im- 
measurably. It  presents  so  many  novel,  bright, 
and  sparkling  thoughts;  such  an  exuberance  of  new 
ideas  on  almost  every  page,  that  we  confess  our- 
selves to  have  become  enamored  with  the  book 
and  its  author ;  and  cannot  withhold  our  congratu- 
lations from  our  Philadelphia  confreres,  that  such  a 
teacher  is  in  their  service.— N.  Y.  Med.  Gazette. 


BY   THE   SAME   AUTHOR. 

ON    THE    NATURE,    SIGNS,    AND    TREATMENT    OF    CHILDBED 

FEVER.    In  a  Series  of  Letters  addressed  to  the  Students  of  his  Class.    In  one  handsome 

octavo  volume,  extra  cloth,  of  365  pages.     $2  50. 

lectable  book.  *  *  *  This  treatise  upon  child- 
bed fevers  will  have  an  extensive  sale,  being  des- 
tined, as  it  deserves,  to  find  a  place  in  the  library 


The  instructive  and  interesting  author  of  this 
work,  whose  previous  labors  have  placed  his  coun- 
trymen under  deep  and  abiding  obligations,  again 
challenges  their  admiration  in  the  fresh  and  vigor- 
ous, attractive  and  racy  pages  before  us.  It  is  a  de- 


of  every  practitioner  who  scorns  to  lag  in  the  rear. — 
Nashville  Journal  of  Medicine  and  Surgery. 


BY  THE  SAME  AUTHOR  J  WITH  COLORED  PLATES. 

A  TREATISE  ON  ACUTE  AND  CHRONIC  DISEASES  OF  THE  NECK 

OF  THE  UTERUS.    With  numerous  plates,  drawn  and  colored  from  nature  in  the  highest 
style  oi  art.    In  one  handsome  octavo  volume,  extra  cloth.     $4  50. 


MAYNE'S  DISPENSATORY  AND  THERA- 
PEUTICAL REMEMBRANCER.  With  every 
Practical  Formula  contained  in  the  three  British 
Pharmacopoeias.  Edited,  with  the  addition  of  the 
Farmulse  of  the  U.S.  Pharmacopoeia,  by  R.  E. 
GRIFFITH,  M.D.  112mo.vol.ex.cl.,300pp.  75  c. 


MALGAIGNE'S  OPERATIVE  SURGERY,  based 
on  Normal  and  Pathological  Anatomy.  Trans- 
lated from  the  French  by  FREDERICK  BRITTAN, 
A .  B . ,  M .  D .  Wi  th  numerous  illustrations  on  wood . 
In  one  handsome  octavo  volume,  extra  cloth,  of 
nearly  six  hundred  pages.  $2  25. 


BLANCHARD    &    LEA'S    MEDICAL 


MACLISE   (JOSEPH),    SURGEON. 

SURGICAL  ANATOMY.  Forming  one  volume,  very  large  imperial  quarto. 
With  sixty-eight  large  and  splendid  Plates,  drawn  in  the  best  style  and  beautifully  colored.  Con- 
taining one  hundred  and  ninety  Figures,  many  of  them  the  size  of  life.  Together  with  copious 
and  explanatory  letter-press.  Strongly  and  handsomely  bound  in  extra  cloth,  being  one  of  the 
cheapest  and  best  executed  Surgical  works  as  yet  issued  in  this  country.  $11  00. 
*„,*  The  size  of  this  work  prevents  its  transmission  through  the  post-office  as  a  whole,  but  those 

who  desire  to  have  copies  forwarded  by  mail,  can  receive  them  in  five  parts,  done  up  in  stout 

wrappers.     Price  $9  00. 


One  of  the  greatest  artistic  triumphs  of  the  age 
in  Surgical  Anatomy. — British  American  Medical 
Journal. 

No  practitioner  whose  mea»s  will  admit  should 
fail  to  possess  it. — Ranking's  Abstract. 


A  work  which  has  no  parallel  in  point  of  accu- 
racy and  cheapness  in  the  English  language. — N.  Y. 
Journal  of  Medicine. 

We  are  extremely  gratified  to  announce  to  the 
profession  the  completion  of  this  truly  magnificent 


Too  much  cannot  be  said  in  its  praise;  indeed,  I  work    which    ag    -  whol      certainly 'stands  unri- 
we  have  not  language  to  do  it  justice.— Ohio  Medi-  '  >    .-    '-  •         ,._...__    -- 


cal  and  Surgical  Journal. 

The  most  accurately  engraved  and  beautifully 
colored  plates  we  have  ever  seen  in  an  American 
book — one  of  the  best  and  cheapest  surgical  works 
ever  published. — Buffalo  Medical  Journal. 

It  is  very  rare  that  so  elegantly  printed,  so  well 
illustrated,  and  so  useful  a  work,  is  offered  at  BO 
moderate  a  price.— Charleston  Medical  Journal. 

Its  plates  can  boast  a  superiority  which  places 
them  almost  beyond  the  reach  of  competition. — Medi- 
cal Examiner. 

Country  practitioners  will  find  these  plates  of  im- 
mense value. — N.  Y.  Medical  Gazette. 


vailed,  both  for  accuracy  of  drawing,  beauty  of 
coloring,  and  all  the  requisite  explanations  of  the 
subject  in  hand. — Tht  Neu>  Orleans  Medical  and 
Surgical  Journal. 

This  is  by  far  the  ablest  work  on  Surgical  Ana- 
tomy that  has  come  under  our  observation.  We 
know  of  no  other  work  that  would  justify  a  stu- 
dent, in  any  degree,  for  neglect  of  actual  dissec- 
tion. In  those  sudden  emergencies  that  so  often 
arise,  and  which  require  the  instantaneous  command 
of  minute  anatomical  knowledge,  a  work  of  this  kind 
keeps  the  details  of  the  dissecting-room  perpetually 
fresh  in  the  memory. — The  Western  Journal  of  Medi- 
cine and  Surgery. 


MILLER  (HENRY),  M.  D., 

Professor  of  Obstetrics  and  Diseases  of  Women  and  Children  in  the  University  of  Louisville. 

PRINCIPLES  AND  PRACTICE  OF  OBSTETRICS,  &o. ;  including  the  Treat- 

ment  of  Chronic  Inflammation  of  the  Cervix  and  Body  of  the  Uterus  considered  as  a  frequent 
cause  of  Abortion.  With  about  one  hundred  illustrations  on  wood.  In  one  very  handsome  oc- 
tavo volume,  of  over  600  pages.  (Lately  Published.)  $3  75. 

The  reputation  of  Dr.  Miller  as  an  obstetrician  is  too  widely  spread  to  require  the  attention  oi 
the  profession  to  be  specially  called  to  a  volume  containing  the  experience  of  his  long  and  extensive 
practice.  The  very  favorable  reception  accorded  to  his  "  Treatise  on  Human  Parturition,"  issued 
some  years  since,  is  an  earnest  that  the  present  work  will  fulfil  the  author's  intention  of  providing 
within  a  moderate  compass  a  complete  and  trustworthy  text-book  for  the  student,  and  book  of  re- 
ference for  the  practitioner. 

tion  to  which  its  merits  justly  entitle  it.  The  style 
is  such  that  the  descriptions  are  clear,  and  each  sub- 
ject is  discussed  and  elucidated  with  due  regard  to 
its  practical  bearings,  which  cannot  fail  to  make  it 
acceptable  and  valuable  to  both  students  and  prac- 
titioners. We  cannot,  however,  close  this  brief 
notice  without  congratulating  the  author  and  the 
profession  on  the  production  of  such  an  excellent 
treatise.  The  author  is  a  western  man  of  whom  we 


We  congratulate  the  author  that  the  task  is  done. 
We  congratulate  him  that  he  has  given  to  the  medi- 
cal public  a  work  which  will  secure  for  him  a  high 
and  permanent  position  among  the  standard  autho- 
rities on  the  principles  and  practice  of  obstetrics. 
Congratulations  are  not  less  due  to  the  medical  pro- 
fession of  this  country,  on  the  acquisition  of  a  trea- 
tise embodying  the  results  of  the  studies,  reflections, 
and  experience  of  Prof.  Miller.  Few  men,  if  any, 
in  this  country,  are  more  competent  than  he  to  write 


on  this  department  of  medicine.  Engaged  for  thirty- 
five  years  in  an  extended  practice  of  obstetrics,  for 
many  years  a  teacher  of  this  branch  of  instruction 
in  one  of  the  largest  of  our  institutions,  a  diligent 
student  as  well  as  a  careful  observer,  an  original  and 
independent  thinker,  wedded  to  no  hobbies,  ever 
ready  to  consider  without  prejudice  new  views,  and 
to  adopt  innovations  if  they  are  really  improvements, 
and  withal  a  clear,  agreeable  writer,  a  practical 
treatise  from  his  pen  could  not  fail  to  possess  great 
value. — Buffalo  Med  Journal,  Mar.  1858. 

In  fact,  this  volume  must  take  its  place  among  the 
standard  systematic  treatises  on  obstetrics ;  a  posi- 


feel  proud,  and  we  cannot  but  think  that  his  book 
will  find  many  readers  and  warm  admirers  wherever 
obstetrics  is  taught  and  studied  as  a  science  and  an 
art.— The  Cincinnati  Lancet  and  Observer,  Feb.  1858. 
A  most  respectable  and  valuable  addition  to  our 
home  medical  literature,  and  one  reflecting  credit 
alike  on  the  author  and  the  institution  to  which  he 
is  attached.  The  student  will  find  in  this  work  a 
most  useful  guide  to  his  studies ;  the  country  prac- 
titioner, rusty  in  his  reading,  can  obtain  from  its 
pages  a  fair  resume  of  the  modern  literature  of  the 
science;  and  we  hope  to  see  this  American  produc- 
tion generally  consulted  by  the  profession. —  Va. 
Med.  Journal,  Feb.  1858. 


MACKENZIE  (W.),    M.  D., 

Surgeon  Oculist  in  Scotland  in  ordinary  to  Her  Majesty,  &c.  &c. 

A  PRACTICAL   TREATISE  ON   DISEASES   AND  INJURIES  OF   THE 

EYE.  To  which  is  prefixed  an  Anatomical  Introduction  explanatory  of  a  Horizontal  Section  of 
the  Human  Eyeball,  by  THOMAS  WHARTON  JONES,  F.  R.  S.  From  the  Fourth  Revised  and  En- 
larged  London  Edition.  With  Notes  and  Additions  by  ADDINELL  HEWSON,  M.  D.,  Surgeon  to 
Wills  Hospital,  &c.  &c.  In  one  very  large  and  handsome  octavo  volume,  leather,  raised  bands,  with 
plates  and  numerous  wood-cuts.  $5  25. 
The  treatise  of  Dr.  Mackenzie  indisputably  holds 


spi 

the  first  place,  and  forms,  in  respect  of  learning  and 
research,  an  Encyclopaedia  unequalled  in  extent  by 
any  other  work  of  the  kind,  either  English  or  foreign. 
—Dixon  on  Diseases  of  the  Eye. 

Few  modern  books  on  any  department  of  medicine 
or  surgery  have  met  with  such  extended  circulation, 
or  have  procured  for  their  authors  a  like  amount  of 
European  celebrity.  The  immense  research  which 
it  displayed,  the  thorough  acquaintance  with  the 
•ubiect,  oracticallv  as  well  as  iU— ^"nlly.and  the 


able  manner  in  which  the  author's  stores  of  learning 
and  experience  were  rendered  available  for  general 
use,  at  once  procured  for  the  first  edition,  as  well  on 
the  continent  as  in  this  country,  that  high  position 
as  a  standard  work  which  each  successive  edition 
has  more  firmly  established.  We  consider  it  the 
duty  of  ever 
and  the  w 

self  familiar  with  this  the  most  complete  work  in 
the  English  language  upon  the  diseases  of  the  eye. 
—  Med.  Times  and  Gazette. 


ery  one  who  has  the  love  of  his  profession 
elfare  of  his  patient  at  heart,  to  make  him- 


AND   SCIENTIFIC    PUBLICATIONS. 


MILLER  (JAMES),   F.  R.  S.  E., 

Professor  of  Surgery  in  the  University  of  Edinburgh,  &c. 

PRINCIPLES  OF  SURGERY.  Fourth  American,  from  the  third  and  revised 
Edinburgh  edition.  In  one  large  and  very  beautiful  volume,  leather,  of  700  pages,  with  two 
hundred  and  forty  illustrations  on  wood.  $3  75. 


The  work  of  Mr.  Miller  is  too  well  and  too  favor- 
ably known  among  us,  as  one  of  our  best  text-books, 
to  render  any  further  notice  of  it  necessary  than  the 
announcement  of  a  new  edition,  the  fourth  in  our 
country,  a  proof  of  its  extensive  circulation  among 
us.  As  a  concise  and  reliable  exposition  of  the  sci- 
ence of  modern  surgery,  it  stands  deservedly  high — 
we  know  not  its  superior. — Boston  Med.  and  Surg. 
Journal . 


The  work  takes  rank  with  Watson's  Practice  of 
Physic;  it  certainly  does  not  fall  behind  that  great 
work  in  soundness  of  principle  or  depth  of  reason- 
ing and  research.  No  physiciaji  who  values  his  re- 
putation, or  seeks  the  interests  of  his  clients,  can 
acquit  himself  before  his  God  and  the  world  without 
making  himself  familiar  with  the  sound  and  philo- 
sophical views  developed  in  the  foregoing  book. — 
New  Orleans  Med.  and  Surg.  Journal. 


BY   THE   SAME   AUTHOR.      (Just  Issued.) 

THE  PRACTICE  OF  SURGERY.  Fourth  American  from  the  last  Edin- 
burgh edition.  Revised  by  the  American  editor.  Illustrated  by  three  hundred  and  sixty- four 
engravings  on  wood.  In  one  large  octavo  volume,  leather,  of  nearly  700  pages.  $3  75. 


No  encomium  of  ours  could  add  to  the  popularity 
of  Miller's  Surgery.  Its  reputation  in  this  country 
is  unsurpassed  by  that  of  any  other  work,  and,  when 
taken  in  connection  with  the  author's  Principles  of 
Surgery,  constitutes  a  whole,  without  reference  to 
to  which  no  conscientious  surgeon  would  be  willing 
practice  his  art. —  Southern  Med.  and  Surg.  Journal. 

It  is  seldom  that  two  volumes  have  ever  made  so 
profound  an  impression  in  so  short  a  time  as  the 
"Principles"  and  the  "  Practice"  of  Surgery  by 
Mr.  Miller — or  so  richly  merited  the  reputation  they 
have  acquired.  The  author  is  an  eminently  sensi- 
ble, practical,  and  well-informed  man,  who  knows 
exactly  what  he  is  talking  about  and  exactly  how  to 
talk  it.— Kentucky  Medical  Recorder. 

By  the  almost  unanimous  voice  of  the  profession, 


his  works,  both  on  the  principles  and  practice  of 
surgery  have  been  assigned  thehighest  rank.  If  we 
were  limited  to  but  one  work  on  surgery,  that  one 
should  be  Miller's,  as  we  regard  it  as  superior  to  all 
others. — St.  Louis  Med.  and  Surg.  Journal. 

The  author  has  in  this  and  his  "  Principles,"  pre- 
sented to  the  profession  one  of  the  most  complete  and 
reliable  systems  of  Surgery  extant.  His  style  of 
writing  is  original,  impressive,  and  engaging,  ener- 
getic, concise,  and  lucid.  Few  have  the  faculty  of 
condensing  so  much  in  small  space,  and  at  the  same 
time  so  persistently  holding  the  attention.  Whether 
as  a  text-book  for  students  or  a  book  of  reference 
for  practitioners,  it  cannot  be  too  strongly  recom- 
mended.— Southern  Journal  of  Med.  and  Physical 
Sciences. 


MORLAND  (W.  WJ,   M.  D., 

Fellow  of  the  Massachusetts  Medical  Society,  &c. 

DISEASES  OF  THE  URINARY  ORGANS;  a  Compendium  of  their  Diagnosis, 

Pathology,  and  Treatment.     With  illustrations.     In  one  large  and  handsome  octavo  volume,  of 
about  600  pages,  extra  cloth.     (Just  Issued.)    $3  50. 


Taken  as  a  whole,  we  can  recommend  Dr.  Mor- 
land's  compendium  as  a  very  desirable  addition  to 
the  library  of  every  medical  or  surgical  practi- 
tioner.— Brit,  and  For.  Med.-Chir.  Rev.,  April,  1859. 

Every  medical  practitioner  whose  attention  has 
been  to  any  extent  attracted  towards  the  class  of 
diseases  to  which  this  treatise  relates,  must  have 
often  and  sorely  experienced  the  want  of  some  full, 
yet  concise  recent  compendium  to  which  he  could 
refer.  This  desideratum  has  been  supplied  by  Dr. 
Morland,  and  it  has  been  ably  done.  He  has  placed 
before  us  a  full,  judicious,  and  reliable  digest. 
Each  subject  is  treated  with  sufficient  minuteness, 


yet  in  a  succinct,  narrational  style,  such  as  to  render 
the  work  one  of  great  interest,  and  one  which  will 
prove  in  the  highest  degree  useful  to  the  general 
practitioner.  To  the  members  of  the  profession  in  the 
country  it  will  be  peculiarly  valuable,  on  account 
of  the  charasteristics  which  we  have  mentioned, 
and  the  one  broad  aim  of  practical  utility  which  is 
kept  in  view,  and  which  shines  out  upon  every  page, 
together  with  the  skill  which  is  evinced  in  the  com- 
bination of  this  grand  requisite  with  the  utmost 
brevity  which  a  just  treatment  of  the  subjects  would 
admit.— N.  Y.  Journ.  of  Medicine,  Nov.  1858. 


MONTGOMERY  (W.  F.),   M.  D.,  M.  R.  I.  A.,  &c., 

Professor  of  Midwifery  in  the  King  and  Queen's  College  of  Physicians  in  Ireland,  &c. 

AN  EXPOSITION  OF  THE  SIGNS  AND  SYMPTOMS  OF  PREGNANCY. 

With  some  other  Papers  on  Subjects  connected  with  Midwifery.  From  the  second  and  enlarged 
English  edition.  With  two  exquisite  colored  plates,  and  numerous  wood-cuts.  In  one  very 
handsome  octavo  volume,  extra  cloth,  of  nearly  600  pages.  (Lately  Published.)  $3  75. 


A  book  unusually  rich  in  practical  suggestions. — 
Am.  Journal  Med.  Sciences,  Jan.  1857. 

These  several  subjects  so  interesting  in  them- 
selves, and  so  important,  every  one  of  them,  to  the 
most  delicate  and  precious  of  social  relations,  con- 
trolling often  the  honor  and  domestic  peace  of  a 
family,  the  legitimacy  of  offspring,  or  the  life  of  its 
parent,  are  all  treated  with  an  elegance  of  diction, 
fulness  of  illustrations,  acutenessand  justice  of  rea- 
soning, unparalleled  in  obstetrics,  and  unsurpassed  in 
medicine.  The  reader's  interest  can  never  flag,  so 
fresh,  and  vigorous,  and  classical  is  our  author's 
style;  and  one  forgets,  in  the  renewed  charm  of 
every  page,  that  it,  and  every  line,  and  every  word 


has  been  weighed  and  reweighed  through  years  of 
preparation ;  that  this  is  of  all  others  the  book  of 
Obstetric  Law,  on  each  of  its  several  topics ;  on  all 
points  connected  with  pregnancy,  to  be  everywhere 
received  as  a  manual  of  special  jurisprudence^  at 
once  announcing  fact,  affording  argument,  establish- 
ing precedent,  and  governing  alike  the  juryman,  ad- 
vocate, and  judge.  It  is  not  merely  in  its  le^al  re- 
lations that  we  find  this  work  so  interesting.  Hardly 
a  page  but  that  has  its  hints  or  facts  important  to 
the  general  practitioner ;  and  not  a  chapter  without 
especial  matter  for  the  anatomist,  physiologist,  or 
pathologist.  —  N.  A.  Med.-Chir.  Review,  March, 
1857. 


MOHR  (FRANCIS),  PH.  D.,  AND  REDWOOD  (TH  EOPHI  LUS). 
PRACTICAL    PHARMACY.     Comprising  the  Arrangements,  Apparatus, 


and 


Manipulations  of  the  Pharmaceutical  Shop  and  Laboratory.  Edited,  with  extensive  Additions, 
by  Prof.  WILLIAM  PROCTER,  of  the  Philadelphia  College  of  Pharmacy.  In  one  handsomely 
printed  octavo  volume,  extra  cloth,  of  570  pages,  with  over  500  engravings  on  wood.  $2  75, 


24  BLANCHARD    &    LEA'S    MEDICAL 

NEILL  (JOHN),   M.  D., 

Surgeon  to  the  Pennsylvania  Hospital, &c. ;  and 

FRANCIS  GURNEY  SMITH,   M.D., 

Professor  of  Institutes  of  Medicine  in  the  Pennsylvania  Medical  College. 

AN  ANALYTICAL  COMPENDIUM  OF  THE  VARIOUS  BRANCHES 

OF  MEDICAL  SCIENCE  ;  for  the  Use  and  Examination  of  Students.    A  new  edition,  revised 
and  improved.    In  one  very  large  and  handsomely  printed  royal  12mo.  volume,  of  about  one 
thousand  pages,  with  374  wood-cuts.     Strongly  bound  in  leather,  with  raised  bands.     $3  00. 
The  very  flattering  reception  which  has  been  accorded  to  this  work,  and  the  high  estimate  placed 
upon  it  by  the  profession,  as  evinced  by  the  constant  and  increasing  demand  which  has  rapidly  ex- 
hausted two  large  editions,  have  stimulated  the  authors  to  render  the  volume  in  its  present  revision 
more  worthy  of  the  success  which  has  attended  it.    It  has  accordingly  been  thoroughly  examined, 
and  such  errors  as  had  on  former  occasions  escaped  observation  have  been  corrected,  and  whatever 
additions  were  necessary  to  maintain  it  on  a  level  with  the  advance  of  science  have  been  introduced. 
The  extended  series  of  illustrations  has  been  still  further  increased  and  much  improved,  while,  by 
a  slight  enlargement  of  the  page,  these  various  additions  have  been  incorporated  without  increasing 
the  bulk  of  the  volume. 

The  work  is,  therefore,  again  presented  as  eminently  worthy  of  the  favor  with  which  it  has  hitherto 
been  received.  As  a  book  for  daily  reference  by  the  student  requiring  a  guide  to  his  more  elaborate 
text-books,  as  a  manual  for  preceptors  desiring  to  stimulate  their  students  by  frequent  and  accurate 
examination,  or  as  a  source  from  which  the  practitioners  of  older  date  may  easily  and  cheaply  acquire 
a  knowledge  of  the  changes  and  improvement  in  professional  science,  its  reputation  is  permanently 
established. 

the  students  is  heavy,  and  review  necessary  for  an 


The  best  work  of  the  kind  with  which  we  are 
acquainted. — Med.  Examiner. 

Having  made  free  use  of  this  volume  in  our  ex- 
aminations of  pupils,  we  can  speak  from  experi- 
ence in  recommending  it  as  an  admirable  compend 
for  students,  and  as  especially  useful  to  preceptors 
who  examine  their  pupils.  It  will  save  the  teacher 
much  labor  by  enabling  him  readily  to  recall  all  of 
the  points  upon  which  his  pupils  should  be  ex- 
amined. A  work  of  this  sort  should  be  in  the  hands 
of  every  one  who  takes  pupils  into  his  office  with  a 
view  of  examining  them ;  and  this  is  unquestionably 
the  best  of  its  class. — Transylvania  Med.  Journal. 

In  the  rapid  course  of  lectures,  where  work  for 


examination,  a  compend  is  not  only  valuable,  but 
it  is  almost  a  sine  qua  non.  The  one  before  us  is, 
in  most  of  the  divisions,  the  most  unexceptionable 
of  all  books  of  the  kind  that  we  know  of.  The 
newest  and  soundest  doctrines  and  the  latest  im- 
provements and  discoveries  are  explicitly,  though 
concisely,  laid  before  the  student.  There  is  a  class 
to  whom  we  very  sincerely  commend  this  cheap  book 
as  worth  its  weight  in  silver — that  class  is  the  gradu- 
ates in  medicine  of  more  than  ten  years'  standing, 
who  have  not  studied  medicine  since.  Tliey  will 
perhaps  find  out  from  it  that  the  science  is  not  exactly 
now  what  it  was  when  they  left  it  off.— The  Stetho- 
scope. 


NELIGAN  (J.   MOORE),  M.  D.,  M.  R.  I. A.,  &c. 

(A  splendid  work.    Just  Issued.) 

ATLAS  OF  CUTANEOUS  DISEASES.     In  one  beautiful  quarto  volume,  extra 

cloth,  with  splendid  colored  plates,  presenting  nearly  one  hundred  elaborate  representations  of 

disease.    $4  50. 

This  beautiful  volume  is  intended  as  a  complete  and  accurate  representation  of  all  the  varieties 
of  Diseases  of  the  Skin.  While  it  can  be  consulted  in  conjunction  with  any  work  on  Practice,  it  has 
especial  reference  to  the  author's  "  Treatise  on  Diseases  of  the  Skin,"  so  favorably  received  by  the 
profession  some  years  since.  The  publishers  feel  justified  in  saying  that  few  more  beautifully  exe- 
cuted plates  have  ever  been  presented  to  the  profession  of  this  country. 

Neligan's  Atlas  of  Cutaneous  Diseases  supplies  a  [  give,  at  a  coup  d'ail,  the  remarkable  peculiarities 
long  existent  desideratum  much  felt  by  the  largest 
class  of  our  profession.  It  presents,  in  quarto  size, 
16  plates,  each  containing  from  3  to  6  figures,  and 
forming  in  all  a  total  of  90  distinct  representations 
of  the  different  species  of  skin  affections,  grouped 
together  in  genera  or  families.  The  illustrations 
have  been  taken  from  nature,  and  have  been  copied 
with  such  fidelity  that  they  present  a  striking  picture 
of  life ;  in  which  the  reduced  scale  aptly  serves  to 

BY  THE  SAME  AUTHOR. 

A   PRACTICAL   TREATISE    ON   DISEASES   OF  THE   SKIN.      Third 

American  edition.    In  one  neat  royal  12mo.  volume,  extra  cloth,  of  334  pages.    $1  00. 
The  two  volumes  will  be  sent  by  mail  on  receipt  of  Five  Dollars. 


of  each  individual  variety.  And  while  thus  the  dis- 
ease is  rendered  more  definable,  there  is  yet  no  loss 
of  proportion  incurred  by  ttte  necessary  concentra- 
tion. Each  figure  is  highly  colored,  and  so  truthful 
has  the  artist  been  that  the  mostfastid'ous  observer 
could  not  justly  take  exception  to  the  correctness  of 
the  execution  of  the  pictures  under  his  scrutiny. — 
Montreal  Med.  Chronicle. 


OWEN   ON   THE  DIFFERENT   FORMS   OF  I     One  vol.  royal  12mo.,  extra  cloth  with  numerous 
THE  SKELETON,  AND   OF   THE   TEETH.  |     illustrations.    81  25 


PIRRIE  (WILLIAM),  F.  R.  S.  E., 

Professor  of  Surgery  in  the  University  of  Aberdeen. 

THE   PRINCIPLES  AND  PRACTICE  OF  SURGERY.    Edited  by  JOHN 

NEILL,  M.  D.,  Professor  of  Surgery  in  the  Penna.  Medical  College,  Surgeon  to  the  Pennsylvania 

Hospital,  &c.   In  one  very  handsome  octavo  volume,  leather,  ot  780  pages,  with  316  illustrations. 

$3  75. 

We  know  of  no  other  surgical  work  of  a  reason-  J  rately  discussed  the  principles  of  surgery,  and  a 
able  size,  wherein  there  is  so  much  theory  and  prac-  j  safe  and  effectual  practice  predicated  upon  them, 
tice,  or  where  subjects  are  more  soundly  or  clearly  j  Perhaps  no  work  upon  this  subject  heretofore  issued 
taught. — The  Stethoscope.  \  is  so  full  upon  the  science  of  the  art  of  surgery. — 

Prof.  Pirrie,  in  the  work  before  us,  has  elabo-  |  Nashville  Journal  of  Medicine  and  Surgery. 


AND   SCIENTIFIC    PUBLICATIONS. 


2f> 


PARRISH    (EDWARD), 

Lecturer  on  Practical  Pharmacy  and  Materia  Medica  in  the  Pennsylvania  Academy  of  Medicine,  &c. 

AN  INTRODUCTION  TO  PRACTICAL  PHARMACY.    Designed  as  a  Text- 

Book  for  the  Student,  and  as  a  Guide  for  the  Physician  and  Pharmaceutist.  With  many  For- 
mulae and  Prescriptions.  Second  edition,  greatly  enlarged  and  improved.  In  one  handsome 
octavo  volume  of  720  pages,  with  several  hundred  Illustrations,  extra  cloth.  $3  50.  (Now 
Ready.) 

During  the  short  time  in  which  this  work  has  been  before  the  profession,  it  has  been  received 
with  very  great  favor,  and  in  assuming  the  position  of  a  standard  authority,  it  has  filled  a  vacancy 
which  had  been  severely  felt.  Stimulated  by  this  encouragement,  the  author,  in  availing  himself 
of  the  opportunity  of  revision,  has  spared  no  pains  to  render  it  more  worthy  of  the  confidence  be- 
stowed'upon  it,  and  his  assiduous  labors  have  made  it  rather  a  new  book  than  a  new  edition,  many 
portions  having  been  rewritten,  and  much  new  and  important  matter  added.  These  alterations  and 
improvements  have  been  rendered  necessary  by  the  rapid  progress  made  by  pharmaceutical  science 
during  the  last  few  years,  and  by  the  additional  experience  obtained  in  the  practical  use  of  the 
volume  as  a  text-book  and  work  of  reference.  To  accommodate  these  improvements,  the  size  of 
the  page  has  been  materially  enlarged,  and  the  number  of  pages  considerably  increased,  presenting 
in  all  nearly  one- half  more  matter  than  the  last  edition.  The  work  is  therefore  now  presented  as  a 
complete  exponent  of  the  subject  in  its  most  advanced  condition.  From  the  most  ordinary  matters 
in  the  dispensing  office,  to  the  most  complicated  details  of  the  vegetable  alkaloids,  it  is  hoped  that 
everything  requisite  to  the  practising  physician,  and  to  the  apothecary,  will  be  found  fully  and 
clearly  set  forth,  and  that  the  new  matter  alone  will  be  worth  more  than  the  very  moderate  cost  of 
the  work  to  those  who  have  been  consulting  the  previous  edition. 


That  Edward  Parrish,  in  writing  a  book  upon 
practical  Pharmacy  some  few  years  ago — one  emi- 
nently original  and  unique — did  the  medical  and 
pharmaceutical  professions  a  great  and  valuable  ser- 
vice, no  one,  we  think,  who  has  had  access  to  its 
pages  will  deny;  doubly  welcome,  then,  is  this  new 
edition,  containing  the  added  results  of  his  recent 
and  rich  experience  as  an  observer,  teacher,  and 
practicil  operator  in  the  pharmaceutical  laboratory. 
The  excellent  plan  of  the  first  is  more  thoroughly, 
and  in  detail,  carried  out  in  this  edition. — Peninsular 
Med.  Journal,  Jan.  1860. 

We  know  of  no  work  on  the  subject  which  would 
be  more  indispensable  to  the  physician  or  student 
desiring  information  on  the  subject  of  which  it  treats. 
With  Griffith's  "  Medical  Formulary"  and  this,  the 
practising  physician  would  be  supplied  with  nearly 
or  quite  all  the  most  useful  information  on  the  sub- 
ject.— Charleston  Med.  Journal  and  Review,  Jan. 
1860. 

This  edition,  now  much  enlarged,  is  one  of  the 
most  useful  works  of  the  past  year. — N.  O.  Med. 
and  Surg.  Journal,  Jan.  1860. 

The  whole  treatise  is  eminently  practical;   and 


there  is  no  production  of  the  kind  in  the  English 
language  so  well  adapted  to  the  wants  of  the  phar- 
maceutist and  druggist.  To  physicians,  also,  it  can 


phys 

.ble, 


not  fail  to  be  highly  valuable,  especially  to  those 
who  are  obliged  to  prepare  and  compound  many  of 
their  own  medicines. — N,  Am.  Med.  Chir.  Review , 
Jan. 1860. 

Of  course,  all  apothecaries  who  have  not  already 
a  copy  of  the  first  edition  will  procure  one  of  this; 
it  is,  therefore,  to  physicians  residing  in  the  country 
and  in  small  towns,  who  cannot  avail  themselves  of 
the  skill  of  an  educated  pharmaceutist,  that  we 
would  especially  commend  this  work.  In  it  they 
will  find  all  that  they  desire  to  know,  and  should 
know,  but  very  little  of  which  they  do  really  Know 
in  reference  to  this  important  collateral  branch  of 
their  profession ;  for  it  is  a  well  established  fact, 
that,  in  the  education  of  physicians,  while  the  sci- 
ence of  medicine  is  generally  well  taught,  very 
little  attention  is  paid  to  the  art  of  preparing  them 
for  use,  and  we  know  not  how  this  defect  can  be  so 
well  remedied  as  by  procuring  and  consulting  Dr. 
Patrish's  excellent  work. — St.  Louis  Med.  Journal. 
Jan.  1860. 


PEASLEE  (E.  R.),  M.  D., 

Professor  of  Physiology  and  General  Pathology  in  the  New  York  Medical  College. 

HUMAN  HISTOLOGY,  in  its  relations  to  Anatomy,  Physiology,  and  Pathology; 
for  the  use  of  Medical  Students.    With  four  hundred  and  thirty- four  illustrations.    In  one  hand- 
some octavo  volume,  of  over  600  pages.    (Lately  Published.}    $3  75. 
It  embraces  a  library  upon  the  topics  discussed 

within  itself,  and  is  just  what  the  teacher  and  learner 

need.    Another  advantage,  by  no  means  to  be  over- 


UCCU.         AUUM1V1    aUVClUMUCCj    uy    I1U    JIlC'MiB    (*U    UC    UVC1- 

looked,  everything  of  real  value  in  the  wide  range 
which  it  embraces,  is  with  great  skill  compressed 
into  an  octavo  volume  of  but  little  more  than  six 
hundred  pages.  We  have  not  only  the  whole  sub- 
ject of  Histology,  interesting  in  itself,  ably  and  fully 
discussed,  but  what  is  of  infinitely  greater  interest 
to  the  student,  because  of  greater  practical  value, 
are  its  relations  to  Anatomy,  Physiology,  and  Pa- 
thology, which  are  here  fully  and  satisfactorily  set 
forth.— Nashville  Journ.  of  Med.  and  Surgery,  Dec. 
1857. 


We  would  recommend  it  to  the  medical  student 
and  practitioner  ?  as  containing  a  summary  of  all  that 
is  known  of  the  important  subjects  which  it  treats ; 
of  all  that  is  contained  in  the  great  works  of  Simon 
and  Lehmann,  and  the  organic  chemists  in  general. 
Master  this  one  volume,  we  would  say  to  the  medical 
student  and  practitioner — master  this  book  and  you 
know  all  that  is  known  of  the  great  fundamental 
principles  of  medicine,  and  we  have  EO  hesitation 
in  saying  that  it  is  aa  honor  to  the  American  medi- 
cal profession  that  one  of  its  members  should  have 
produced  it.— St.  Louis  Mtd.  and  Surg.  Journal, 
March,  1858. 


PEREIRA  (JONATHAN),  M.  D.,  F.  R.  S.f  AND  L.  S. 
THE    ELEMENTS    OF   MATERIA    MEDICA    AND    THERAPEUTICS. 

Third  American  edition,  enlarged  and  improved  by  the  author ;  including  Notices  of  most  of  the 
Medicinal  Substances  in  use  in  the  civilized  world,  and  forming  an  Encyclopaedia  of  Materia 
Medica.  Edited,  with  Additions,  by  JOSEPH  CARSON,  M.  D.,  Professor  of  Materia  Medica  and 
Pharmacy  in  the  University  of  Pennsylvania.  In  two  very  large  octavo  volumes  of  2100  pages, 
on  small  type,  with  about  500  illustrations  on  stone  and  wood,  strongly  bound  in  leather,  with 
raised  bands.  $9  00. 
x**  Vol.  II.  will  no  longer  be  sold  separate. 


PARKER  (LANGSTON), 

Surgeon  to  the  Queen's  Hospital,  Birmingham. 

THE  MODERN  TREATMENT  OF  SYPHILITIC  DISEASES,  BOTH  PRI- 
MARY AND  SECONDARY;  comprising  the  Treatment  of  Constitutional  and  Confirmed  Syphi- 

I.    With  numerous  Cases,  Formulae,  and  Clinical  Observa- 


lis,  by  a  safe  and  successful  method. 

tions.    From  the  Third  and  entirely  rewritten  London  edition. 

extra  cloth,  of  316  pages.    $1  75. 


In  one  neat  octavo  volume, 


BLANCHARD  &  LEA'S  MEDICAL 


RAMSBOTHAM  (FRANCIS  H.),  M.D. 
THE  PRINCIPLES  AND  PRACTICE  OF  OBSTETRIC  MEDICINE  AND 

SURGERY,  in  reference  to  the  Process  of  Parturition.  A  new  and  enlarged  edition,  thoroughly 
revised  by  the  Author.  With  Additions  by  W.  V.  KEATING,  M.  D.  In  one  large  and  handsome 
imperial  octavo  volume,  of  650  pages,  strongly  bound  in  leather,  with  raised  bands;  with  sixty- 
four  beautiful  Plates,  and  numerous  Wood-cuts  in  the  text,  containing  in  all  nearly  two  hundred 
large  and  beautiful  figures.  $5  00. 

From  Prof.  Hodge,  of  the  University  of  Pa. 
To  the  American  public,  it  is  most  valuable,  from  its  intrinsic  undoubted  excellence,  and  as  being 

the  best  authorized  exponent  of  British  Midwifery.    Its  circulation  will,  I  trust,  be  extensive  throughout 

our  country. 

to  the  |  truly  elegant  style  in  which  they  have  brqught  it 


It  is  unnecessary  to  say  anything  in  regard 
utility  of  this  work.  It  is  already  appreciated 
country  for  the  value  of  the  matter,  the  clearness  of 
its  style,  and  the  fulness  of  its  illustrations.  To  the 
physician's  library  it  is  indispensable,  while  to  the 
student  as  a  text-book,  from  which  to  extract  the 
material  for  laying  the  foundation  of  an  education  on 
obstetrical  science,  it  has  no  superior. — Ohio  Med. 
and  Surg.  Journal. 

The  publishers  have  secured  its  success  by  the 


out,  excelling  themselves  in  its  production,  espe- 
cially in  its  plates.    It  is  dedicated  to  Prof.  Meigs, 


and  has  the  emphatic  endorsement  of  Prof.  Hod 
as  the  best  exponent  of  British  Midwifery. 
knc.w  of  no  text-book  which  deserves  in  all  respects 
to  be  more  highly  recommended  to  students,  and  we 


ge, 
e 


could  wish  to  see  it  in  the  hands  of  ever 


ry 

for  they  will  find  it  invaluable  for  reference 
Gazette. 


practitioner, 
Med. 


RICORD  (P.),  M.  D. 
A  TREATISE  ON  THE  VENEREAL  DISEASE.    By  JOHN  HUNTER,  F.  R.  S. 

With  copious  Additions,  by  PH.  RICORD,  M.D.    Translated  and  Edited,  with  Notes,  by  FREEMAN 
J.  BUMSTEAD,  M.  D.,  Lecturer  on  Venereal  at  the  College  of  Physicians  and  Surgeons,  New  York. 
Second  edition,  revised,  containing  a  resume  of  RICORD'S  RECENT  LECTURES  ON  CHANCRE.     In 
one  handsome  octavo  volume,  extra  cloth,  of  550  pages,  with  eight  plates.   $3  25.   (Just  Issued.} 
In  revising  this  work,  the  editor  has  endeavored  to  introduce  whatever  matter  of  interest  the  re- 
cent investigations  of  syphilographers  have  added  to  our  knowledge  of  the  subject.     The  principal 
source  from  which  this  has  been  derived  is  the  volume  of  "Lectures  on  Chancre,"  published  a  few 
months  since  by  M.  Ricord,  which  affords  a  large  amount  of  new  and  instructive  material  on  many 
controverted  points.     In  the  previous  edition,  M.  Ricord's  additions  amounted  to  nearly  one-third 
of  the  whole,  and  with  the  matter  now  introduced,  the  work  may  be  considered  to  present  his  views 
and  experience  more  thoroughly  and  completely  than  any  other. 


Every  one  will  recognize  the  attractiveness  and 
value  which  this  work  derives  from  thus  presenting 
the  opinions  of  these  two  masters  side  by  side.  But, 
it  must  be  admitted,  what  has  made  the  fortune  of 
the  book,  is  the  fact  that  it  contains  the  "  most  com- 
plete embodiment  of  the  veritable  doctrines  of  the 
Hopital  du  Midi,"  which  has  ever  been  made  public. 
The  doctrinal  ideas  of  M.  Ricord,  ideas  which,  if  not 
universally  adopted,  are  incontestably  dominant,  have 
heretofore  only  been  interpreted  by  more  or  less  skilfu  1 


secretaries,  sometimes  accredited  and  sometimes  not. 
In  the  notes  to  Hunter,  the  master  substitutes  him- 
self forhis  interpreters,  and  gives  hisoriginal  thoughts 
to  the  world  in  a  lucid  and  perfectly  intelligible  man- 
ner. In  conclusion  we  can  say  that  this  is  incon- 
testably the  best  treatise  on  syphilis  with  which  we 
are  acquainted,  and,  as  we  do  not  often  employ  the 
phrase,  we  may  be  excused  for  expressing  the  hope 
that  it  may  find  a  ' 
sician 


may  find  a  place  in  the  library  of  every  phy- 
.—  Virginia  Med.  and  Surg.  Journal. 


BY  THE  SAME  AUTHOR. 

RICORD'S  LETTERS  ON  SYPHILIS.   Translated  by  W.  P.  LATTIMORE,  M.  D. 

In  one  neat  octavo  volume,  of  270  pages,  extra  cloth.    $2  00. 

ROYLE'S   MATERIA    MEDIC  A    AND   THERAPEUTICS;   including  the 

Preparations  of  the  Pharmacopoeias  of  London,  Edinburgh,  Dublin,  and  of  the  United  States. 
With  many  new  medicines.  Edited  by  JOSEPH  CARSON,  M.  D.  With  ninety-eight  illustrations. 
In  one  large  octavo  volume,  extra  cloth,  of  about  700  pages.  $3  00. 

ROKITANSKY  (CARL),    M..D., 

Curator  of  the  Imperial  Pathological  Museum,  and  Professor  at  the  University  of  Vienna,  &c. 

A    MANUAL   OF  PATHOLOGICAL    ANATOMY.    Four  volumes,   octavo, 

bound  in  two,  extra  cloth,  of  about  1200  pages.  Translated  by  W.  E.  SWAINE,  EDWARD  SIEVE- 
KING,  C.  H.  MOORE,  and  G.  E.  DAY.  $5  50 


The  profession  is  too  well  acquainted  with  the  re- 
putation of  Rokitansky's  work  to  need  our  assur- 
ance that  this  is  one  of  the  most  profound,  thorough, 
and  valuable  books  ever  issued  from  the  medical 
press.  It  is  sui  generis,  and  has  no  standard  of  com- 
parison. It  is  only  necessary  to  announce  that  it  is 
issued  in  a  form  as  cheap  as  is  compatible  with  its 
size  and  preservation,  and  its  sale  follows  as  a 
matter  of  course.  No  library  can  be  called  com- 
plete without  it.— Buffalo  Med.  Journal. 

An  attempt  to  give  our  readers  any  adequate  idea 
of  the  vast  amount  of  instruction  accumulated  in 
these  volumes,  would  be  feeble  and  hopeless.  The 
effort  of  the  distinguished  author  to  concentrate 
in  a  small  space  his  great  fund  of  knowledge,  has 


so  charged  his  text  with  valuable  truths,  that  any 
attempt  of  a  reviewer  to  epitomize  is  at  once  para- 
lyzed, and  must  end  in  a  failure. — Western  Lancet. 

As  this  is  the  highest  source  of  knowledge  upon 
the  important  subject  of  which  it  treats,  no  real 
student  can  afford  to  be  without  it.  The  American 
publishers  have  entitled  themselves  to  the  thanks  of 
the  profession  of  their  country,  for  this  timeous  and 
beautiful  edition. — Nashville  Journal  of  Medicine. 

As  a  book  of  reference,  therefore,  this  work  must 
prove  of  inestimable  value,  and  we  cannot  too  highly 
recommend  it  to  the  profession. — Charleston  Med. 
Journal  and  Review. 

This  book  is  a  necessity  to  every  practitioner.— 
Am.  Med.  Monthly. 


RIGBY    (EDWARD),    M.  D., 

Senior  Physician  to  the  General  Lying-in  Hospital,  Sec. 

A    SYSTEM    OF    MIDWIFERY.     With  Notes  and  Additional  Illustrations. 

Second  American  Edition.     One  volume  octavo,  extra  cloth,  422  pages.     $2  50. 
BY  THE  SAME  AUTHOR.     (Lately  Published.} 

ON  THE   CONSTITUTIONAL  TREATMENT  OF  FEMALE  DISEASES. 

In  one  neat  royal  12mo.  volume,  extra  cloth,  of  about  250  pages.    $1  00. 


AND    SCIENTIFIC    PUBLICATIONS. 


27 


STILLE  (ALFRED),    M.  D. 
THERAPEUTICS  AND  MATERIA  MEDIC  A;  a  Systematic  Treatise  on  the 

Action,  and  Uses  of  Medicinal  Agents,  including  their  Description  and  History.    In  two  large  and 
handsome  octavo  volumes,  of  1789  pages.     (Now  Ready ;  1860.)    $8  00. 

This  work  is  designed  especially  for  the  student  and  practitioner  of  medicine,  and  treats  the  various 
articles  of  the  Materia  Medica  from  the  point  of  view  of  the  bedside,  and  not  of  the  shop  or  of  the 
lecture-room.  While  thus  endeavoring  to  give  all  practical  information  likely  to  be  useful  with 
respect  to  the  employment  of  special  remedies  in  special  affections,  and  the  results  to  be  anticipated 
from  their  administration,  a  copious  Index  of  Diseases  and  their  Remedies  renders  the  work  emi- 
nently fitted  for  reference  by  showing  at  a  glance  ihe  different  means  which  have  been  employed, 
and  e'nabling  the  practitioner  to  extend  his  resources  in  difficult  ca-es  with  all  that  the  experience 
of  the  profession  has  suggested.  At  the  same  time  particular  care  has  been  given  to  the  subject 
of  General  Therapeutics,  and  at  the  commencement  of  each  class  of  medicines  there  is  a  chapter 
devoted  to  the  consideration  of  their  common  influence  upon  morbid  conditions.  The  action  of 
remedial  agents  upon  the  healthy  economy  and  on  animals  has  likewise  received  particular  notice, 
from  the  conviction  that  their  physiological  effects  will  afford  frequent  explanations  of  their  patho- 
logical influence,  and  in  many  cases  lead  to  new  and  important  suggestions  as  to  their  practical  use 
in  disease.  Within  the  scope  thus  designed  by  the  auihor,  no  labor  has  been  spared  to  accumulate 
all  the  facts  which  have  accrued  from  the  experience  of  the  profession  in  all  ages  and  all  countries  ; 
and  the  vast  amount  of  recent  researches  recorded  in  the  periodical  literature  of  both  hemispheres 
has  been  zealously  laid  under  contribution,  resulting  in  a  mass  of  practical  information  scarcely 
attempted  hitherto  in  any  similar  work  in  ihe  language. 


Our  expectations  of  the  value  of  this  work  were 
based  on  the  well-known  reputation  and  character 
of  the  author  as  a  man  of  scholarly  attainments,  an 
elegant  writer,  a  candid  inquirer  after  truth,  and  a 
philosophical  thinker  ;  we  knew  that  the  task  would 
be  conscientiously  performed,  and  that  few,  if  any, 
among  the  distinguished  medical  teachers  in  this 
country  are  better  qualified  than  fce  to  prepare  a 
systematic  treatise  on  therapeutics  in  accordance 
wilh  the  present  requirements  of  medical  science. 
Our  preliminary  examination  of  the  work  has  satis- 
fied us  that  we  were  not  mistaken  in  our  anticipi- 
tions.  In  congratulating  the  author  on  the  comple- 
tion of  the  great  labor  which  such  a  work  involves, 
we  are  happy  in  expressing  the  conviction  that  its 
merits  will  receive  that  reward  which  is  above  all 
price—  the  grateful  appreciation  of  his  medical  bre- 
thren.— New  Orleans  Medical  News,  March,  I860. 

We  think  this  work  will  do  much  to  obviate  the 
reluctance  to  a  thorough  investigation  of  this  branch 
of  scientific  study,  for  in  the  wide  range  of  medical 
literature  treasured  in  the  English  tongue,  we  shall 
hardly  find  a  work  written  in  a  style  more  clear  and 
simple,  conveying  forcibly  the  facts  taught,  and  yet 
free  from  turgidity  and  redundancy.  There  is  a  fas- 
cination in  its  pages  that  will  insure  to  it  a  wide 
popularity  and  attentive  perusal,  and  a  degree  of 
usefulness  not  often  attained  through  the  influence 
of  a  single  work.  The  author  has  much  enhanced 


the  practical  utility  of  his  book 


hriefly 


over  the  physical,  botani  ?al ,  arid  commercial  history 
of  medicines,  and  directing  attention  chiefly  to  their 
physiological  action,  and  their  application  for  the 
amelioration  or  cure  of  disease.  He  ignores  hypothe- 
sis and  theory  which  are  so  alluring  to  many  medical 
writers,  and  so  liable  to  lead  them  astray,  and  con- 
fines hiimelf  to  such  facts  as  have  been  tried  in  the 
crucible  of  experience. — Chicago  Medical  Journal, 
March,  1860. 

The  plan  pursued  by  the  author  in  these  very  ela- 
borate volumes  is  not  strictly  one  of  scientific  unity 
and  precision;  he  has  rather  subordinated  these  to 
practical  utility.  Dr.  Stille  has  produced  a  work 
which  will  he  valuable  equally  to  the  student  of 
medicine  and  the  busy  practitioner.—  London  Lan- 
cet, March  10,  1860. 

Wilh  Pereira,  Dunglison,  Mitchell,  and  Wood  be- 
fore us,  we  may  well  ask  if  there  was  a  necessity 
for  a  new  book  on  the  subject.  After  examining  this 
work  with  some  care,  we  can  answer  affirmatively. 
Dr.  Wood's  book  is  well  adapted  for  students,  while 
Dr.  Stille's  will  be  more  satisfactory  to  the  practi- 
tioner, who  desires  to  study  the  action  of  medicines . 
The  author  needs  no  encomiums  from  us,  for  he  is 
well  known  as  a  ripe  scholar  and  a  man  of  the  most 
extensive  reading  in  his  profession.  This  work  bears 


evidence  of  this  fact  on  every  page. 
Lancet,  April,  Is60. 


-Cincinnati 


SMITH    (HENRY    H.),   M.  D. 

MINOR  SURGERY;  or,  Hints  on  the  Every-day  Duties  of  the  Surgeon.  With 
247  illustrations.  Third  edition.  1  vol.  royal  12mo.,  pp.  456.  In  leather,  $2  25 ;  cloth,  $2  00. 

BY   THE   SAME   AUTHOR,   AND 

HORNER  (WILLIAM  E.),  M.D., 
Late  Professor  of  Anatomy  in  the  University  of  Pennsylvania. 

AN  ANATOMICAL  ATLAS,  illustrative  of  the  Structure  of  the  Human  Body. 
In  one  volume,  large  imperial  octavo,  extra  cloth,  with  about  six  hundred  and  fifty  beautiful 
figures.  $3  00. 


These  figures  are  well  selected,  and  present  a 
complete  and  accurate  representation  of  that  won- 
derful fabric,  the  human  body.  The  plan  of  this 

convenient 
execution, 
have  been  already  pointed  out.     We  must  congratu- 


Atlas,  which  renders  it  so  peculiarly 
for  the  student,  and  its  superb  artistical 


late  the  student  upon  the  completion  of  this  Atlas, 
as  it  is  the  most  convenient  work  of  the  kind  that 
has  yet  appeared  ;  and  we  must  add,  the  very  beau- 
tiful manner  in  which  it  is  "got  up"  is  so  creditable 
to  the  country  as  to  be  flattering  to  our  national 
pride. — American  Medical  Journal. 


SHARPEY  (WILLIAM),   M.  D.,   JONES   QUAIN,   M.  D.,  AND 

RICHARD  QUAIN,   F.  R.  S.,  &c. 
HUMAN  ANATOMY.     Revised,  with  Notes  and  Additions,  by  JOSEPH  LEIDT, 

M.  D.,  Professor  of  Anatomy  in  the  University  of  Pennsylvania.  Complete  in  two  large  octavo 
volumes,  leather,  of  about  thirteen  hundred  pages.  Beautifully  illustrated  with  over  five  hundred 
engravings  on  wood.  $6  00. 

SIMPSON  (J.  Y.  ,   M.  D.f 

Professor  of  Midwifery,  &c.,  in  the  University  of  Edinburgh,  &c. 

CLINICAL  LECTURES  ON  THE  DISEASES  OF  FEMALES.     With  numc- 

rotis  illustrations. 

This  valuable  series  of  practical  Lectures  is  now  appearing  in  the  "MEDICAL,  NEWS  AND 
LIBRARY"  for  1860,  and  can  thus  be  had  without  cost  by  subscribers  to  the  "AMERICAN  JOURNAL 
OF  THE  MEDICAL  SCIENCES."  See  p.  2. 


28 


BLANCHARD    &   LEA'S    MEDICAL 


SARGENT  (F.  W.),   M.  D. 

ON  BANDAGING  AND  OTHER  OPERATIONS  OF  MINOR  SURGERY. 

Second  edition,  enlarged.     One  handsome  royal  12mo.  vol.,  of  nearly  400  pages,  with  182  wood- 
cuts.    Extra  cloth,  $1  40 ;  leather,  $1  50. 


Sargent's  Minor  Surgery  has  always  been  popular, 
and  deservedly  so.  It  furnishes  that  knowledge  of  the 
most  frequently  requisite  performances  of  surgical 
art  which  cannot  be  entirely  understood  by  attend- 
ing clinical  lectures.  The  art  of  bandaging,  which 
is  regularly  taught  in  Europe,  is  very  frequently 
overlooked  by  teachers  in  this  country  ;  the  student 
and  junior  practitioner,  therefore,  may  often  require 
that  knowledge  which  this  little  volume  so  tersely 
and  happily  supplies.  —  Charleston  Med.  Journ.  and 
Review,  M 


arch,  1856. 


A  work  that  has  been  so  long  and  favorably  known 
to  the  profession  as  Dr.  Sargent's  Minor  Surgery, 
needs  no  commendation  from  us.  We  would  remark, 
however,  in  this  connection,  that  minor  surgery  sel- 
dom gets  that  attention  in  our  schools  that  its  im- 
portance deserves.  Our  larger  works  are  also  very 
defective  in  their  teaching  on  these  small  practical 
points.  This  little  book  will  supply  the  void  which 
all  must  feel  who  have  not  studied  its  pages. — West- 
ern Lancet,  March,  1856. 


SMITH  (W.  TYLER),  M.  D., 

Physician  Accoucheur  to  St.  Mary's  Hospital,  &c. 

ON   PARTURITION,   AND   THE    PRINCIPLES    AND   PRACTICE   OF 

OBSTETRICS.    In  one  royal  12mo.  volume,  extra  cloth,  of  400  pages.    $1  25. 

BY  THE  SAME  AUTHOR. 

A  PRACTICAL  TREATISE  ON  THE  PATHOLOGY  AND  TREATMENT 

OF  LEUCORRHCEA.    With  numerous  illustrations.    In  one  very  handsome  octavo  volume, 
extra  cloth,  of  about  250  pages.    $1  50. 


SOLLY  ON  THE  HUMAN  BRAIN;  its  Structure, 
Physiology,  and  Diseases.  From  the  Second  and 
much  enlarged  London  edition.  In  one  octavo 
volume,  extra  cloth,  of  500  pages,  with  120  wood- 
cuts. $2  00. 

SKEY'S  OPERATIVE  SURGERY.    In  one  very 


handsome  octavo  volume,  extra  cloth,  of  over  650 
pages,  with  about  one  hundred  wood-cuts.  $3  '25. 
SIMON'S  GENERAL  PATHOLOGY,  as  conduc- 
ive to  the  Establishment  of  Rational  Principles 
for  the  prevention  ano  Cure  of  Disease.  In  one 
octavo  volume,  extra  cloth,  of  212  pages.  $1  25. 


TODD  (R.  B.),  M.D.,    F.  R.  S.,  &c. 
CLINICAL  LECTURES  ON  CERTAIN  DISEASES  OF  THE  URINARY 

ORGANS  AND  ON  DROPSIES.    In  one  octavo  volume,  284  pages.  .  $1  50. 
BY  THE  SAME  AUTHOR.     (Now  Ready.) 

CLINICAL  LECTURES  ON  CERTAIN  ACUTE  DISEASES.     In  one  neat 

octavo  volume,  of  320  pages,  extra  cloth.     $1  75. 

The  subjects  treated  in  this  volume  are — RHEUMATIC  FEVER,  CONTINUED  FEVER,  ERYSIPELAS, 
ACUTE  INTERNAL  INFLAMMATION,  PYJEMIA,  PNEUMONIA,  and  the  THERAPEUTICAL  ACTION  OF  ALCO- 
HOL. The  importance  of  these  matters  in  the  daily  practice  of  every  physician,  and  the  sound 
practical  nature  of  Dr.  Todd's  writings,  can  hardly  fail  to  attract  to  this  work  the  general  attention 
that  it  merits. 


TANNER  (T.    H.),    M.  D., 

Physician  to  the  Hospital  for  Women,  &c. 

A  MANUAL  OF  CLINICAL  MEDICINE  AND  PHYSICAL  DIAGNOSIS. 

To  which  is  added  The  Code  of  Ethics  of  the  American    Medical  Association.     Second 
American  Edition.    In  one  neat  volume,  small  12mo.,  extra  cloth,  87$  cents. 


TAYLOR  (ALFRED  S.),  M.  D.,  F.  R.  S., 

Lecturer  on  Medical  Jurisprudence  and  Chemistry  in  Guy's  Hospital. 

MEDICAL  JURISPRUDENCE.  Fourth  American  Edition.  With  Notes  and 
References  to  American  Decisions,  by  EDWARD.HARTSHORNE,  M.  D.  In  one  large  octavo  volume, 
leather,  of  over  seven  hundred  pages.  $3  00. 


No  work  upon  the  subject  can  be  put  into  the 
hands  of  students  either  of  law  or  medicine  which 
will  engage  them  more  closely  or  profitably ;  and 
none  could  be  oflered  to  the  busy  practitioner  oi 
either  calling,  for  the  purpose  of  casual  or  hasty 
reference,  that  would  be  more  likely  to  afford  the  aid 
desired.  We  therefore  recommend  it  as  the  best  and 
safest  manual  for  daily  use.— American  Journal  oj 
Medical  Sciences. 

BY   THE   SAME   AUTHOR 


It  is  not  excess  of  praise  to  say  that  the  volume 
before  us  is  the  very  best  treatise  extant  on  Medical 
Jurisprudence.  In  saying  this,  we  do  not  wish  to 
be  understood  as  detracting  from  the  merits  of  the 
excellent  works  of  Beck,  Ryan,  Traill,  Guy,  and 
others;  but  in  interest  and  value  we  think  it  must 
be  conceded  that  Taylor  is  superior  to  anything  that 
has  preceded  it.— N.  W.  Medical  and  Surg.  Journal. 


(New  Edition,  just  issued.) 

ON  POISONS,  IN  RELATION  TO  MEDICAL  JURISPRUDENCE  AND 

MEDICINE.    Second  American,  from  a  second  and  revised  London  edition.     In  one  large 

octavo  volume,  oi  755  pages,  leather.    $3  50. 

Since  the  first  appearance  of  this  work,  the  rapid  advance  of  Chemistry  has  introduced  into 
use  many  new  substances  which  may  become  fatal  through  accident  or  design  —  while  at  the 
same  time  it  has  likewise  designated  new  and  more  exact  modes  of  counteracting  or  detecting  those 
previously  treated  of.  Mr.  Taylor's  position  as  the  leading  medical  jurist  of  England,  has  during 
this  period  conferred  on  him  extraordinary  advantages  in  acquiring  experience  on  these  subjects, 
nearly  all  cases  of  moment  being  referred  to  him  for  examination,  as  an  expert  whose  testimony 
is  generally  accepted  as  final.  The  results  of  his  labors,  therefore,  as  gathered  together  in  this 
volume,  carefully  weighed  and  sifted,  and  presented  in  the  clear  and  intelligible  style  for  which 
he  is  noted,  may  be  received  as  an  acknowledged  authority,  and  as  a  guide  to  be  followed  with 
implicit  confidence. 


AND    SCIENTIFIC    PUBLICATIONS.  29 

TODD  (ROBERT  BENTLEY),  M.  D.,  F.  R.  S., 

Professor  of  Physiology  in  King's  College,  London;  and 

WILLIAM  BOWMAN,  F.  R.  S., 

Demonstrator  of  Anatomy  in  King's  College,  London. 

THE  PHYSIOLOGICAL  ANATOMY  AND  PHYSIOLOGY  OF  MAN.    With 

about  three  hundred  large  and  beautiful  illustrations  on  wood.    Complete  in  one  large  octavo 

volume,  of  950  pages,  leather.     Price  $4  50. 

£3F°  Gentlemen  who  have  received  portions  of  this  work,  as  published  in  the  "  MEDICAL  NEWS 
AND  LIBRARY,"  can  now  complete  their  copies,  if  immediate  application  be  made.  It  will  be  fur- 
nished as  follows,  free  by  mail,  in  paper  covers,  with  cloth  backs. 

PARTS  I.,  II.,  III.  (pp.  25  to  552),  $2  50. 

PART  IV.  (pp.  553  to  end,  with  Title,  Preface,  Contents,  &c.),  $2  00. 

Or,  PART  IV.,  SECTION  II.  (pp.  725  to  end,  with  Title,  Preface,  Contents,  &c.),  $1  25. 


A  magnificent  contribution  to  British  medicine, 
and  the  American  physician  who  shall  fail  to  peruse 
it,  will  have  failed  to  read  one  of  the  most  instruc- 
tive books  of  the  nineteenth  century.— N.  O.  Med. 
and  Surg.  Journal,  Sept.  1857. 

It  is  more  concise  than  Carpenter's  Principles,  and 
more  modern  than  the  accessible  edition  of  Muller's 
Elements;  its  details  are  brief,  but  sufficient;  its 
descriptions  vivid ;  its  illustrations  exact  and  copi- 
ous ;  and  its  language  terse  and  perspicuous.— 
Charleston  Med.  Journal,  July,  1857. 

We  know  of  no  work  on  the  subject  of  physiology 


so  well  adapted  to  the  wants  of  the  medical  student. 
Its  completion  has  been  thus  long  delayed,  that  the 
authors  might  secure  accuracy  by  personal  observa- 
tion.—St.  Louis  Med.  and  Surg.  Journal,  Sept.  '57. 

Our  notice,  though  it  conveys  but  a  very  feeble 
and  imperfect  idea  of  the  magnitude  and  importance 
of  the  work  now  under  consideration,  already  tran- 
scends our  limits  ;  and,  with  the  indulgtnce  of  our 
readers,  and  the  hope  that  they  will  peruse  the  book 
for  themselves,  as  we  feel  we  can  with  confidence 
recommend  it,  we  leave  it  in  their  hands.  —  The 
Northwestern  Med.  and  Surg.  Journal. 


TOYNBEE  (JOSEPH),   F.  R.  S., 

Aural  Surgeon  to,  and  Lecturer  on  Surgery  at,  St.  Mary's  Hospital. 

A  PRACTICAL  TREATISE  ON  DISEASES  OF  THE  EAR;  their  Diag- 
nosis, Pathology,  and  Treatment.  Illustrated  with  one  hundred  engravings  on  wood.  In  one 
very  handsome  octavo  volume,  extra  cloth,  $3  00.  (Now  Ready.) 

Mr.  Toynbee's  name  is  too  widely  known  as  the  highest  authority  on  all  matters  connected  with 
Aural  Surgery  and  Medicine,  to  require  special  attention  to  be  called  to  anything  which  he  may 
communicate  to  the  profession  on  the  subject.  Twenty  years'  labor  devoted  to  the  present  work 
has  embodied  in  it  the  results  of  an  amount  of  experience  and  observation  which  perhaps  no  other 
living  practitioner  has  enjoyed.  It  therefore  cannot  fail  to  prove  a  complete  and  trustworthy  guide 
on  all  matters  connected  with  this  obscure  and  little  known  class  of  diseases,  which  so  frequently 
embarrass  the  general  practitioner. 

The  volume  will  be  found  thoroughly  illustrated  with  a  large  number  of  original  wood-engrav- 
ings, elucidating  the  pathology  of  the  organs  of  hearing,  instruments,  operations,  &c.,  and  in  every 
respect  it  is  one  of  the  handsomest  specimens  of  mechanical  execution  issued  from  the  American 
press. 

The  following  condensed  synopsis  of  the  contents  will  show  the  plan  adopted  by  the  author,  and 
the  completeness  with  which  all  departments  of  the  subject  are  brought  under  consideration. 
CHAPTER  I.  Introduction — Mode  cf  Investigation — Dissection.  II.  The  External  Ear — Ana- 
tomy— Pathology — Malformations  —  Diseases.  III.  The  External  Meatus  —  Its  Exploration. 
IV.  The  External  Meatus — Foreign  Bodies  and  Accumulations  of  Cerumen.  V.  The  External 
Meatus— The  Dermis  and  its  Diseases.  VI.  The  External  Meatus— Polypi.  VII.  The  External 
Meatus — Tumors.  VIII.  The  Membrana  Tympani — Structure  and  Functions.  IX.  The  Mem- 
brana Tympani — Diseases.  X.  The  Membrana  Tympani — Diseases.  XI.  The  Eustachian 
Tube — Obstructions.  XII.  The  Cavity  of  the  Tympanum — Anatomy — Pathology — Diseases. 
XIII.  The  Cavity  of  the  Tympanum— Diseases.  XIV.  The  Mastoid  Cells— Diseases.  XV. 
The  Diseases  of  the  Nervous  Apparatus  of  the  Ear,  producing  what  is  commonly  called  "  Nerv- 
ous Deafness."  XVI.  The  Diseases  of  the  Nervous  Apparatus,  continued.  XVII.  Malignant 
Disease  of  the  Ear.  XVIII.  On  the  Deaf  and  Dumb.  XIX.  Ear-Trumpets  and  their  uses. 
APPENDIX. 

WILLIAMS  (C.  J.  B.),    M.D.,    F.  R.  S., 

Professor  of  Clinical  Medicine  in  University  College,  London,  &c. 

PRINCIPLES  OF  MEDICINE.     An  Eleraentaiy  View  of  the  Causes,  Nature, 

Treatment,  Diagnosis,  and  Prognosis  of  Disease ;  with  brief  remarks  on  Hygienics,  or  the  pre- 
servation of  health.  A  new  American,  from  the  third  and  revised  London  edition,  in  one  octavo 
volume,  leather,  of  about  500  pages.  $2  50.  (Just  Issued.) 

We  find  that  the  deeply-interesting  matter  and  expressed.  It  is  a  judgment  of  almost  unqualified 
style  of  this  book  have  so  far  fascinated  us,  that  we  praise.— London  Lancet. 


have  unconsciously  hung  upon  its  pages 
long,  indeed,  for  our  own  profit,  but  longer  than  re- 
viewers can  be  permitted  to  indulge.  We  leave  the 
further  analysis  to  the  student  and  practitioner.  Our 
judgment  of  the  work  has  already  been  sufficiently 


A  text-book  to  which  no  other  in  our  language  is 
comparable. — Charleston  Medical  Journal. 

No  work  has  ever  achieved  or  maintained  a  more 
deserved  reputation.— Va.  Med.  and  Surg.  Journal. 


WHAT   TO   OBSERVE 
AT    THE    BEDSIDE    AND    AFTER   DEATH,   IN    MEDICAL   CASES. 

Published  under  the  authority  of  the  London  Society  for  Medical  Observation.    A  new  American, 

from  the  second  and  revised  Londou  edition.    In  one  very  handsome  volume,  royal  12mo.,  extra 

cloth.    $1  00. 

To  the  observer  who  prefers  accuracy  to  blunders  I  One  of  the  finest  aids  to  a  young  practitioner  we 
and  precision  to  carelessness,  this  little  book  is  in-  I  have  ever  seen. — Peninsular  Journal  of  Medici**. 
valuable.— N.  H,  Journal  of  Medicint.  [ 


30  BLANCHARD   &    LEA'S   MEDICAL 

New  and  much  enlarged  edition — (Just  Issued.) 

WATSON   (THOMAS),    M.D.,    &c., 

Late  Physician  to  the  Middlesex  Hospital,  &c. 

LECTURES    ON    THE   PRINCIPLES    AND    PRACTICE   OP   PHYSIC. 

Delivered  at  King's  College,  London.     A  new  American,  from  the  last  revised  and  enlarged 

English  edition,  with  Additions,  by  D.  FRANCIS  CONDIE,  M.  D.,  author  of"  A  Practical  Treatise 

on  the  Diseases  of  Children,"  &c.     With  one  hundred  and  eighty.five  illustrations  on  wood.     In 

one  very  large  and  handsome  volume,  imperial  octavo,  of  over  ]200  closely  printed  pages  in 

small  type ;  the  whole  strongly  bound  in  leather,  with  raised  bands.     Price  $4  2o. 

That  the  high  reputation  of  this  work  might  be  fully  maintained,  the  author  has  subjected  it  to  a 

thorough  revision ;  every  portion  has  been  examined  with  the  aid  of  the  most  recent  researches 

in  pathology,  and  the  results  of  modern  investigations  in  both  theoretical  and  practical  subjects 

have  been  carefully  weighed  and  embodied  throughout  its  pages.     The  watchful  scrutiny  of  the 

editor  has  likewise  introduced  whatever  possesses  immediate  importance  to  the  American  physician 

in  relation  to  diseases  incident  to  our  climate  which  are  little  known  in  England,  as  well  as  those 

points  in  which  experience  here  has  led  to  different  modes  of  practice  ;  and  he  has  also  added  largely 

to  the  series  of  illustrations,  believing  that  in  this  manner  valuable  assistance  may  be  conveyed  to 

the  student  in  elucidating  the  text.     The  work  will,  therefore,  be  found  thoroughly  on  a  level  with 

the  most  advanced  state  of  medical  science  on  both  sides  of  the  Atlantic. 

The  additions  which  the  work  has  received  are  shown  by  the  fact  that  notwithstanding  an  en- 
largement in  the  size  of  the  page,  more  than  two  hundred  additional  pages  have  been  necessary 
to  accommodate  the  two  large  volumes  of  the  London  edition  (which  sells  at  ten  dollars),  within 
the  compass  of  a  single  volume,  and  in  its  present  form  it  contains  the  matter  of  at  least  three 
ordinary  octavos.  Believing  it  to  be  a  work  which  should  lie  on  the  table  of  every  physician,  and 
be  in  the  hands  of  every  student,  the  publishers  have  put  it  at  a  price  within  the  reach  of  all,  making 
it  one  of  the  cheapest  books  as  yet  presented  to  the  American  profession,  while  at  the  same  time 
the  beauty  of  its  mechanical  execution  renders  it  an  exceedingly  attractive  volume. 

The  fourth  edition  now  appears,  so  carefully  re-  j  The  lecturer's  skill,  his  wisdom,  his  learning,  are 
vised,  as  to  add  considerably  to  the  value  of  a  book  ;  equalled  by  the  ease  of  his  graceful  diction,  his  elo- 
already  acknowledged,  wherever  the  English  Ian-  |  quence,  and  the  far  higher  qualities  of  candor,  of 
guage  is  read,  to  be  beyond  all  comparison  the  best  i  courtesy,  of  modesty,  and  of  generous  appreciation 
systematic  work  on  the  Principles  and  Practice  of;  of  merit  in  others.  May  he  long  remain  to  instruct 
Physic  in  the  whole  range  of  medical  literature.  !  us,  and  to  enjoy,  in  the  glorious  sunset  of  his  de- 
Every  lecture  contains  proof  of  the  extreme  anxiety  !  clining  years,  the  honors,  the  confidence  and  love 
of  the  author  to  keep  pace  with 'he  advancing  know- j  gained  during  his  useful  life. — N.  A.  Med-Chir. 
ledge  of  the  day,  and  to  bring  the  results  of  the  Review,  July,  1858. 
labors,  not  only  of  physicians,  b 


)ut  of  chemists  and 

histologists,  before  his  readers,  wherever  they  can 
be  turned  to  useful  account.  And  this  is  done  with 
such  a  cordial  appreciation  of  the  merit  due  to  the 
industrious  observer,  such  a  generous  desire  to  en- 
courage younger  and  rising  men,  and  such  a  candid 
acknowledgment  of  his  own  obligations  to  them, 
that  one  scarcely  knows  whether  to  admire  most  the 
pure,  simple,  forcible  English  —  the  vast  amount  of 
useful  practical  information  condensed'  into  the 
Lectures  —  or  the  manly,  kind-hearted,  unassuming 
character  of  the  lecturer  shining  through  his  work. 
—London  Med.  Times  and  Gazette,  Oct.  31,  1857. 

Thus  these  admirable  volumes  come  before  the 
profession  in  their  fourth  edition,  abounding  in  those 


Watson's  unrivalled,  perhaps  unapproachable 
work  on  Practice  —  the  copious  additions  made  to 
which  (the  fourth  edition)  have  given  it  all  the  no- 
velty and  much  of  the  interest  of  a  new  book.  — 
Charleston  Med.  Journal,  July,  1858. 

Lecturers,  practitioners,  and  students  of  medicine 
will  equally  hail  the  reappearance  of  the  work  of 
Dr.  Watson  in  the  form  of  anew  —  a  fourth  —  edition. 
We  merely  do  justice  to  our  own  feelings,  and,  we 
are  sure,  of  the  whole  profession,  if  we  thank  him 
for  having,  in  the  trouble  and  turmoil  of  a  large 
practice,  made  leisure  to  supply  the  hiatus  caused 
by  the  exhaustion  of  the  publisher's  stock  of  the 
third  edition,  which  has  been  severely  felt  for  the 


distinguished  attributes  of  moderation,  judgment,  I  last  three  years.    For  Dr.  Watson  has  not  merely 
...     .__,,.=—  i;__    -i  -------   .,_j  -i  --------   ..-.-^u  I 


erudite  cultivation,  clearness,  and  eloquence,  with 
which  they  were  from  the  first  invested,  but  yet 
richer  than  before  in  the  results  of  more  prolonged 
observation,  and  in  the  able  appreciation  of  the 
latest  advances  in  pathology  and  medicine  by  one 
of  the  most  profound  medical  thinkers  of  the  day.  — 
London  Lancet,  Nov.  14,  1857. 


cause(i  tne  lectures  to  be  reprinted,  but  scattered 
through  the  whole  work  we  find  additions  or  altera- 
tions which  prove  that  the  author  has  in  every  way 
sought  to  bring  up  his  teaching  to  the  level  of  che 
most  recent  acquisitions  in  science.  —  Brit,  and  For, 
Medico-Chir.  Review,  Jan.  1858. 


WALSHE  (W.    H.),   M.  D., 

Professor  of  the  Principles  and  Practice  of  Medicine  in  University  College,  London,  &c. 

A  PRACTICAL  TREATISE  ON  DISEASES  OF  THE  LUNGS;  including 

the  Principles  of  Physical  Diagnosis.    A  new  American,  from  the  third  revised  and  much  en- 
larged London  edition.     In  one  vol.  octavo,  of  468  pages.    (Just  Issued,  June,  1860.)     $225. 
The  present  edition  has  been  carefully  revised  and  much  enlarged,  and  may  be  said  in  the  main 
to  be  rewritten.    Descriptions  of  several  diseases,  previously  omitted,  are  now  introduced;  the 
causes  and  mode  of  production  of  the  more  important  affections,  so  far  as  they  possess  direct  prac- 
tical significance,  are  succinctly  inquired  into;  an  effort  has  been  made  to  bring  the  description  of 
anatomical  characters  to  the  level  of  the  wants  of  the  practical  physician  ;  and  the  diagnosis  and 
prognosis  of  each  complaint  are  more  completely  considered.     The  sections  on  TREATMENT  and 
the  Appendix  (concerning  the  influence  of  climate  on  pulmonary  disorders),  have,  especially,  been 
largely  extended. — Author's  Preface. 
%*%  To  be  followed  by  a  similar  volume  on  Diseases  of  the  Heart  and  Aorta. 

WILSON  (ERASMUS),   F.  R.  S., 

Lecturer  on  Anatomy,  London. 

THE    DISSECTOR'S  MANUAL;  or,  Practical  and  Surgical  Anatomy.     Third 

American,  from  the  last  revised  and  enlarged  English  edition.  Modified  and  rearranged,  by 
WILLIAM  HUNT,  M.  D.,  Demonstrator  of  Anatomy  in  the  University  of  Pennsylvania.  In  one 
large  and  handsome  royal  12mo.  volume,  leather,  of  582  pages,  with  154  illustrations.  $2  00. 


AND    SCIENTIFIC    PUBLICATIONS 


31 


New  and  much  enlarged  edition — (Just  Issued.) 

WILSON    (ERASMUS),  F.  R.  S. 

A  SYSTEM  OF  HUMAN  ANATOMY,  General  and  Special.  A  new  and  re- 
vised American. from  the  last  and  enlarged  English  Edition.  Edited  by  W.  H.GOBRECHT,  M.  IX, 
Professor  of  Anatomy  in  the  Pennsylvania  Medical  College,  &c.  Illustrated  with  three  hundred 
and  ninety-seven  engravings  on  wood.  In  one  large  and  exquisitely  printed  octavo  volume,  of 
over  600  large  pages;  leather.  $3  25. 

The  publishers  trust  that  the  well  earned  reputation  so  long  enjoyed  by  this  work  will  be  more 
than  maintained  by  the  present  edition.  Besides  a  very  thorough  revision  by  the  author,  it  has  been 
mo*t  carefully  examined  by  the  editor,  and  the  efforts  of  both  have  been  directed  to  introducing 
everything  which  increased  experience  in  its  use  has  suggested  as  desirable  to  render  .it  a  complete 
text-book  for  those  seeking  to  obtain  or  to  renew  an  acquaintance  with  Human  Anatomy.  The 
amount  of  additions  which  it  has  thus  received  may  be  estimated  from  the  fact  that  the  present 
edition  contains  over  one-fourth  more  matter  than  the  last,  rendering  a  smaller  type  and  an  enlarged 
page  requisite  to  keep  the  volume  within  a  convenient  size.  The  author  has  not  only  thus  added 
largely  to  the  work,  but  he  has  also  made  alterations  throughout,  wherever  there  appeared  the 
opportunity  of  improving  the  arrangement  or  style,  so  as  to  present  every  fact  in  its  most  appro- 
priate manner,  and  to  render  the  whole  as  clear  and  intelligible  as  possible.  The  editor  has 
exercised  the  utmost  caution  to  obtain  entire  accuracy  in  the  text,  and  has  largely  increased  the 
number  of  illustrations,  of  which  there  are  about  one  hundred  and  fifty  more  in  this  edition  than 
in  the  last,  thus  bringing  distinctly  before  the  eye  of  the  student  everything  of  interest  or  importance. 


It  may  be  recommended  to  the  student  as  no  less 
distinguished  by  its  accuracy  and  clearness  of  de- 
scription than  by  its  typographical  elegance.  The 
wood-cuts  are  exquisite. — Brit,  and  For.  Medical 
Review. 

An  elegant  edition  of  one  of  the  most  useful  and 
accurate  systems  of  anatomical  science  which  has 
been  issued  from  the  press  The  illustrations  are 
really  beautiful.  In  its  style  the  work  is  extremely 
concise  and  intelligible.  No  one  can  possibly  take 
up  this  volume  without  being  struck  with  the  great 


beauty  of  its  mechanical  execution,  and  the  clear- 
ness of  the  descriptions  which  it  contains  is  equally 
evident.  Let  students,  by  all  means  examine  tne 
claims  of  this  work  on  their  notice,  before  they  pur- 
chase a  text-book  of  the  vitally  important  science 

which  this  volume  so  fully  and  easily  unfolds. 

Lancet. 

We  regard  it  as  the  best  system  now  extant  for 
students. — Western  Lancet. 

It  therefore  receives  our  highest  commendation. — 
Southern  Med.  and  Surg.  Journal. 


BY   THE   SAME   AUTHOR.      (Just  Issued.) 

ON  DISEASES  OF  THE  SKIN.     Fourth  and  enlarged  American,  from  the  last 
and  improved  London  edition.     In  one  large  octavo  volume,  of  650  pages,  extra  cloth,  $2  75. 


The  writings  of  Wilson,  upon  diseases  of  the  skin, 
are  by  far  tne  most  scientific  and  practical  that 
have  ever  been  presented  to  the  medical  world  on 
this  subject.  The  presenteditionisa  great  improve- 
ment on  all  its  predecessors.  To  dwell  upon  all  the 
great  merits  arid  high  claims  of  the  work  before  us, 
seriatim,  would  indeed  be  an  agreeable  service;  it 
would  be  a  mental  homage  which  we  could  freely 
offer,  but  we  should  thus  occupy  an  undue  amount 
of  space  in  this  Journal.  We  will,  however,  look 


at  some  of  the  more  salient  points  with  which  it 
abounds,  and  which  make  it  incomparaoiy  superior  in 
excellence  to  all  other  treatises  on  the  subject  of  der- 
matology. No  mere  speculative  views  are  allowed 
a  place  in  this  volume,  which,  without  a  doubt,  will, 
for  a  very  long  period,  be  acknowledged  as  the  chief 
standard  work  on  dermatology.  The  principles  of 
an  enlightened  and  rational  therapeia  are  introduced 
on  every  appropriate  occasion. — Am.  Jour.  Med 
Science,  Oct.  1857. 


of 


ALSO,  NOW  READY, 

A  SERIES  OF  PLATES  ILLUSTRATING  WILSON  ON  DISEASES   OF 

THE  SKIN ;  consisting  of  nineteen  beautifully  executed  plates,  of  which  twelve  are  exquisitely 
colored,  presenting  the  Normal  Anatomy  and  Pathology  of  the  Skin,  and  containing  accurate  re- 
presentations of  about  one  hundred  varieties  of  disease,  most  of  them  the  size  of  nature.  Price 
in  cloth  $4  25. 

In  beauty  of  drawing  and  accuracy  and  finish  of  coloring  these  plates  will  be  found  equal  to 
anything  of  the  kind  as  yet  issued  in  this  country. 

The  plates  by  which  this  edition  is  accompanied 
leave  nothing  to  be  desired,  so  far  as  excellence  of 
delineation  and  perfect  accuracy  of  illustration  are 
concerned. — Medico-Chirurgical  Revieto. 

Of  these  plates  it  is  impossible  to  speak  too  highly 
The  representations  of  the  various  forms  of  cutane- 
ous disease  are  singularly  accurate,  and  the  color- 
ing exceeds  almost  anything  we  have  met  with  in 
point  of  delicacy  and  finish. — British  and  Foreign 
Medical  Review. 

BY   THE   SAME   AUTHOR. 

ON    CONSTITUTIONAL    AND    HEREDITARY    SYPHILIS,   AND    OiN 

SYPHILITIC  ERUPTIONS.  In  one  small  octavo  volume,  extra  cloth,  beautifully  printed,  with 
four  exquisite  colored  plates,  presenting  more  than  thirty  varieties  of  syphilitic  eruptions.  $2  25, 

BY    THE   SAME   AUTHOR. 

HEALTHY  SKIN;  A  Popular  Treatise  on  the  Skin  and  Hair,  their  Preserva- 
tion and  Management.  Second  American,  from  the  fourth  London  edition.  One  neat  volume, 
royal  12mo.5  extra  cloth,  of  about  300  pages,  with  numerous  illustrations.  $1  00 ;  paper  cover, 
75  cents. 


We  have  already  expressed  our  high  appreciation 
Mr.  Wilson's  treatise  on  Diseases  of  the  Skin. 
The  plates  are  comprised  in  a  separate  volume, 
which  we  counsel  all  those  who  possess  the  text  to 
purchase.  It  is  a  beautiful  specimen  of  color  print- 
ing, and  the  representations  of  the  various  forms  of 
skin  disease  are  as  faithful  as  is  possible  in  plates 
of  the  size.— Boston  Med.  and  Surg.  Journal.  April 
8,  1858. 


VVHITEHEAD  ON  THE  CAUSES  AND  TREAT- 
MENT OF    ABORTION    AND  STERILITY. 


Second  American  Edition.    In  one  volume,  octa- 
vo extra  cloth,  pp.  308.    SI    75. 


32 


BLANCHARD   &    LEA'S    MEDICAL    PUBLICATIONS. 


ON  OBSCURE  DISEASES  OF  THE  BRAIN  AND  DISORDERS  OF  THE 

MIND;  their  incipient  Symptoms,  Pathology,  Diagnosis,  Treatment,  and  Prophylaxis.  la  one 
handsome  octavo  volume,  of  nearly  600  pages.  (Just  Issued,  June,  1660.)  $3  00. 
The  momentous  questions  discussed  in  this  volume  have  perhaps  not  hitherto  been  so  ably  and 
elaborately  treated.  Dr.  Window's  distinguished  reputation  and  long  experience  in  everything  re- 
lating to  insanity  invest  his  teachings  with  the  highest  authority,  and  in  this  carefully  considered 
volume  he  has  drawn  upon  the  accumulated  resources  of  a  life  of  observation.  His  deductions 
are  founded  on  a  vast  number  of  cases,  the  peculiarities  of  which  are  related  in  detail,  rendering 
the  work  not  only  one  of  sound  instruction,  but  of  lively  interest;  the  author's  main  object  being 
to  point  out  the  connection  between  organic  disease  and  insanity,  tracing  the  lalter  through  all  its 
stages  from  mere  eccentricity  to  mania,  and  urging  the  necessity  of  early  measures  of  prophylaxis 
and  appropriate  treatment.  A  subject  of  greater  importance  to  society  at  large  could  scarcely  be 
named;  while  to  the  physician  who  may  at  any  moment  be  called  upon  for  interference  in  the  most 
delicate  relations  of  life,  or  for  an  opinion  in  a  court  of  justice,  a  work  like  the  present  Kay  be  con- 
sidered indispensable. 

The  treatment  of  the  subject  may  be  gathered  from  the  following  summary  of  the  contents  : — 
CHAPTER  I.  Introduction. — II.  Morbid  Phenomena  of  Intelligence.  III.  Premonitory  Symptoms 
of  Insanity. — IV.  Confessions  of  Patients  after  Recovery. — V.  State  of  the  Mind  during  Re- 
covery.— VI.  Anomalous  and  Masked  Affections  of  the  Mind. — VII.  The  Stage  of  Consciousness. 
— Vlil.  Stage  of  Exaltation. — IX.  Stage  of  Mental  Depression. — X.  Stage  of  Aberration. — XL 
Impairment  of  Mind. — XII.  Morbid  Phenomena  of  Attention. — XIII.  Morbid  Phenomena  of 
Memory — XIV.  Acute  Disorders  of  Memory. — XV.  Chronic  Affections  of  Memory. — XVI. 
Perversion  and  Exaltation  of  Memory. — XVII.  Psychology  and  Pathology  of  Memory. — XVIII. 
Morbid  Phenomena  of  Motion. — XIX.  Morbid  Phenomena  of  Speech. — XX.  Morbid  Phenomena 
of  Sensation. — XXI.  Morbid  Phenomena  of  the  Special  Senses. — XXII.  Morbid  Phenomena  of 
Vision,  Hearing,  Taste,  Touch,  and  Smell. — XXIII.  Morbid  Phenomena  of  Sleep  and  Dreaming. 
— XXIV.  Morbid  Phenomena  of  Organic  and  Nutritive  Life. — XXV.  General  Principles  of  Pa- 
thology, Diagnosis,  Treatment,  and  Prophylaxis. 

WEST   (CHARLES),    M.  D., 

Accoucheur  to  and  Lecturer  on  Midwifery  at  St.  Bartholomew's  Hospital,  Physician  to  the  Hospital  for 

Sick  Children,  &c. 

LECTURES  ON  THE  DISEASES  OF  WOMEN.     Now  complete  in  one  hand- 
some octavo  volume,  extra  cloth,  of  about  500  pages ;  price  $2  50. 
Also,  for  sale  separate,  PART  II,  being  pp.  309  to  end,  with  Index,  Title  matter, 

&c.,  8vo.,  cloth,  price  $1. 


We  must  now  conclude  this  hastily  written  sketch 
with  the  confident  assurance  to  our  readers  that  the 
work  will  well  repay  perusal.  The  conscientious, 
painstaking,  practical  physician  is  apparent  on  every 
page.— N.  Y.  Journal  of  Medicine,  March,  1858. 

We  know  of  no  treatise  of  the  kind  so  complete 
and  yet  so  compact.— Chicago  Med.  Journal,  Janu- 


and  children  is  not  to  be  found  in  any  country. — 
Southern  Med.  and  Surg.  Journal,  January  1858. 

We  gladly  recommend  his  Lectures  as  in  the  high- 
est degree  instructive  to  all  who  are  interested  in 
obstetric  practice. — London  Lancet. 

We  have  to  say  of  it,  briefly  and  decidedly,  that 
it  is  the  best  work  on  the  subject  in  any  language; 
and  that  it  stamps  Dr.  West  as  the  facile  princeps 
of  British  obstetric  authors. — Edinb.  Med.  Journ. 


ary,  1858. 

A  fairer,  more  honest,  more  earnest,  and  more  re- 
liable investigator  of  the  many  diseases  of  women 

BY  THE  SAME  AUTHOR.     (Now  Ready.) 

LECTURES   ON   THE   DISEASES   OF  INFANCY  AND   CHILDHOOD. 

Third  American,  from  the  fourth  enlarged  and  improved  London  edition.     In  one  handsome 
octavo  volume,  extra  cloth,  of  about  six  hundred  and  fifty  pages.     $2  75. 

The  continued  favor  with  which  this  work  has  been  received  has  stimulated  the  author  to  ren- 
der it  in  every  respect  more  complete  and  more  worthy  the  confidence  of  the  profession.  Con- 
taining nearly  two  hundred  pages  more  than  the  last  American  edition,  with  several  additional 
Lectures  and  a  careful  revision  and  enlargement  of  those  formerly  comprised  in  it,  it  can  hardly 
fail  to  maintain  its  reputation  as  a  clear  and  judicious  text-book  for  the  student,  and  a  safe  and 
reliable  guide  for  the  practitioner.  The  fact  stated  by  the  author  that  these  Lectures  '•  now  embody 
the  results  of  900  observations  and  288  post-mortem  examinations  made  among  nearly  30,000 
children,  who,  during  the  past  twenty-years,  have  come  under  my  care,"  is  sufficient  to  show  their 
practical  value  as  the  result  of  an  amount  of  experience  which  few  physicians  enjoy. 


The  three  former  editions  of  the  work  now  before 
us  have  placed  the  author  in  tne  foremost  rank  of 
those  physicians  who  have  devoted  special  attention 
to  the  diseases  of  early  life  We  attempt  no  ana- 
1>  sis  of  this  edition,  but  may  refer  the  reader  to  some 
of  the  chapters  to  which  the  largest  additions  have 
been  made — those  on  Diphtheria,  Disorders  of  the 
Mind,  and  Idiocy,  for  instance— as  a  prooi  that  the 
work  is  really  a  new  edition ;  not  a  mere  roprint. 
In  its  preient  shape  it  will  be  lound  of  the  greatest 
possible  service  in  the  every-day  practice  of  nine- 
tenths  of  the  profession.— Med.  Times  and  Gazette, 
London,  Dec.  10,  1859. 

All  things  considtred,  this  book  of  Dr.  West  is 
by  far  the  best  treatise  in  our  language  upon  such 
modifications  of  morbid  action  and  disease  as  are 
witnessed  when  we  have  to  deal  with  infancy  and 
childhood.  It  is  true  that  it  confines  itself  to  such 
disorders  as  come  within  the  province  of  the  phy- 
sician, and  even  with  respect  to  these  it  is  unequal 
as  regards  minuteness  of  consideration,  and  some 


diseases  it  omits  to  notice  altogether.  But  those 
who  know  anything  of  the  present  condition  of 
paediatrics  will  readily  admit  that  it  would  be  next 
to  impossible  to  effect  more,  or  effect  it  better,  than 
the  accoucheur  of  St.  Bartholomew's  has  done  i-n  a 
single  volume.  The  lecture  (XVI.)  upon  Disorders 
of  the  Mind  in  children  is  an  admirable  specimen  of 
the  value  of  the  later  information  conveyed  in  the 
Lectures  of  Dr.  Charles  West.— London  Lancet, 
Oct.  22,  1859. 

Since  the  appearance  of  the  first  edition,  about 
eleven  years  ago,  the  experience  of  the  author  has 
doubled;  so  that,  whereas  the  lectures  at  first  were 
founded  on  six  hundred  observations,  and  one  hun- 
dred and  eighty  dissections  made  among  nearly  four- 
teen thousand  children,  they  now  embody  the  results 
of  nine  hundred  observations,  and  two  hundred  and 
eighty-eight  post- mortem  examinations  made  among 
nearly  thirty  thousand  children,  who,  during  the 
past  twenty  years,  have  been  under  his  care. — 
British  Med.  Journal,  Oct.  1,  1859. 


BY  THE  SAME  AUTHOR. 


AN  ENQUIRY  INTO  THE  PATHOLOGICAL  IMPORTANCE  OF  ULCER- 

ATION  OF  THE  OS  UTERI.    In  one  neat  octavo  volume,  extra  cloth.    $1  00. 


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